A n a l y t i c a l E d i t i o n V o l .
5, No. 3
M a y
15, 1933
I n d u s t r i a l
A N D E N G I N E E R I N G
C h e m i s t r y
VOL. 25, CONSECUTIVE NO. 17
P u b l i s h e d b y t h e Am e r i c a n Ch e m i c a l So c i e t y
Ha r r i s o n E . Ho w e, Ed i t o r
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18,400 Copies of This Issue Printed Use of Stannous Chloride in Evaluation of Dye Mixtures . .
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M a y
15, 1933
Use of Stannous Chloride in Evaluation of Dye Mixtures
J . A. Ki m e
C olor C ertificatio n L a b o ra to ry , F ood an d D rug A d m in istratio n , U. S. D e p a rtm e n t of A griculture, W ashington, D . C.
I
N T H E evaluation of dye mixtures advantage is often taken of the difference in stability or ease of reduction in the presence of reducing agents. Titanous chloride is widely used in the estimation of dyes and under generally favorable conditions most dye mixtures are reduced with but slight evidence of selective action. Holmes (S), however, investigated a number of dye mixtures and demonstrated th a t under less favorable conditions the reduction of one dye often takes precedence over the other, or the one dye may not be reduced a t all. Certain of the permitted food colors also exhibit this property. The azo dyes appeared to take precedence over the triphenylmethane group under the influence of titanous chloride, but as Holmes pointed out with several dye mixtures, the difficulty in the visual detection of the end point limited the practical utility. From this apparent difference in ease of reduction it appeared th a t many simple food dye mixtures might be evaluated by means of a set of reducing agents. In employing two such reagents there are two limiting conditions: (1) one agent shall be selective in its action, and (2) the reaction products of the first reduction shall not interfere with the action of the second agent. Even- son and Nagel (2) utilized ammonium sulfide in selectively reducing am aranth (C. I. 184) in presence of tartrazine (C. 1 .640), the excess being destroyed by lowering the pH with sodium bitartrate, which also acted as a buffer for the estima
tion of the tartrazine with titanous chloride.
Stannous chloride and titanous chloride fulfil the require
ments for a desirable set of reducing agents. The former is selective in its action and the stannic ions which are formed in a reduction do not react with the titanous ions in the subse
quent titration of the second dye. Knecht and Hibbert (5) state th a t stannous chloride will reduce diamine sky blue (C. I. 518) in the presence of ehrysophenin (C. I. 365), two azo dyes, and (4) th a t titanous chloride does not appear to react on stannic chloride nor stannous chloride with titanium tetrachloride, yet so far as the author is aware, these properties have never been employed in the evaluation of dye mixtures.
Among the permitted food dyes tested in this laboratory, it was interesting th a t the triphenylmethane group was sufficiently stable and four of the azo dyes sufficiently sus
ceptible to the action of stannous chloride to permit evalua
tion of a simple mixture, one from each group.
Ma t e r i a l s
For convenience, the stannous chloride was made up ap
proximately normal; 113 grams of tin chloride (SnCl2.2H20) were dissolved in an equal number of cubic centimeters of hydrochloric acid and diluted to one liter. The dyes were of certified grade, made by various food color manufacturers.
The titanous chloride solution was made and standardized according to the usual laboratory procedure (4). The sodium bitartrate used as a buffer was U. S. P. or c. p. quality.
Se l e c t i v e Re d u c t i o n Ex p e r i m e n t s
One per cent pure dye solutions of the food colors were made and standardized. Amaranth, ponceau SX, orange I (C. I. 85), and sunset yellow FC F were found to be rather readily reduced with stannous chloride. The reduction may be completed a t room temperatures, and increasing the temperature only serves to speed up the reaction rate. Fast green FCF, guinea green B (C. I. 666), light green SF yellow
ish (C. I. 670), and brilliant blue FC F are but slightly at
tacked by stannous chloride, even on continued heating.
Amaranth, particularly, reduced more rapidly in the presence of hydrochloric acid, but sodium bitartrate proved generally better. Buffers such as borax cannot be used in these dye mixtures, as the triphenylmethane dyes are not sufficiently stable toward stannous chloride a t such an alkaline pH.
