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H,P0
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Mol. Wt. 98 04A C T U A L B A T C H A N A L Y S I S (Not merely maximum Impurity valuu)
EBatch No. 17631
Chloride (Cl)... 0.00015%
Sulphate (SOr)... 0.002%
Nitrate (NOi) ... 0.0005%
Lead (Pb) ... 550.0002%
Iron (Fe) ... ? 0.001% j Calcium and Magnesium (Ca +Mg) .. 0.003%
Oxygen Absorbed (O)... 0.0002%
Arsenic (AS
2
O3
)... 1.0 p.p.m.T h e above analysts Is based on the results, n o t o f o u r o w n C o n tr o l L a b o ra to rie s alone, b u t also on the c o n firm a to ry A n a ly tic a l C e rtific a te issued by Independent
C o n su lta n ts o f in te rn a tio n a l repute
T h e G e n e r a l C h e m i c a l & P h a r m a c e u t i c a l C o . , L t d . C h e m i c a l M a n u f a c t u r e r s , J u d e x W o r k s , S u d b u r y , M i d d l e s e x
SEPTEMBER, 1944 V o l. 69, N o . 822
T H E A N A L Y S T
P R O C E E D I N G S O F T H E S O C I E T Y O F P U B L I C A N A L Y S T S A N D O T H E R A N A L Y T I C A L C H E M I S T S
T a n n i n a s a S e l e c t i v e R e a g e n t f o r Z i r c o n i u m
By W . R. S C H O E L L E R , Ph.D., F.R.I.C.
It will be recalled that in the course of our investigations into the Analytical Chemistry of Tantalum, Niobium, a n d their Mineral Associates,1'2 w e have introduced the use of tannin as a reagent of p a r a m o u n t importance for the quantitative separation a n d gravimetric deter
mination of a n u m b e r of elements. T h e principal applications proposed b y us m a y be grouped under 4 heads according to the m e d i u m in which the reaction is m a d e to take place: (1) In oxalate soln., tantalum, titanium a n d niobium (Group A) are quantitatively separable from G r o u p B (zirconium, thorium, aluminium, uranium, etc.)3; within. G r o u p A, tantalum is quantitatively separable from niobium.4 (2) F r o m neutralised tartrate soln., tannin quanti
tatively ppts. all the above elements5; a n u m b e r of others, beryllium, rare earths, manganese, etc. (Group C) are pptd. only from a m m o n i a c a l tartrate soln.® (3) A soln. of tannin in dilute sulphuric acid, in contact with a bisulphate melt, dissolves titanium, zirconium, a n d metals yielding soluble sulphates, leaving an insol. residue containing tantalum a n d niobium.7 (4) Hydrochloric acid add e d to an alkaline tungstate soln. containing tannin ppts. the tungsten complex, cinchonine soln. being a d d e d for the sole purpose of completing its flocculation, not as a precipitant for tungstic acid (tannin-cinchonine m e t h o d 8). T h e present paper describes tannin reactions occurring in hydrochloric acid soln. a n d their bearing o n the analytical chemistry of zirconium.
T h e tannin-cinchonine m e t h o d provides a separation of tungsten from uranium, aluminium, a n d beryllium, if present in the alkaline soln., the tannin complexes of these elements being soluble in hydrochloric acid.9 Zirconium, on the other hand, has been s h o w n to interfere in the determination of tungsten b y the s a m e m e t h o d in • tungstate solns. containing a little sodi u m zirconate.10 T h e positive tungsten errors were ascribed to zirconia "occluded in the tannin-cinchonine ppt.” While re-investigating this rather unexpected result I found that zirconium chloride solns. containing free hydrochloric acid are actually pptd. b y tannin, the ppt. flocculating readily a n d filtering well. Further study revealed that the pptn. is quantitative a n d affords a separation from m a n y elements, the tannin complexes of which
are soluble in acid. •
E x p e r i m e n t a l — (1) Gravimetric determination of Zirconium— T w o solns. of pure re
crystallised zirconyl chloride in 0-1 N hydrochloric acid were standardised b y pptn. with a m m o n i a a n d ignition of the ppt. to Z r 0 2; 25-ml portions gave 0-0521 a n d 0-0655 g respec
tively. T h e pptn. with tannin w a s carried out as follows: 25-ml portions were diluted to ca. 50 ml, heated to boiling, a n d treated with 10 m l of saturated a m m o n i u m chloride soln.
a n d 1 g of tannin in strong freshly-made soln. T h e whitish flocculent ppts. were m i x e d with filter pulp, collected o n 11-cm. No. 41 W h a t m a n filters, w a s h e d with 2 % a m m o n i u m chloride soln., a n d ignited to Z r 0 2. F o u n d > 0 - 0 5 2 1 a n d 0-0654 g respectively, in excellent agreement with the values obtained b y a m m o n i a pptn.
(2) Separations— T h e tannin separation of zirconium from other metals hinges on the acidity of the soln., as w a s proved b y the test separations described below, in which the
•zirconia w a s pptd. in presence of other metallic chlorides at relatively high concns. A t or below 0-1 N acidity there is extensive co-pptn. of, e.g., aluminium, while at higher acid concns.
(e.g., 0-5 N ) the zirconia recovery is incomplete ( T P = ignited tannin ppt.):
Expt. ZrO», g A120 3, g Acidity T P Zr02 Error, g
1 0-0521 0-3890 ca. 0-1 N 0 0725 +0-0204
2 0-0521 0-3890 ca. 0-5 N 0-0477 — 0 0044
A separation m e t h o d involving re-treatment of T P appeared undesirable, since this w o u l d require bisulphate fusion followed b y conversion of the sulphate into chloride soln. I avoided
2 5 9
260 s c h o e l l e r:t a n n i n a s a SELECTIVE r e a g e n t f o r z i r c o n i u m
the re-treatment b y pptng. the ma j o r zirconia fraction ( T P 1) at 0-25 to 0-5 N acidity a n d recovering the balance ( T P 2) from the filtrate b y partial neutralisation. This simple pro
cedure w a s found to yield uniformly g o o d results.
Procedure— Treat the chloride soln. free from sulphate (0-1 g Z r 0 2) with 20 m l of saturated a m m o n i u m chloride soln., dilute to 200 ml, a n d adjust the hydrochloric acid concn: to 0-25 N (if v a n a d i u m or thorium is present, to 0-5 N). Boil, stir, a n d a d d a freshly-made strong soln. of 1 g of tannin. Gently boil for 1 min., set aside to cool, collect after 2 hr. o n an 11-cm. filter containing a little crea m e d filter-pulp, using a cold wash-liquor containing 50 m l of saturated a m m o n i u m chloride soln. a n d 25 m l of strong hydrochloric acid in 500 ml;
measure the v o l u m e of this wash-liquor before a n d after use. Ignite the w e t ppt., T P 1,
■in a tared porcelain crucible.