From the standardized pure dye solutions, mixtures were made and the following procedure adopted:
Samples of a known mixture were pipetted into 1-liter wide- mouth Florence flasks. Fifteen grams of sodium bitartrate and water to bring the volume to 200 cc. were added. The solution was then heated and the number of cubic centimeters of 0.1 N titanous chloride for the reduction of both dyes determined under an atmosphere of carbon dioxide.
To one sample was added approximately 100 per cent excess of the amount of stannous chloride equivalent to the azo dye present (component A). The reaction was carried on according to the conditions given in Table I. The triphenylmethane dye (component B) was then determined by the usual titanous chloride titration. In order to approximate the excess for the azo dye, a trial reduction was first made, using twice the quantity of stannous chloride that would be necessary for the reduction of both dyes as indicated by the titanous chloride equivalent of the mixture. For example, 25.0 cc. of 0.1 JV titanous chloride are required for a given mixture. Therefore, 5.0 cc. of normal 151
152 A N A L Y T I C A L E D I T I O N Vol. 5, No. 3
T a b l e I. T y p i c a l R e s u l t s b y S e l e c t i v e R e d u c t i o n S t a n n o u s C h l o r i d e R e d u c t i o n
Se r i e s Co m p o n e n t Dy e s
A , p onceau S X B , g u in ea green B
2 A , s u n s e t y ello w F C F B , briLliant b lu e F C F
3 A, orange I
B , lig h t green S F y ello w ish
4 A , a m aran th
B , fa s t green F C F
stannous chloride are used in the trial reduction. Subsequently, this trial reduction showed that component B used 15.0 cc. of 0.1 iV titanous chloride and component A, by difference, needed 10.0 cc. Therefore, 2.0 cc. of 1.0 N stannous chloride represented approximately 100 per cent excess. The amounts of stannous chloride used, appearing in Table I, were ascertained by such a procedure.
In a mixture containing less than 40 to 45 per cent of compo
nent B, an additional step was necessary, because (1) estimations of small amounts of triphenylmethane dyes with titanous chloride invariably give high results; and (2) if a larger sample of mixture is taken, the high concentration of the stannic ions formed in the reduction of the azo dye restores the color of component B so rapidly after reduction by titanous chloride that quantitative estimation is uncertain. A known amount of component B was therefore added prior to the stannous chloride and subsequently subtracted from the final titration value obtained with titanous chloride.
Co n c l u s i o n
The general utility of the set of reducing agents, stannous chloride and titanous chloride, in the evaluation of dye mix
tures, has been investigated. Experimental evidence showed
1 % Dy e So l u t i o n
N S n C l i
ad d e d T im e T em p eratu re
C o m p o n e n t A P r e s e n t
C o m p o n e n t B P r e s e n t
C o m p o n e n t B F o u n d Di f k e r e n
Cc. Cc. A fin . ° C. % % % %
30 4 . 5 3 8 0 -1 0 0 9 1 .2 8 . 8 1 1 .0 + 2 . 2
30 3 . 8 3 8 0 -1 0 0 7 5 .2 2 4 .8 2 5 .8 + 1 .0
50 4 .6 3 8 0 -1 0 0 5 4 .2 4 5 .8 4 8 .4 + 2 . 6
50 2 .1 3 8 0 -1 0 0 2 5 .5 7 4 .5 7 4 . 4 - 0 . 1
100 1 .8 3 8 0 -1 0 0 1 0 .3 8 9 .7 8 9 .8 + 0 . 1
100 0 . 6 3 8 0 -1 0 0 3 . 5 9 6 .5 9 6 .5 0 . 0
40 5 .1 18 hr. 2 0 -2 5 7 5 . 8 2 4 .2 2 5 .6 + 1 .4
110 1 .7 18 hr. 2 0 -2 5 9 . 5 9 0 .5 9 0 .5 0 . 0
60 2 . 0 24 hr. 2 0 - 2 5 1 9 .3 8 0 .7 7 9 . 3 - 1 . 4
50 6 . 0 12 8 0 -1 0 0 9 0 .9 9 .1 1 1 .0 + 1 .9
100 1 .2 12 8 0 -1 0 0 9 .1 9 0 .9 9 1 .7 + 0 . 8
th a t mixtures of azo and triphenylmethane food dyes were satisfactorily analyzed. No doubt there are many dye mix
tures whose components differ in ease of reduction in such a manner as to permit evaluation by a similar procedure.
The author wishes to thank 0 . L. Evenson, of this labora
tory, for his helpful suggestions in performing and presenting this work.
Li t e r a t u r e Ci t e d
(1) Ambler, J. A ., Clarke, W . F ., E venson , O. L ., W ales, H ., U . S.
D ep t. A gr., B u ll. 1390, 26 (1925).
(2) E venson , O. L., and N agel, R . H . , I n d . E.vg. C h e m ., A nal. Ed., 3, 260 (1931).
(3) H olm es, W . C ., A m . D yestuff R e p t r . , 14, 415 (1925).
(4) K n ech t and H ibbert, “ N ew R eduction M ethods in V olumetric A n alysis,” 2nd ed., p. 6, Longm ans, 1925.
(5 ) Ib id ., p . 7 .
R e c e i v e d N o v em b er 2 5 , 1932. P r esen te d b efo re th e D iv is io n o f D y e C h e m istr y a t th e 8 5 th M e etin g o f th e A m erican C h e m ica l S o c ie ty , W a sh in g to n , D . C ., M arch 26 to 3 1 , 1933. C o n tr ib u tio n 10 from th e C olor C e rti6 c a tio n L a b o ra to ry , F o o d a n d D ru g A d m in istr a tio n , U . S. D e p a r t
m e n t of A g riculture, W a sh in g to n , D . C.
Volumetric Determination of Nitroglycerol and of Nitroglycerol and Dinitrotoluene
in Admixture
Wa l t e r W . Be c k e r, E x p erim en t S tatio n , H ercules P ow der C om pany, W ilm ington, D el.
N
ITROGLYCEROL may be conveniently determined by means of the du Pont nitrometer; the results obtained, however, are usually slightly low. Certain aromatic nitro compounds, such as dinitrotoluene, do not i n t e r f e r e in this nitrometerd e t e r m i n a t i o n ; compounds such as diethyldiphenylurea or diphenylamine, however, do in
terfere, b e c a u s e they undergo n i t r a t i o n in the decomposing bulb, and cause low results.
The nitrate groups in nitro
glycerol m a y be r e d u c e d by suitable reagents, and the ni
trogen determined as ammonia.
In th e method of S ilb e r a a d (7) the nitroglycerol in e th e r solution is partially re d u c e d w ith sodium e t h y l a t e , then metallic zinc and iron are added, and the r e s u ltin g a m m o n ia
Practically theoretical results fo r nitroglycerol and fo r elhyleneglycol dinilrate m a y be obtained by dissolving the sam ple in acetic acid, adding an excess o f ferrous chloride, hydrochloric acid, boiling, and then titrating the resulting ferric iron ivith standard titanous chloride solution, using am m onium thiocyanale as the indicator.
W hen present in adm ixture, 2,¿¡-dinitrotoluene does not interfere, and m a y be accurately de
termined in the residual solution by reduction ivith titanous chloride in the regular way.
W hen present in sm all amount, diethyldi
phenylurea or diphenylam ine does not interfere.
is distilled into standard acid. In the method of Muraour (6) the nitroglycerol in acetone solution is treated with hydrogen peroxide, sodium hydroxide, and sodium perborate;
after standing overnight Devarda’s alloy is added, and the resulting a m m o n ia d is tille d into standard acid. Muraour obtained results which closely c h e c k e d t h e t h e o r e t i c a l . Excellent results on the analy
sis of nitroglycerol w ere a lso obtained in this laboratory, but because of the time r e q u ir e d for a determination, the method was not exactly suited to control analysis.
D ic k s o n a n d Easterbrook (2) determined dinitrotoluene in admixture with nitroglycerol by r e d u c in g the l a t t e r with f e r r o u s c h lo r id e in s tr o n g hydrochloric acid solution, then
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 153 extracting the unchanged dinitrotoluene with ether. Methyl-
sulfuric acid m ay be used in place of ferrous chloride. Huff and Leitch (/,) attem pted to determine nitroglycerol in the presence of aromatic nitro compounds. The nitroglycerol and nitro compound were dissolved in acetic acid, hydro
chloric acid was added, and then a known excess of ferrous sulfate. After boiling and evaporating to a small volume, the excess ferrous iron was titrated with standard potassium permanganate solution. Low results were obtained on nitroglycerol alone and in admixture with p-nitrotoluene.