Min o r zirconia fraction: the quantity of acid in the filtrate.plus washings being k n o w n , stir the boiling liquid a n d a d d a m m o n i a (1 : 1) so as to leave 2-3 m l of free acid (determine the equivalence of the strong acid a n d 1 : 1 a m m o n i a used). A s neutralisation proceeds the liquid b e c o m e s turbid, a delicate test for minute a m o u n t s of zirconia; flo.cculation normally occurs almost at once. If too m u c h a m m o n i a is a d d e d a h e a v y flocculent ppt. begins to form, wh i c h readily dissolves in a few m l of 1 : 1 hydrochloric acid kept ready at hand.
Cool to r o o m temperature, collect a n d w a s h the ppt. as before, ignite w e t a n d weigh. T h e t w o ppts. m a y be ignited together or separately; I prefer the latter, should a purity test on T P2 appear desirable.
Test separations— -The above procedure gave the following results, the weight of metallic ride ad d e d being calculated to oxide (except in Expt. 15) :
Zr02 HCl Zr02 Zr02
Expt. taken added concn. T P 1 T P 2 found Error
g . g s g g g
3 0-0521 A120 3 0-3890 0-25 N 0-0511 0-0014 0-0525 + 0-0004
4 0-0521 Fe20 3 0-3600 do. 0-0500 0-0019 0-0519 -0-0002
5 0-0521 Cr20 3 0-2337 do. 0-0518 nil 0-0518 -0-0003
6 0-0655 R %0 3* 0-2060 do. 0-0635 0-0014 0-0649 -0-0006
7 0-0521 u 3o 8 0-2800 do. 0-0516 0-0009 0-0525 + 0-0004
8 0-0655 B e O 0-1185 do. 0-0615 0-0035 0-0650 -0-0005
9 0-0655 M n O 0-3140 do. 0-0642 0-0010 0-0652 -0-0003
10 0-0521 v 2o 4 0-2496 do. 0-0555 0-0012 0-0567 + 0 0046
11 0-0521 do. 0-2496 0-5 N 0-0443 0-0078 0-0521 0-0000 .
12 0-0521 ThO, 0-1005 0-25 N 0-0544 0-0041 0-0585 + 0-0064
13 0-0655 do. 0-1005 0-5 N 0-0583 0-0122 0-0705 + 0-0050
14 0-0655 do. 0-1005 do. 0-0561 0-0100 0-0661 + 0-0006
15 0-0655 Ni 1-00 do. 0-0628 0-0024 0-0652 -0-0003
* 1?20 3 = 0-1046 g N d 20 3.+ 0-1014 g Y 20 3.
O b s e r v a t i o n s — (1) Separation from iron— W h e n tannin is a d d e d to the hot hydrochloric acid soln., the zirconium c o m e s d o w n a& a b r o w n ppt. T h e discoloration w a s at first believed to be caused b y co-pptn. of iron, but o n ignition the b r o w n ppts. yield pure white Z r 0 2;
hence it m u s t be due to an organic impurity in the reagent, probably oxidised b y the ferric salt, since in the other test separations the zirconium ppts. were not und u l y discoloured.
A s a general rule, elements with colourless tannin complexes give white ppts. w h i c h b e c o m e buff or dirty white on further addition of reagent. T a n n i n a d d e d to hot acid ferric chloride solns. changes the colour to a reddish-brown, further addition of hydrochloric acid not re
storing the original yellow colour. Dr. Nierenstein, w h o m I consulted o n the question, suggests that these colour reactions m a y be d ue to the cyclogallipharic acid present in c o m mercial tannins.*
(2) F r o m vanadium— Solutions of vanadyl chloride were prepared from pure a m m o n i u m m e t a v a n a d a t e b y evaporation with strong hydrochloric acid. T h e separation m u s t be carried out in 0-5 N acid (Expt. II): at 0-25 N acidity a high result w a s obtained, a n d th#
ignited T P 1 w a s yellow (Expt. 10).
: 1 : —
* Cyclogallipharic acid, C 20H 31. O H . C O O H (Kunz-Krause, /. prakt. Chem., 1904, 69, 385, 387; Kunz- Krauseand Richter, J. prakt. Chem., 19.07, 75, 306; Arch. Pharm., 1907, 245, 28; Manicke, thesis, Dresden, 1910), crystallises from light petroleum in needles, m.p. S9° C. The cyclogallipharic acid placed by Heilbron and Bunbury (Dictionary of Organic Compounds) under the heading Hydroginkolic Acid is 6-hydroxy-2- pcntadecylbenzoic acid. It contains in its molecule one C atom more than Kunj-Krause’s compound and melts at 86°-S8° C. These data are taken from Sci. Papers, Inst. Phys. Chem. Res., Tokyo; presumably the cyclogallipharic acid derived from Japanese galls differs from that from European galls.
s c h o e l l e r:t a n n i n a s a SELECTIVE r e a g e n t f o r z i r c o n i u m 261 (3) F r o m thorium— A s w a s to be expected, the separation of zirconium from thorium is slightly m o r e delicate than the other operations here described. In Expt. 12, pptn. at 0-25 N acidity gave a positive error. In the next test (13), the higher acid concn. gave a T P1 free from thoria; the positive error w a s incurred wholly in the recovery of T P 2 through over-neutralisation, w h i c h necessitated re-acidification. O n c e formed, the thorium comp l e x evidently re-dissolves in acid m o r e sluggishly than the other tannin complexes. It is therefore essential to conduct the partial neutralisation so-that the soln. remains slightly acid; the v o l u m e of 1 : 1 a m m o n i a required should be carefully calculated as prescribed, a n d a strip of litmus paper partly i m m e r s e d in the soln. as a control. In Expt. 14, with an error of + 0 - 0 0 0 6 g, the c o m b i n e d ppts. gave a w e a k thorium reaction w h e n the bisulphate melt w a s extracted with oxalic acid.11
(4) F r o m nickel— This test separation from a large a m o u n t of nickel w a s undertaken with a view to using tannin in the determination of zirconium in materials subjected to sod i u m peroxide fusion in nickel crucibles. T h e separation w a s easily accomplished.
In t e r f e r e n c e s— T antalum, niobium, a n d tungsten cannot normally be present in simple chloride soins. Tin12 a n d titanium are pptd.; the tin c o m p l e x appears to be m o r e insoluble than those of zirconium a n d titanium. In acid soins, containing ferric chloride titanium yields a brownish-red ppt. of m u c h darker h u e than the vermilion c o m p l e x obtained in oxalate or tartrate soln. G e r m a n i u m gives a white tannin c o m p l e x insoluble in dilute sulphuric acid,13 the pptn. of which m a y serve for its separation from zirconium; hence g e r m a n i u m is no doubt pptd. b y tannin from hydrochloric acid soln. However, it is so rare that the possibility of its association with zirconium in m o r e than traces is quite remote.