Several suggestions for methods of attack were given in their article, however. Their method was tried out in this laboratory, and low results for nitroglycerol were obtained consistently.
I t was thought th a t the oxidized iron might be determined accurately without interference from glycerol, traces of nitric oxide, or possibly other substances. Knecht and Hibbert (5) first determined nitric acid by reduction to nitric oxide with ferrous iron, and titrating the resulting ferric iron with a standard solution of titanous chloride. The procedure finally developed was to dissolve the sample of nitroglycerol in acetic acid, add an excess of ferrous chloride, hydrochloric acid, and boil under a reflux condenser.
After cooling, the ferric iron was titrated with standard titanous chloride solution, using ammonium thiocyanate as the indicator. An atmosphere of carbon dioxide was of course maintained in the titration flask during the entire procedure.
Pr e p a r a t i o n a n d St a n d a r d i z a t i o n o f So l u t i o n s
A 0.2 N titanous chloride solution was prepared according to the directions of English (3). For each liter of solution, 150 cc. of 20 per cent titanous chioride and 100 cc. of concen
trated hydrochloric acid were used.
For control work, a quick and accurate method of standardi
zation is essential. Ferrous ammonium sulfate was used by Knecht and H ibbert (.5). Thornton and Wood (8) recom
mend the use of Sibley iron ore. For the analysis of aromatic nitro compounds, Callan and Henderson (I) used titanous sulfate which had been standardized against recrystallized p-nitraniline. English (3) gave a comparison of this standard and ferrous ammonium sulfate. In the results reported in this article, ferrous ammonium sulfate was used as the ulti
mate standard.
The ferrous iron of the standard may be oxidized by several different methods. Potassium chlorate in hydrochloric acid solution was selected, as the excess oxidizing agent may be easily and completely destroyed by two evaporations to dryness. Knecht and H ibbert (5) suggest the use of a supple
m entary ferric alum solution for quickly checking the nor
m ality of the titanous chloride solution.
0.7 N F e r r o u s C h l o r i d e . For each liter of solution, 200 grams of ferrous chloride (FeCl2.4H20) and 50 cc. of concentrated hy
drochloric acid were used.
0.15 N F e r r i c A l u m . F o r e a c h l i t e r o f s o l u t i o n , 75 g r a m s o f f e r r i c a l u m a n d 25 c c . o f 95 p e r c e n t s u l f u r i c a c i d w e r e u s e d .
Ap p a r a t u s
The volumetric solutions were protected from the air, using an arrangement similar to th a t given by Knecht and Hibbert (5). Instead of hydrogen, carbon dioxide was used.
A special Florence-type titration flask with ground-in condenser was designed for this determination. The capacity of the flask is about 300 cc.; carbon dioxide is admitted by means of a short inlet tube on the shoulder. The ground- glass joint is No. 25 T , rendering the'condenser and flasks interchangeable. For a titration, the flask is fitted with a narrow one-hole rubber stopper, through which is inserted a short piece of 8-mm. glass tubing.
De t e r m i n a t i o n o f Ni t r o g l y c e r o l
Weigh 1.8 to 2.0 grams of nitroglycerol, dissolve in glacial acetic acid, and make up to volume in a 250-cc. volumetric flask. Displace the air in a special titration flask by passing in a current of carbon dioxide for about 5 minutes. By means of an accurate pipet, transfer a 25-cc. aliquot portion of the acetic acid solution to the flask. Add 15 cc. of ferrous chloride solution and 25 cc. of hydrochloric acid (1 to 1) in the order named, connect the flask to the reflux condenser, and boil gently for 5 minutes. A few glass beads may be added to prevent bumping.
Increase the current of carbon dioxide, then cautiously im
merse the flask in a large beaker of cold water, keeping the index finger over the top of the condenser until the hot vapors are condensed. After cooling to room temperature, disconnect the condenser, insert the one-hole rubber stopper, and titrate with the titanous chloride solution until near the end point.
Add 5 cc. of 20 per cent ammonium thiocyanate solution and continue the titration until the red color of ferric thiocyanate just disappears.
Determine the small amount of ferric iron in the ferrous chloride solution and traces of interfering impurities in the reagents by running a blank under the same conditions as for a sample, and make the necessary correction.