Stannic a n d germanic chlorides are volatile; the t w o metals can be pptd. as sulphides. A s regards hafnium, w e m a y take it for granted that it follows zirconium in this as in all its other reactions. T h e h a f n i u m content of the zirconium preparation used in this w o r k is not k n o w n to me, but it m a y be recorded that the salt w a s prepared from purified zircon sand derived from the Travancore beach deposit. Sulphuric acid or sulphates should not be present; the former prevents, the latter retard a n d impair pptn. T o s u m up, titanium is the one element that m u s t always be tested a n d allowed for in zirconia obtained b y tannin pptn., but this is the rule in zirconium analysis b y other processes; if small, the titanium content is determined colorimetrically, if large, b y the gravimetric tannin process in oxalate soln.14
A n a l y t i c a l A p plication— T h e quantitative pptn. of zirconium b y tannin from acid chloride soln. is not merely of theoretical interest in establishing a n e w serial order of precipita- bility (probably Sn-Zr-Ti-Th-V-M"'-U-M"), as against the oxalate series,15 Ta-Ti-Nb-V-Fe-Zr- T h - A l - U - M " . Tan n i n undoubtedly has certain practical advantages over other précipitants, which should secure its adoption as one of the selective reagents for zirconium ; the operation is one of great simplicity, the reagent is cheap a n d stable a n d the ignition products of the ppt.
are innocuous. Single pptn. separates zirconium from preporiderating a m o u n t s of other metals; it is true that the recovery takes place in t w o fractions, but even so the operation is still simpler than, e.g., the arsenate16 or selenious acid17 process, both of wh i c h involve double pptn. as well as conversion of the sulphate into chloride soln.
T h e zirconium-tannin c o m plex produced in hydrochloric acid soln. contains less tannin a n d is less bulky than those pptd. from neutrahor a m m o n i a c a l soins. ; it flocculates readily, but hardly adheres tp the glass at all. It includes a n y titania present, which m u s t be deter
m i n e d a n d deducted. In the cupferron m e t h o d 18 the zirconium ppt. contains the whole of the titania a n d m i n o r quantities of thoria a n d rare earths. T h e ppt. obtained b y the arsenate m e t h o d 16 m u s t be tested colorimetrically for titania. T h e selenite ppt.17 includes a n y thoria present, a h d a colorimetric test for titania is advised. T h e phosphate m e t h o d 13 is the only one that yields a zirconium ppt. quite frée from titania; but it is unsuitable for substantial quantities, and* the ignited ppt. is not of constant composition. O n ignjtion the cupferron, arsenate, a n d selenite ppts. give off poisonous fumes.
This investigation is still in progress, a n d I h o p e to submit data on the tannin pptn.
of titanium a n d tin from chloride soins., the treatment of sulphate soins., the resolution of G r o u p B b y tannin without the use of cupferron,20 a n d the application of the tannin m e t h o d in the analysis of zirconium ores.
S u m m a r y — Zirconium is quantitatively pptd. b y tannin from chloride soln. containing free hydrochloric acid below O-liV concentration. T h e reaction affords a separation of
> zirconium from sesquioxides, monoxides, uranium, vanadium, a n d thorium, the bulk of the zirconium being first pptd. at 0-025 to 0-5 N acidity, a n d the small balance in the filtrate b y
262 A L D R I D G E : A N E W M E T H O D F O R T H E ESTIMATION O F MI C R O QUANTITIES O F
partial neutralisation. Titanium a n d tin arę pptd. with the zirconium. T h e advantages , of tannin as a selective reagent for zirconium are discussed. T h e application of tannin in zirconium analysis is under investigation.
Re f e r e n c e s 1. Schoeiler, W . R., An a l y s t, 1936, 61, 811.
2. --- "The Analytical Chemistry of Tantalum and Niobium," London, 1937.
3. --- and Powell, A. R., An a l y s t, 1932, 57, 551.
a ____
77
/ lOa“’ 57 7505. --- and Webb, H. W.,' Id.,1929, 54, 709.
6 . ---Id., 1934, 59, 669.
7. --- Id., 1929, 54, 454.
8. ---- and Jahn, C., Id., 1927, 52, 505.
9. .--- op. cit., 151.
10. ---- op. cit., 92.
11. --- and Powell, A. R., "The Analysis of Minerals and Ores of the Rarer Elements," 2nd Ed., London, 1940, 113.
12. Moser, L., and List, F., Monatsh. Client., 1929, 51, 1139.
13. Davies, G. It., and Morgan, G., An a l y s t, 1938, 63, 391.
14. Schoeiler and Powell, op. cit., 90.
15. ---- op. cit., 148.
16. Schumb, W . C., and Nolan, E. J.; Schoeiler and Powell, op. cit., 113.
17. Simpson, S. G., and Schumb, W . C.; Schoeiler and Powell, op. cit., 114.
18. Schoeiler and Powell, op. cit., 112.
19 . ---op. cit., 114.
20. ---- op. cit., 135.
' Th e Ta p p i n g Ho u s e
Great Missenden, Bucks. June, 1944
A N e w M e t h o d f o r t h e E s t i m a t i o n o f M i c r o
Q u a n t i t i e s o f C y a n i d e a n d T h i o c y a n a t e
By W . N. A L D R I D G E
I n t r o d u c t i o n — M a n y of the reactions for cyanide are either insensitive or of qualitative interest only, e.g., the picric acid,1 the Prussian blue2 a n d the copper-benzidine3 reactions.
T h e m e t h o d based o n conversion of cyanide into thiocyanate4 a n d determination of the thiocyanate b y the ferric thiocyanate procedure, although of reasonable sensitivity, is extremely unreliable with small a m o u n t s of cyanide, o w i n g largely to instability of the ferric thiocyanate colour. O f the other m e t h o d s of sufficient sensitivity, the phenolphthalein5 a n d the m o r e recent o-cresolphthalein6 methods, both require very careful control a n d are unspecific, the reduced form of these c o m p o u n d s being also oxidised b y m e a n s other than cyanide. T h e m e t h o d here described is based o n the conversion of cyanide a n d thiocyanate into cyanogen bromide, wh i c h is then estimated b y its colour reaction with amines in pyridine solution.7
M e t h o d— In neutral or acid soln. cyanide a n d thiocyanate are converted b y bromine water into cyanogen bromide:
H C N + Br, = C N B r + H B r
• R C N S + 4 B r 2 + 4 H 20 = K B r + C N B r + H 2S 0 4 + 6 H B r
T h e cyanogen bromide thus formed, after removal of excess of b r o mine with s o d i u m arsenite, reacts with a solution of benzidine in dil. pyridine to give an intense orange to red colour proportional to the a m o u n t of cyanogen bromide present.
B y this m e a n s 0-3/xg of hydrocyanic acid a n d 0-6^.g of thiocyanic acid m a y be easily a n d accurately estimated a n d the limit of detectibility o n a rough qualitative basis is of the order of 0-05/xg of H C N .
F o r the purpose for which the m e t h o d w a s required, it w a s desired to differentiate between cyanide a n d thiocyanate. This m a y be easily achieved b y m a k i n g use of the non-volatility of thiocyanic acid.
Reagents— Bromine water— saturated. Sodium arsenite soln.— 1 - 5 % in water. Pyridine reagent— 25 m l of pure redistilled pyridine, together with 2 m l of conc. hydrochloric acid, diluted to 100 m l with v rater. Benzidine hydrochloride soln.— 2 % in water.