The results obtained on a sample of filtered nitroglycerol and on a sample of ethyleneglycol dinitrate are given in Table I.
T a b l e I. P u r i t y o f N i t r o g l y c e r o l a n d o f E t h y l e n e g l y c o l D i n i t r a t e
Et h y l e n e g l y c o l
Me t h o d Ni t r o g l y c e r o l Di n i t r a t e
% %
N itro m e te r 9 9 .2 1 9 9 .2 1
F eC U -T iC li 9 9 .8 4 9 9 .9 2
9 9 .9 4 9 9 .8 7
9 9 .8 8 9 9 .8 6
De t e r m i n a t i o n o f 2 , 4 - Di n i t r o t o l u e n e a n d Ni t r o g l y c e r o l i n Ad m i x t u r e
The determination of dinitrotoluene alone is usually carried out by dissolving the weighed sample in alcohol, acidifying with hydrochloric acid or sulfuric acid, adding a 100 per cent excess of titanous chloride or sulfate, and boiling. After cooling, the excess reducing agent is titrated with a ferric alum solution, using ammonium thiocyanate as the indicator.
In the determination of dinitrotoluene in admixture with nitroglycerol, the latter would interfere if the foregoing procedure were used. I t was found, however, th a t the dinitrotoluene did not interfere in the method developed for the determination of nitroglycerol. Under the conditions of the determination, the 5 minutes’ boiling with ferrous chloride did not reduce the dinitrotoluene. At room tempera
ture, the ferric iron formed by the reduction of the nitro
glycerol could be titrated to a sharp end point by the titanous chloride solution, in the presence of the dinitrotoluene.
I t was also found th a t the dinitrotoluene could then be determined in the residual solution in the usual way.
P r o c e d u r e . Dissolve the weighed sample of nitroglycerol and dinitrotoluene in acetic acid and make up to volume in a 250-cc. volumetric flask. Transfer a 25-cc. aliquot portion to a titration flask, and proceed exactly as in the determination of nitroglycerol alone, taking care to titrate just to the end point.
Add a 100 per cent excess of titanous chloride solution, again connect the flask to the reflux condenser, and boil for 5 minutes.
Cool the flask and contents to room temperature and titrate the excess reducing agent with the ferric alum solution.
Run a blank determination, adding equal quantities of the reagents, and having ascertained the slight loss of titanous chloride on boiling, apply the proper correction in calculating the results.
I t will be observed th a t a few conditions are different from those th a t prevail in the usual determination of dinitro- toluene. The nitro compound is boiled with titanous chloride in the presence of ferrous chloride and ammonium
154 A N A L Y T I C A L E D I T I O N Vol. 5, No. 3 thioeyanate. The indicator was found to have no effect.
Ferrous chloride, however, will reduce the nitro compound to a slight extent, unless some ferric iron is present. The amount of ferric iron necessary to prevent reduction of the dinitrotoluene during the nitroglycerol determination does not appear to be critical, as is shown by the results given in Table II, on two typical laboratory samples.
T a b l e II. E f f e c t o f E x c e s s o f F e r r o u s C h l o r i d e o n D e t e r m i n a t i o n o f D i n i t r o t o l u e n e a n d N i t r o g l y c e r o l
i n A d m i x t u r e
F e C l s Ad d e d Ni t r o g l y c e r o l Di n i t r o t o l u e n e
Cc. % %
15 5 .0 3 6 .6 3
5 .0 7 6 .5 9
T a b l e III. D e t e r m i n a t i o n o f N i t r o g l y c e r o l a n d D i n i t r o t o l u e n e i n A d m i x t u r e
Ni t r o g l y c e r o l Fo u n d
%
9 9 .8 7 9 9 .9 0
Di n i t r o t o l u e n e Fo u n d
%
9 9 .2 8 9 9 .2 2 9 9 .3 6 9 9 .2 6
5 15
20
5 .0 6 2 0 .2 3 2 0 .2 4
6 .5 8 2.00 1 .9 8
The results obtained on a sample of dinitrotoluene and on a known mixture of nitroglycerol and dinitrotoluene are given in Table III.
I t was found th a t small amounts of diethyldiphenylurea or diphenylamine did not interfere in the determination of nitroglycerol and dinitrotoluene by the use of the foregoing method.