C Y A N I D E A N D T H I O C Y A N A T E 263
0 0-71 1-42 2-12 2-83
0 0-230 0-425 0-655 0-870
0 1-24 2-48 3-72 4-96 6-20
0 0-175 0-370 0-545 0-715 0-885
Procedure— T o 1 m l of the soln., containing u p to 3/xg of H C N or 6/xg of H C N S a n d m a d e acid with acetic or trichloroacetic acid, a d d 0-5 m l of saturated bromine water a n d then 0-5 m l of 1 - 5 % s o d i u m arsenite-soln. A t this stage the soln. m a y be left stoppered for 2 hr. A d d 1 m l of this soln. to a mixture of 5 m l of the pyridine reagent a n d 0-2 m l of 2 % benzidine hydrochloride soln. T h e orange colour immediately produced soon changes to a red. Allow 10 m m . for colour development at r o o m temp, a n d read the resulting colour on the Spekker Absorptiometer in 1-cm cells, using an Ilford micro 2-303 blue filter. C o m p a r e the result with a calibration curve prepared in an identical manner.
Cy a n i d e a n d Th i o c y a n a t e i n t h e s a m e So l u t i o n— T hiocyanate a n d cyanide both react with b rom i n e to give cyanogen bromide. If both are present, it m i g h t be important to differentiate between the t w o in view of the relatively low toxicity of thiocyanate as c o m p a r e d with cyanide.
Procedure— C y a nide m a y be easily r e m o v e d b y bubbling air through a n acid soln.
of thiocyanate a n d cyanide, leaving the thiocyanate unchanged. T w o operations are necessary for the determination of both radicles— (1) Determine the Spekker reading for cyanide a n d thiocyanate together as above. (2) B u b b l e air saturated with water vapour through 1 m l of the soln. for 15 min. to r e m o v e cyanide a n d then continue in the usual m a n n e r for the thiocyanate. T h e reading obtained for (2) gives the thiocyanate present a n d the difference between the readings gives the cyanide present. A l t h o u g h throughout this w o r k the readings were taken o n the Spekker Absorptiometer, a n y other photoelectric instrument m a y be used.
E x p e r i m e n t a l — (1) Calibration curves— Solns. of cyanide a n d thiocyanate were standardised b y the usual titrations with silver nitrate. Dilutions w e r e then m a d e to give the dil. solutions used for the calibration curves. Portions of these were submitted to the technique outlined above. A blank gave a zero reading.
H C N . IXg Spekker reading H C N S , pg Spekker reading
If the results for the thiocyanate curve are calculated to cyanide a n d plotted o n the cyanide curve, the t w o curves coincide.
' 2. Determination of thiocyanate and cyanide together— T h e following results were o b tained with k n o w n mixtures of thiocyanate a n d cyanide.
H C N present, fig H C N S present, /rg H C N estimated, ¡xg . H C N S estimated, .¡xg .
3. Reaction,of cyanogen halides with the pyridine reagent and benzidine— Whilst both cyanogen chloride a n d bromide react with great speed with the reagent, cyanogen iodide reacts extremely slowly a n d is of n o value for the estimation in hand. C y a n o g e n bromide has been chosen because its volatility (b.p. 61° C.) is lower than that of the corresponding chloride (b.p. 13° C.).
4. Concentration of water in the colour development— K n o w n a m o u n t s of cyanogen bromide were a d d e d to a pyridine reagent containing various quantities of water. T h e colours developed w e r e read immediately on the Spekker Absorptiometer, using a n Ilford micro 2-303 blue filter.
Water. % .. 13 22 30 39 48 55 65 83 87
Spekker reading .. 1-02 1-06 1 07 1-08 1-08 1-09 1-10 1 10 1-10
Water, % .. 91 95
Spekker reading .. 1 -10 0-30
T h e sensitivity is increased slightly d o w n to a concn. of 1 0 % of pyridine in water;
further, in the dilute pyridine solns. the orange colour first formed rapidly changes to red at r o o m temp., a change w h i c h is accelerated b y hydrochloric acid in the pyridine soln. T h e change to red with 9 5 % pyridine, with the subsequent increase in sensitivity, requires con
trolled heating in. a water-bath. •
A pyridine reagent w a s chosen containing 2 5 % of pyridine a n d 2 % v / v of conc. h y d r o chloric acid. T h e benzidine hydrochloride solution w a s a d d e d separately before each test as the composite reagent rapidly darkens.
0 0-7 1-4 2-1 2-8 3-5
10-3 8-25 6-2 4-1 2-05 0
0-01 0-6 1-3 2-2 2-8 3-5
10-2 8-5 6-5 3-8 1-9 0
264 A L D R I D G E : A N E W M E T H O D F O R T H E ESTIMATION O F M I C R O QUANTITIES OF
5. Conversion into cyanogen bromide— (a) Removal of excess bromine— Excess of bromine reacts vigorously with the pyridine-benzidine reagent producing first the blue quinonoid c o m p o u n d which immediately decomposes with production "of a dark b r o w n colour. A n y substance used for the removal of bromine m u s t react in acid soln. (in alkaline solns. cyanogen b romide is converted into cyanate) a n d m u s t also h ave n o effect o n the subsequent colour development.
Various substances ranging from inorganic reducing agents such as stannous chloride, sod i u m thiosulphate, sod i u m sulphite a n d s o d i u m arsenite, to organic c o m p o u n d s such as metol, hydroquinone a n d 8-hydroxyquinaline were trjed. All except sodi u m arsenite were rejected for one or both of the considerations mentioned.
(b) Speed of conversion of cyanide into cyanogen bromide— Slightly acid cyanide solns.
were treated with bromine water a n d the excess w a s r e m o v e d after various intervals of time b y addition of sodi u m arsenite. T h e Spekker readings indicate that tire reaction is almost instantaneous.
Time of bromination, min. . . 0 2 4 6 10
Spekker reading . . .. 0-695 0-690 0-700 0-690 0-690
(c) Effect of excess sodium arsenite— Different a m o u n t s of 1 - 5 % s o d i u m arsenite soln.
(0-5 to 2-0 ml) were a d d e d to r e m o v e excess of bromine. T h e results indicate that a reasonable excess has n o effect o n the subsequent colour development.
Sodium arsenite, 1-6% soln., ml 0-5 1-0 1-5 2-0 Spekker reading .. . . 0-350 0-350 0-350 0-350
6. Speed of colour development and stability of colour— T h e colour developed w a s read after various intervals of time. T h e readings remained constant from 6 to 24 min. It should be noted that, although the colour alters m a r k e d l y in shade during this period, there is n o alteration in the extinction obtained with the blue filter.
Time, min. .. . . .. 0 Spekker reading .. .. 0-640 Time, min. . . ..
Spekker reading
7. Effect of other substances on the determination— Ferricyanides a n d cyanates have no effect, although it is certain that a n y c o m p o u n d that m a y produce C N ' or C N S ' radicles will be determined b y this method. T h e indications are that organic cyanides a n d thiocyanates c o m e into this class, although n o quantitative examination has been m a d e . N o n e of the nor m a l constituents of trichloroacetic acid blood filtrates has a n y adverse effect. T h e following substances also h a v e n o effect o n the reaction:— potassium a n d s o d i u m phosphates, a m m o n i u m a n d s o d i u m chlorides, potassium oxalate, s o d i u m borate, cobalt acetate in small quantities, phenyl acetic acid a n d ethyl alcohol. Oxidising a n d reducing agents present in small a m o u n t s will be r e m o v e d b y the s o d i u m arsenite a n d brom i n e water respectively.
Ap p l i c a t i o n o f t h e De t e r m i n a t i o n t o Bi o l o g i c a l M a t e r i a l s— T he m e t h o d given above can be readily adapted to the determination of cyanide a n d thiocyanate in biological materials. B o t h cyanide a n d thiocyanate m a y be determined in urine, a n d also in' saliva in m o s t instances without a n y preliminary treatment. Cyanide m a y be determined in whole blood, plasma or serum, a n d in pancreatic juice a n d other protein-containing fluids, after deproteinisation b y m e a n s of trichloroacetic acid. K n o w n a m o u n t s of cyanide- were added to whole blood, 1 m l w a s deproteinised with 2 m l of 5 % trichloroacetic acid, a n d the filtrate w a s analysed for cyanide as described.
H C N per ¡ig of whole blood
Added .. .. 6-33 11-6 16-07 17-48 19-9
Found .. .. 6-3 11-8 16-3 17-5 20-0
In plasma a n d other protein-containing fluids there w a s a 9 3 % recovery of thiocyanate w h e n n o haemoglobin w a s present, but in acid deproteinisation of whole blood S 0 % of the thiocyanate is adsorbed b y the ppt. A s yet, n o efficient m e t h o d for the determination of thiocyanate in whole blood has been developed. In m o s t instances, however, the deter
mination in plasma is quite sufficient, as the thiocyanate appears to be evenly distributed between cells a n d plasma.
I a m indebted to the Director-General, Scientific Research a n d Development, Ministry of Supply, for permission to publish this paper.
2 4 6 8 10 12
0-675 0-700 0-720 0-730 0-730 0-730
16 24 30 67
0-730 0-720 0-700 0-630
C Y A N I D E A N D T H I O C Y A N A T E 265 Re f e r e n c e s
1. Waller, A. D., An a l y s t, 1910, 35, 406.
2. Viehoever, A., and Johns, C. O., J. Amer. Chem. Soc., 1915, 37, 601.
3. Sieverts (A., and Hermsdorf, A., Z . angew. Chem., 1921, 34, 3.
4. Johnson, M. O., J. Amer. Chem. Soc., 1916, 38, 1230; and Francis, C. K., and Connell, W . B., J. Amer. Chem. Soc., 1913, 35, 1624.
5. Kolthoff, I. M., Z . anal. .Chem., 1918, 57, 11.
6. Nicholson, R. I., An a l y s t, 1941, 66, 189.
7. Goris A., and Larsonneau, A., Bull. Sei. Pharmacol., 1921, 28, 497; König, W., Z . angew. Chem., 1905, 115,./. prakt. Chem., 1904, 69, 105.
Cl a t f o r d Oa k c u t t s
N e a r Andover, Hants April, 1944
M e r c u r i c O x y c y a n i d e a s a R e a g e n t i n M i c r o - a n a l y s i s : I t s U s e i n t h e D e t e r m i n a t i o n o f S u l p h u r i n O r g a n i c
S u b s t a n c e s , I o n i c H a l o g e n a n d A l k o x y l *
By
G. I N G R A MIn t r o d u c t i o n a n d Ex p e r i m e n t a l
Su l p h u r— T he Carius m e t h o d for the gravimetric determination of sulphur is very accurate, but d epends o n such factors as complete oxidation, f reedom from glass splinters w h e n the b o m b tube is opened, a n d complete precipitation of b a r i u m sulphate. Errors d u e to co-precipitation h a v e been studied b y Popoff a n d N e u m a n . 1
Tetraliydroxyquinone2 a n d s o d i u m rhodizonate3 have been used as indicators in titrating sulphate-ion with b a r i u m chloride soln. These indicators are satisfactory with 0-1 N, but tend to give inaccurate results with 0-01 N solns. o w i n g to the difficulty of obtaining a correct end-point. This has been confirmed b y Gibson a n d Caulfield.'4
^Methods have beeil described for determining the a d d e d excess of b arium chloride b y pptn. as c hromate followed b y iodimetric titration.4-5 M a n o v a n d Kirk6 have studied the errors— mainly d u e to occlusion— involved in this type of technique.
T h e combustion m e t h o d of Pregl7 provides a satisfactory m e t h o d for the oxidation of organic compounds. W h e n c o m b i n e d with a suitable titration procedure it has the advantage of quickness; a n d it is applicable to a larger range of c o m p o u n d s than the Carius method.
There are, however, s o m e disadvantages. Approx. 7 0 % of the sulphur is converted into S 0 3, which remains in the cool part of the combustion tube, between the platinum contacts a n d spiral. T h e remaining 3 0 % is converted into S 0 2 a n d is retained b y the spiral. This necessitates removal of the combustion tube from the furnace to enable the S 0 3 to be w a s h e d out together with the soln. in the spiral in the final treatment. Loss of S 0 3 also occurs w h e n the c o m p o u n d explodes or burns rapidly, o w i n g to the formation of mist, w h i c h readily passes through the absorbent soln. without being retained.
T w o modifications have been described to enable the absorbed combustion products to be r e m o v e d f r o m the apparatus without removal of the latter from' the furnace,8-9 but both are unsatisfactory. In the former the S 0 3 still remains in the combustion tube, a n d in the latter too m u c h w a s h liquid is required.
In the present investigation errors due to (i) formation of S 0 3 mist, (ii) design of the combustion apparatus, a n d (iii) gravimetric procedures, h a v e been avoided (i) b y carrying out the combustion in water-saturated oxygen, (ii)Jby re-designing the apparatus, a n d (iii) b y utilising mercuric oxycyanide10 to determine the excess of bar i u m chloride.
Mist is prevented b y the combination of S 0 3 with water in the combustion tube to f o r m sulphuric acid, wh i c h is trapped b y the moistened glass beads in the lower portion of the combustion tube (Fig. 1, C). F o r this purpose an inlet w a s h tube is inserted above the m o i s tened beads in the combustion tube, so that the sulphuric acid is conveniently r e m o v e d with a few m l of w a s h liquid. T h e excess of bar i u m chloride is determined with mercuric o x y cyanide (introduced b y Vieböck for halogen determinations) which reacts quantitatively as follows.
2 H g ( O H ) C N + B a C L = 2 H g C l . C N + B a ( O H ) 2
* Read at the symposium held by the Microchemical Club, the South Yorkshire Section of the Royal - Institute of Chemistry and the Sheffield Metallurgical Association, at Sheffield on October 9th, 1943.