A c k n o w l e d g m e n t
The author is indebted to W. H. Fravel for certain of the analytical results used in this article.
Li t e r a t u r e Ci t e d
(1) Callan, T ., and Henderson, J. A. R ., J. Soc. Chem. In d ., 41,
1 5 7-61T (1922).
(2) D ickson, W ., and Easterbrook, W . C ., A n a lyst, 47, 112-17 (1922).
(3) E nglish, F. L., J. In d. Eng. Chem., 12, 9 9 4 -9 7 (1920).
(4) H uff, W . J., and L eitch, R . D ., J . A m . Chem. Soc., 44, 2 6 4 3 -4 5 (1922).
(5) K nech t, E ., and H ibbert, E ., " N ew R ed u ction M eth od s in V olum etric A n alysis,” 2nd ed., L ongm ans, 1925.
(6) M uraour, H ., B ull. soc. chim., 4 5 ,1 1 8 9 -9 2 (1929).
(7) Silberaad, 0 . , Phillips, H . A ., and M errim an, H . J., J . Soc.
Chem. In d ., 25, 6 2 8 -3 0 (1906).
(8) T hornton, W . M ., Jr., and W ood, A . E ., In d. Enq. Chem., 19, 150-52 (1927).
R e c e i v e d D ec em b er 8 , 1932.
Indicators for Determining Chromium and Vanadium in Alloy Steels
Oxidized Diphenylamine Sulfonic Acid and Oxidized Diphenylamine
Ho b a r t H . Wil l a r d a n d Ph i l e n a Yo u n g, University of Michigan, Ann Arbor, Mich.
D
i p h e n y l a m i n e sulfonic acid is an oxida- tion-reduction indicator which may be used in the pres
ence of tungstic acid, but its properties under such conditions have not been thoroughly tested (4). I t has been used in the analysis of chrome-vanadium- tungsten steels for vanadium (S).
Though accurate results were obtained, the indicator blank which had to be applied, because of reduction of some v a n a d ic acid by the indicator, was un
desirably large. I t was noted in
T his paper describes an investigation o f the properties o f diphenylam ine sulfonic acid a s a n indicator fo r chromium and vanadium in tungsten steels. W ith a p relim inary oxidation o f diphenyl
am ine sulfonic acid, the blank correction to be applied when using this indicator is reduced to a very sm all value. I t is sufficiently constant with definite procedures to give very accurate results fo r chromium or vanadium .
A p relim in a ry oxidation o f diphenylam ine completely elim inates a blank correction fo r this indicator when used in vanadium or chromium determinations.
grams of the barium salt (ob
ta in a b le fro m the Eastman Kodak Co., Rochester, N. Y.) in a liter of water, adding to this solution a slight excess of sodium sulfate, and decanting or filter
ing the solution.
P r e p a r a t i o n o f O x i d i z e d I n d i c a t o r . The volume of th e 0.01 M indicator solution speci
fied in a given experiment is placed in a small beaker, 5 cc.
of water, 3 or 4 drops of con
centrated sulfuric acid, and 3 or 4 drops of 0.1 N potassium d ic h r o m a te a r e added, and this paper (7) th a t an indicator
with no blank correction could be prepared by a preliminary oxidation, b u t this is only approximately true.
Since diphenylamine and diphenylbenzidine are of no value in the presence of tungstic acid (£>), it seemed important to study the properties of an indicator not affected by this substance. Methods for chromium or vanadium which do not involve the removal of tungstic acid and the determination of the small am ount of vanadium which inevitably accom
panies the precipitate are obviously much more rapid and simple.
Ex p e r i m e n t a l
P r e p a r a t i o n o f I n d i c a t o r . A 0.01 M solution of di
phenylamine sodium sulfonate is prepared by dissolving 3.2
then very dilute ferrous sulfate (0.01 to 0.02 N ) is added until the purple color, which appears on the addition of the first few drops of ferrous sulfate, just turns to a bluish green. As this purple color begins to dis
appear, the ferrous sulfate should be added in parts of a drop, in order to have no excess present in the oxidized indicator solution. T his bluish green solution is added to the solution to be titrated. In the experiments described in this paper the oxidized indicator was prepared in separate samples for each titration.