266 INGR A M : M E R C U R I C O X Y C Y A N I D E AS A R E A G E N T IN MICRO-ANALYSIS
Removal of Nitric Acid after Combustion.— Brewster a n d R i e m a n 11 have recently described a combustion procedure w h e r e b y nitrogen-containing c o m p o u n d s are analysed for sulphur b y determining the sulphur as sulphuric acid after evaporating off the nitric acid produced b y the combustion. Combustions b y this procedure on c o m p o u n d s containirig sulphur a n d nitrogen gave m e inconsistent results. Loss of sulphuric acid on evaporation w a s suspected.
T o test this, solutions of 0-01 N sulphuric acid containing 2 m l of 0-01 N nitric acid or h y drochloric acid were evaporated o n the water-bath. Loss occurred (possibly owi n g to creep
ing), its a m o u n t depending o n the length of time the dishes were left o n the water-bath after the bulk of the liquid h a d been removed. In these expts. the sulphuric acid w a s determined b y direct titration with 0-01 N s o d i u m hydroxide.
,S04 (0-01 N) N a O H (0-01 N) Time on
taken required water-bath
ml ml min.
5-47 5-44 5
5-47 5-43 10
5-47 5-38 20
5-47 5-31 30
6-47 5-29 40
5-47 4-91 50
Mixtures of 0-01 N sulphuric acid with excess of 0-OliV barium chloride were evaporated, a n d the excess of chloride w a s determined, after removal of free hydrochloric acid b y evapora
tion, b y adding neutral mercuric oxycyanide reagent a n d titrating the liberated barium hydroxide with 0-01 N sulphuric acid. T h e following results were obtained.
H 2SO„ BaCL, 0-01 N H 2S 0 4 for h2s o4
taken taken liberated Ba(OH)2 found
ml ml ml ml
10-65 15-00 4-36 10-64
1000 13-00 3-06 9-94
5-47 7-36 1-91 5-45
5-00 10-00 • 5-02 4-98
3-00 7-00 4-00 3-00
2-30 4-21 1-94 2-27
1-18 1-58 0-41 1*17
0-87 1-05 0-20 0-85
0-60 2-10 1-51 0-59
0-22 1-05 0-85 0-20
results obtained b y evaporating sulphuric acid soins.
of conc. nitric a n d hydrochloric acids, respectively, a n d leaving t h e m o n the water-bath for 5 min. after the bulk of the soln. w a s removed, s h o w that under these conditions the volatile mineral acids are r e m o v e d completely, without loss of sulphuric acid, a n d d o not interfere with the subsequent titration. In these expts. the sulphuric acid w a s determined b y direct titration with 0-01 N bar i u m chloride after addition of 10 m l of neutral mercuric oxycyanide reagent.
0-01 N H , S 0 4 0-01 N BaCL,
taken required
ml ml
'5-47 5-44
0-5 ml H N O a 0-5 ml HC1
5-47 5-46
5-47 5-45
5-47 5-43
T h e titration procedure evolved for the combustion determination can be adapted, therefore, to the Carius oxidation method. T h e following table gives the % of sulphur found in various substances b y Carius oxidation a n d b y combustion.
Substance Sulphonal
¿/-Methionine
N 4-Acetyl sulphanilamide N*-Methyl sulphanilamide Sulphur
* L o w owing to incomplete oxidation.
Carius Combustion Theory
28-24 28-2 28-1 '
28-06 28-05 2S-1
19-96* 21-41 21-5
— 21-7 21-5
15-2 15-2 15-0
17-5 17-32 17-2
— 100-1 100-0
i n g r a m: m e r c u r i c o x y c y a n i d e a s a r e a g e n t i n m i c r o-a n a l y s i s 267 Ha l o g e n s— O rganic ionic halogen c o m p o u n d s have been analysed successfully b y dis
solving in a suitable solvent, adding mercuric oxycyanide reagent, a n d titrating the liberated h y d r o x y c o m p o u n d with standard sulphuric acid.
Al k o x y l Gr o u p s— A ttempts to use this reagent in the determination of m e t h o x y a n d ethoxy groups were successful. T h e alkyl iodides were absorbed in pyridine12 in a modified Zeisel apparatus (Fig. 3) fitted with a water condenser.13
Th e Me t h o d s
R e a g e n t s — Mercuric oxycyanide— Sh a k e 20 g with 1 litre of water, a n d filter into a b r o w n bottle; as the solution is slightly alkaline, neutralise each 10 m l used for the determina
tion with 0-01 N sulphuric acid, using 4 drops of m e t h y l red a n d 2 drops of methylene blue indicator solns. M i xed indicator solutions— Dissolve 50 m g of met h y l red in 100 m l of alcohol a n d 50 m g of methylene blue in 100 m l of alcohol; keep both solns. in dropping bottles a n d a d d separately for each titration. Hydrogen peroxide solution— Dissolve 5 drops of 100 vol. peroxide in 10 m l of water a n d neutralise with s o d i u m hydroxide soln., using 2 drops of met h y l red as indicator. Sodium bisulphite mixture— A d d 5 drops each of sat. solns. of s o d i u m bisulphite a n d carbonate to 10 m l of water. Sulphuric acid, Sodium hydroxide, Ba r i u m chloride— 0-01 N solns. of each. Sodium hydroxide— 0-2 N. Hydrochloric acid— 0-1 N. Barium chloride (BaCl2.2H20 ) — Analytical Reagent quality.
Th e Co m b u s t i o n Ap p a r a t u s— T he combustion apparatus (Fig. 1) consists of a Pregl pressure regulator, a preheater, a bubbler containing water, a n d a quartz combustion tube fitted with a side-arm, all m o u n t e d together on a stand. Connections between the preheater, bubbler a n d combustion tube are m a d e b y B.10 g r o u n d joints, lubricated with phosphoric acid a n d secured b y springs. T h e preheater serves to heat the oxyg e n before it passes through the bubbler, thereby saturating the gas stream with moisture before it enters the combustion tube. T h e oxyg e n m a y be by-passed b y m e a n s of the t w o - w a y tap sealed to the bubbler.
T h e combustion tube is bent at right angles 20 c m from the mout h , fused together with an angle joint. T h e horizontal part A contains the combustion boat a n d quartz baffle, a n d the vertical part B the platinum contact (10 c m long) 25 c m from the bend; at the exit end is a tube C, 8 c m long, filled with glass beads, at the e n d of w h i c h a n A . 10 m a l e joint is sealed. T h e inlet tube D, 10 c m long a n d 4 m m in diam., is sealed at the junction of the b e a d tube a n d combustion tube, a n d protrudes half w a y into the former in the form of a jet. T h e open e n d is closed b y a rubber stopper.
T h a t portion containing the contact a n d 5 c m of the horizontal part adjacent to the bend is covered with w e t asbestos paper a n d w o u n d with nichrome wire. This is covered with 5 m o r e layers of w e t asbestos a n d then w o u n d with thick asbestos string, so that a c o m pact insulated cover is provided. T h e combustion tube is then left in the oven until the
268 i n g r a m: m e r c u r i c o x y c y a n i d e a s a r e a g e n t i n m i c r o-a n a l y s i s
covering is dry. T h e preheater, w h i c h is treated in a similar way, is filled with a 50 : 50 mixture of cerium dioxide a n d lead chromate suspended o n p u m i c e stone (10-14 mesh).
T h e S 0 2 absorber (Fig. 2) is U-shaped, with a tap sealed o n at the b o t t o m to facilitate
Fig. 2. Absorber for
Combustion Fig. 3. Zeisel Apparatus
Apparatus A — Water jacket B — Cork C— Pyridine absorber washing. O n e arm, 7 c m long, of thick walled 2 m m bore tubing has a B.10 female joint sealed o n for connection to the combustion tube. T h e other arm, wh i c h is 7 c m long a nd 11 m m in diam., is filled with glass beads; its upper outlet is constricted a n d terminates with a splash bulb 14 m m in diam.
Ge n e r a l Co m b u s t i o n Pr o c e d u r e— A ssemble the clean a n d dry apparatus and regulate the temperature of the furnace a n d preheater b y rheostats, the former to 800° C.
a n d the latter to 350° C. Fill the bubbler to the bulb of the inlet tube with water, and regulate the oxygen to a flow rate of 20 m l per min. to fill the combustion tube with water vapour. This can be accelerated b y w a r m i n g the bubbler slightly with the small electric heater (E).
Charge the absorber with 1-2 m l of absorbent soln. (neutral hydrogen peroxide for sulphur, sod i u m bisulphite mixture for halogen) a n d connect it to the combustion tube.
Introduce the weighed sample a n d baffle a n d start the combustion, using a gas burner to drive the substance into the m a i n heated part of the tube; ca. 10 min. will be required. After a further 10 min. turn off the oxygen, a n d determine the sulphuric acid b y one of the following three procedures.
(1) For sulphur in compounds containing no nitrogen or halogens14— R e m o v e the absorber a n d rinse the contents, together with the b e a d tube, with water into a 150-ml conical flask.
Boil for 7 sec., cool, a d d 4 drops of methyl red a n d 2 drops of methylene blue indicator, and titrate with 0-01 N s o d i u m hydroxide (1 m l of 0-01 N N a O H = 0-1603 m g of S). A s already indicated, 0-01 N bar i u m chloride can be used as a stable standard soln. instead of 0-01 N s o d i u m hydroxide (see p. 266).
(2) For sulphur in compounds containing halogen*-15— If the c o m p o u n d contains nitrogen also, use the procedure described in (3). If n o nitrogen is present both sulphur a n d halogen are determined. Neutralise the soln. in the conical flask with 0-01 N s o d i u m hydroxide;
this gives the total acid. A d d 10 m l of mercuric oxycyanide reagent a n d titrate the liberated alkali with standard sulphuric acid; this gives the equivalent of halogen (Cl or Br) present.
Subtract this titre from the total acid titre to obtain the sulphur equivalent.
(3) For sulphur in nitrogen-containing compounds— Evaporate the absorbent solution and
I N G R A M : M E R C U R I C O X Y C Y A N I D E AS A R E A G E N T IN MICRO-ANALYSIS 269 washings (approx. 25 ml) contained in the evaporating dish o n the water-bath until almost dry. A d d 1 m l of 0-1 N hydrochloric acid a n d a k n o w n weight of solid bar i u m chloride (20-50 m g ) a n d again evaporate. A d d 4 m l of water a n d evaporate to r e m o v e a n y free acid.
Rinse the mixture of solid bar i u m sulphate a n d chloride into a 150-ml conical flask, a d d 10 m l of neutral mercuric oxycyanide soln., a n d titrate the liberated alkali with N 0-01 sulphuric acid.
U s e the s a m e procedure for the Carius method. Rinse the contents of the b o m b tube into the evaporating dish a n d evaporate. T h e n a d d b a r i u m chloride a n d continue as described above.
Halogen by combustion (Chlorine and Bromine)— U s e the s a m e combustion apparatus.
Charge the absorber with 1-2 m l of s o d i u m bisulphite mixture a n d rinse the liquid containing absorbed halogen acids into the conical flask. A d d 3 drops of 100 vol. hydrogen peroxide to oxidise the bisulphite, boil for 2 min., cool, neutralise with 0-2 N s o d i u m hydroxide, adjusting the neutral point with 0-01 N sulphuric acid, a d d 10 m l of oxycyanide reagent, a n d titrate the liberated alkali.
Ionic H a l o g e n (Chlorine, B r o m i n e a n d Iodine)— Dissolve 3-10 m g of the substance in a suitable solvent, e.g., water, a d d 10 m l of the reagent, a n d titrate the liberated hydroxyl c o m p o u n d with 0-01 N sulphuric acid. Boil c o m p o u n d s insoluble in water a n d dil. alcohol with 10 m l of 0-01 A r N a O H sol., cool a n d neutralise before treatment with the reagent.
A l k o x y l — U s e the normal procedure with the apparatus s h o w n (Fig. 3). Transfer the pyridine containing the absorbed alkyl iodide to a 100-ml round-bottomed flask fitted with an outlet tube for connection to the water-pump, a n d an inlet tube reaching halfway into the flask. Distil off the pyridine under reduced pressure o n the water-bath a n d dissolve the residue of pyridinium iodide in 5 m l of water. A d d 10 m l of oxycyanide reagent soln. a n d titrate the liberated pyridinium base with 0-01 N sulphuric acid (1 m l of 0-01 N H 2S 0 4 = 0-3100 m g of O C H 3 or 0-4500 m g of O C 2H 5).
Su m m a r y— T he technique described for sulphur determinations w a s developed to enable a titration procedure to be carried out o n c o m p o u n d s containing nitrogen also.
B y evaporating mixtures of .iV/100 solns. of nitric a n d sulphuric acids on the water-bath it w a s found that the former is r e m o v e d completely with only slight loss of sulphuric acid.
This loss depends on the length of time the acid is left on the water-bath after the bulk of the solution has been r e m o v e d b y evaporation. If prolonged heating is avoided at this stage the sulphuric acid can be accurately determined b y titration with alkali.
A n alternative m e t h o d is described in wh i c h excess of barium chloride is a d d e d a n d after evaporation the excess is determined with mercuric oxycyanide. T h e m e t h o d can be applied with equal accuracy to Carius sulphur determinations. It has the advantage of speed over the gravimetric method.
T h e combustion apparatus is modified so that errors due to the formation of S 0 3 are eliminated.
T h e use of mercuric oxycyanide has been extended to the determination of ionic halogen, a n d to the Zeisel method, thereby replacing the gravimetric procedure b y a rapid a n d simple acid-alkaline titration.
I a m indebted to the Director General of Scientific Research a n d Development, Ministry of Supply, for permission to publish this paper.