A stock solution of the oxidized indicator is often more con
venient and may be prepared as follows:
One hundred cubic centimeters of 0.01 M diphenylamine sodium sulfonate and 25 cc. of concentrated sulfuric acid are diluted to 900 cc. in a liter volumetric flask. To this solution 25 cc. of
0.1 N potassium dichromate are added slowly with frequent shak
ing, followed by 0.1 N ferrous sulfate with repeated shaking, until one drop causes a visible change in color from bluish green to a clear deep Kreen in the liquid when viewed through the neck of the flask. This will require approximatelv 6.5 cc. of ferrous sulfate.
Since the indicator will slowly settle out of this solution, it must be shaken before using. To avoid this as well as the necessity for adding an exact amount of ferrous sulfate, the precipitate may be washed free from acid and salts and then stirred up with water in which it remains as a colloidal solu
tion. This is done as follows:
One hundred cubic centimeters of 0.01 M diphenylamine sodium sulfonate and 5 cc. of concentrated sulfuric acid are diluted to about 300 cc. To this liquid 25 cc. of Oil N potassium dichromate are added slowly, followed by 8 cc. of 0.1 N ferrous sulfate. This green solution is allowed to stand 3 or 4 days, until a portion of the supernatant liquid gives no color when added to 100 cc. of water containing 2 cc. of 0.1 N potassium dichromate and 5 cc. of concentrated sulfuric acid. Then the supernatant liquid should be siphoned off slowly, care being taken not to disturb the green pre
cipitate which is the indicator. Three hundred cubic centimeters of water and 15 cc. of concentrated sulfuric acid are added to the precipitate, the latter is allowed to settle again, and the super
natant liquid is siphoned off. More thorough and rapid washing can be obtained by centrifuging. The green precipitate is then shaken up with 100 cc. of water. In the absence of electrolyte the precipitate will not settle out appreciably; even when not centrifuged, it settles very slowly and requires only occasional shaking. One-half cubic centimeter of this mixture gives the same color intensity in a titration as 0.3 cc. of 0.01 M diphenyl
amine sulfonic acid, and therefore may be substituted for it.
R e l a t i v e B l a n k s f o r O x i d i z e d a n d U n o x i d i z e d I n d i c a t o r . Twenty cubic centimeter portions of 0.05 N potas
sium dichromate solution were treated with 5 cc. of sulfuric acid (specific gravity 1.5) and 5 cc. of phosphoric acid (specific gravity, 1.37), and diluted with water to 200 cc. Three of these samples were titrated with 0.025 N ferrous sulfate after adding 0.3,0.6, and 0.9 cc. of 0.01 M unoxidized indicator, and three more were titrated after adding these same volumes of 0.01 M indicator which had been oxidized. For the first three samples the blanks in 0.025 N ferrous sulfate were 0.30, 0.56, and 0.79 cc., respectively; for those containing the same volumes of indicator in oxidized form, the blanks were 0.15, 0.38, and 0.57 cc.
The theory has been advanced U) th a t the first step in the oxidation of diphenylamine sulfonic acid is to diphenylbenzi
dine sulfonic acid, and the next to diphenylbenzidine sulfonic acid violet, which in turn may form a meriquinoid compound, diphenylbenzidine sulfonic acid green, with unoxidized di
phenylbenzidine sulfonic acid, or may form higher oxidation , products if any excess of oxidizing agent is present. If this explanation is correct, the use of the oxidized indicator in ti
trations of chromic or vanadic acid with ferrous sulfate should eliminate any blank correction due to the first step in the oxidation of the indicator, an irreversible reaction, and, since the next step in the oxidation represents a reversible reaction, should cause any blank correction to be due mainly to the irre
versible reactions represented by the oxidation of diphenyl
benzidine sulfonic acid violet, which means destruction of the indicator, some loss of which is inevitable.
To determine the indicator blanks with variations in con
centration of oxidizing agent, volume of solution, acidity, and volume of oxidized indicator, measured portions of a standard solution of potassium dichromate prepared from material of accurately determined oxidizing power were taken, treated with a definite volume of sulfuric, and of hydrofluoric or phosphoric acid, and diluted to the volume specified in Table I.