Re f e r e n c e s
1. Popofi, S., and Neuman, E. W., Ind. Eng. Chem., Anal. Ed., 1930, 2, 45.
2. Schroeder, W . C., Id., 1933, 5, 403.
3. Abrahamczik, E., and Bliimel, F., Mikrochim. Acta, 1937, 1, 354.
4. Gibson, D. T„ and Caulfield, T. H„ Analyst, 1935, 60, 522.
5. Josephson, B., Id., 1939, 64, 181.
G. Manov, G. C., and Kirk, P. L., Ind. Eng. Chcm., Anal. Ed., 1937, 9, 198.
7. Pregl, F., “Quantitative Organic Micro-analysis," 1924, p. 113..
8. Beasley, C. W., Ind. Eng. Cliem., Anal. Ed., 1938,*10, 005.
9. Hallett, L. T., and Kuipers, J. W., Id., 1940, 12, 357. . 10. Viebock, F., Ber., 1932, 496.
11. Brewster, E. L., and Kieman, W., Ind. Eng. Chon., Anal. Ed., 1942, 14, 820.
12. Kirpal, A., and Biihn, T „ Monatsh., 1915, 36, 853.
13. Elek, A., Ind. Eng. Chem., Anal. Ed., 1939, 11, 174.
14. Friedrich, A., and Watzlaweck, O., Z. anal. Chem., 1932, 89, 401.
15. Beet, A. E., and Belcher, R., Fuel, 1940, 19, 42.
22, Bo u r n e Av e n u e
Salisbury April, 1944
270 NO T E S
N o t e s
T H E D E T E R M I N A T I O N O F S M A L L A M O U N T S O F S I L V E R IN C O P P E R A N D B R A S S * Th e following investigation was undertaken to meet a demand for accurate information respecting the amount of certain low percentages of silver in copper and soine copper alloys. It was found unnecessary to go outside the classical precipitation as chloride, and within the acidity and amount of chloride prescribed precipitation appeared to be complete. It was necessary to work on a large sample and the final deter
mination was done cyanometrically. The following method was worked out.
Me t h o d— D issolve 50 g of the sample (cleaned by pickling) in 200 ml of nitric acid (sp. gr. 1-42);
it is better to have the sample in lump form and it is desirable to add the acid a little at a time. After action has practically ceased add a further 50 ml of nitric acid (sp.gr. 1-2), which will dissolve the remainder of the sample; boil out the nitrous fumes and dilute with 100 ml of hot water. A d d 5 ml of 2 0 % sodium chloride soln., boil for 10 min. and leave overnight. Filter off the pptd. silver chloride, cold, on a tightly pressed pulp filter and wash with cold water until copper is completely removed; then, having rinsed the outside of the funnel and stem to remove splashes, transfer it to a clean flask. R e m o v e any adherent ppt.
from the precipitation beaker by rinsing with 20 ml of dil. (1 : 3) ammonia, pouring these rinsings through the ppt. on the filter and wash two or three times with water; wash once with 20 ml of boiling dilute nitric acid (sp.gr. 1-2) and again two or three times with water. Finally remove all residual traces of silver from the filter by a second treatment with 20 ml of 1 : 3 ammonia followed by a third washing with water.
Neutralise the filtrate with ammonia or nitric acid as required and add an excess of 10 ml of 1 : 1 ammonia;
dilute the filtrate, which should be quite bright, to 200 ml and cool.
Titration— -Run in an excess (say, ca. 10 ml) of standard potassium cyanide soln., add 10 ml of 4 % potassium iodide soln. and titrate carefully to the first faint permanent opalescence with standard silver nitrate soln. The silver equivalent of the potassium cyanide is found by adding a measured vol. of cyanide soln. to the exactly titrated soln. and again titrating; if this volume is the same as that used originally it saves any need of factors in the conversion. The difference between the silver equivalent of the potassium cyanide originally used and that of the silver soln. required in the titration gives a measure of the silver present.
Trials made by this method with two samples of pure copper to which varying amounts of silver had been added gave the following results.
Silver Material
Wt.
taken
Silver added
Titration A g N 0 3 soln.
Silver found
(corr. for blank) Added
■A.
Found
g g ml g % %
Copper A tt tt
50-0 — 5-10-5-10= — — — _
50-0 0-001S4 5-00-4-50=0-50 0-00184 0-00368 0-00368
tt tt 50-0 0-00368 5-10-4-10=1-00 0-00368 0-00736 0-00736
B 50-0 0-00184 5-10-4-50=0-50* 0-00184 0-00368 0-00368
St tt 50-0 0-00368 4-90-3-80=1-00* 0-00368 0-00736 0-00736
st tt 50-0 0-00736 5-05-2-95 = 2-00* 0-00736 0-01472 0-01472 tt tt 50-0 0-02945 11-00-2-90=8-00* 0-02945 0-05890 0-05890
tt tt 50-0 — 5-00-4-90 = 0-10 — — —
* Corrected for blank.
Solutions used— (i) Potassium Cyanide— 16-8 g of AnalaR potassium cyanide and 8-0 g of potassium hydroxide dissolved in water and made up to 3-5 litres.
(ii) Silver Nitrate (1ml = 0-00368 g of Ag) — 11-584 g of silver nitrate dissolved and made up to 2 litres with distilled water.
Similar trials with two samples of brass, one 70 : 30 and the other 60 : 40 containing 2-30% of lead and 0-23% of tin gave:
Material
Wt.
taken g
Silver added
g
Titration ml A g N 0 3 soln.
Silver found g corr. for blank
Per cent, of silver t A
Added Found
70:30 Brass 50-0 — 5-50-5-35 = 0-15 _ _ _
,, 50-0 0-00184 4-50-3-80=0-70 0-00202 0-00368 0-00404
,, 50-0 0-00368 5-30— 4-15= 1-15 0-00368 0-00736 0-00736
50-0 0-00736 8-70-6-50=2-20 0-00755 0-01472 0-01510
tt 50-0- 0-01472 6-75-2-60=4-15 0-01472 0-02944 0-02944
60:40 Brass 50-0 — ' 5-50-4-70=0-80 — _ _
tt 50-0 0-00184 5-25-3-95=1-30 0-00184 0-00368 0-00368
ff 50-0 0-00368 4-40-2-60=1-80 0-00368 0-00736 0-00736
50-0 0-00736 4-70-1-90=2-80 0-00736 0-01472 0-01472
,, 50-0 0-01472 6-50-1-75 = 4-75 0-01453 0-02944 0-02906
Large weights of both brass samples were dissolved, made up to a known volume and an aliquot part taken before the addition of the silver. This procedure was adopted because of earlier slightly erratic results obtained for the blank by taking separate sample weights. The only explanation of tins would appear to be segregation of the silver present in the brass, although apparently there is no metallurgical support for this.
“Metastannic acid” had to be filtered off from the 60 : 40 solution before proceeding.
The author wishes to thank the Director-General of Scientific Research and Development, Ministry
of Supply, for permission to publish this paper. D. G. Higgs
11, Ha r r i e t t St r e e t, Ca t h a y s, Ca r d i f f /«/y, 1944
* Communication from Armament Research Department, formerly the Research Department, Woolwich.-