The stated volume of 0.01 M indicator, oxidized, was added and the chromic acid titrated with 0.025 N ferrous sulfate standardized electrometrically against standard eerie sulfate
155 (8), the strength of which had been determined against Bureau of Standards sodium oxalate (<?). From the normality of the ferrous sulfate the volume of reducing agent equivalent to the dichromate present was calculated, and the difference between this value and the actual value obtained in the titra tion, using the indicator, was recorded as the apparent indi
cator blank. The results are given in Table I. Some of the values in the last column in this table involve more than the true indicator blank, as the volume of ferrous sulfate re
quired to titrate a given amount of dichromate varies with the concentration of the latter (1, 5) and is reduced slightly by hydrofluoric acid if present in sufficient concentration {!).
T a b l e I. T i t r a t i o n o f D i c h r o m a t e w i t h D i p h e n y l a m i n e S u l f o n i c A c i d a s I n d i c a t o r
KîCrîCb H1S O4 S p . G r . H F
H3P O4 S p . G r .
I n d i c a t o r 0.01 M . In i t i a l
A p p a r e n t I n d i c a t o r B l a n k f o r 0 .0 2 5 N
Ex p t. 0.05JV 1 .5 48% 1 .3 7 Ox i d i z e d Vo l u m e FeSO<
Cc. Cc. Cc. Cc. Cc. Cc. Cc.
1 25 5 5 0 . 3 200 0 .2 8
2 25 5 5 0 . 3 100 0 .3 1
3 25 5 5 0 . 3 300 0 .2 6
4 15 5 5 0 . 3 2 00 0 .2 6
5 5 5 5 0 . 3 2 00 0 . 2 2
6 25 5 5 0 . 5 2 0 0 0 .5 0
7 2 5 5 5 0 . 3 200 0 .1 3
8 25 5 10 0 . 3 2 00 0 .1 5
9 25 5 20 0 . 3 2 0 0 0 .1 7
10 25 10 5 0 . 3 2 0 0 0 .1 2
11 25 20 5 0 . 3 200 0 .1 3
12 25 5 5 0 .6 2 0 0 0 .4 0
13 25 5 5 0 . 9 2 00 0 .5 9
With the solutions containing hydrofluoric acid, there was a slight return of purple after the end point was apparently reached, and an additional 0.04 to 0.08 cc. of 0.025 N ferrous sulfate was required to obtain a permanent end point. This latter volume of ferrous sulfate is the one recorded above.
From experiments 1 to 5 it is seen th a t variations in volume and in concentration of dichromate cause only a very slight change in the indicator blank and from 7 to 11 th a t the blank in phosphoric acid solutions is consistently lower than in hydrofluoric acid solutions. Experiments 7 to 11 also show th a t the blank is independent of acid concentration, while 6, 12, and 13 indicate th a t the blank is greatly influenced by the volume of indicator used, because the destruction of the latter is greater the greater the concentration. The end
point color change was less sensitive in 10 and 11, in which considerable acid was present.
In d i c a t o r Bl a n k i n Ti t r a t i o n s o f Va n a d i c Ac i d i n Tu n g s t e n St e e l s
Since one of the most important uses of diphenylamine sul
fonic acid is for titrations of vanadic acid in chrome-vanadium- tungsten steels, the effect on the indicator blank of changes in concentration of vanadium was studied.
To 0.8-gram samples of ingot iron, potassium dichromate equivalent to 0.035 gram of chromium, measured portions of an ammonium vanadate solution, and 5 ec. of sulfuric acid (specific gravity, 1.83) were added. After the iron was dissolved, nitric acid (specific gravity, 1.42) was added, drop by drop, to the hot solution, about 5 cc. being used in all, and the solution was boiled 2 to 3 minutes to remove oxides of nitrogen. After diluting to 160—175 cc. and cooling to room temperature, 5 cc. of hydrofluoric acid (48 per cent) and sodium tungstate solution equivalent to 0.2 gram of tungsten were added. The permanganate-azide method (7) was used to convert the vanadyl salt into vanadic acid. Then 3 cc. more of hydrofluoric acid (48 per cent) and 0.5 cc. of 0.01 M indicator, oxidized, were added, and the vanadic acid titrated with 0.025 N ferrous sulfate standardized electrometri
cally against standard eerie sulfate.
The results obtained are given in Table II, each value in the second and third columns representing an average of three duplicate determinations.
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
