By J . J . T . G raham and C. M . Sm ith
INSE C T IC ID E A ND F u N O IC ID E L ABO R ATO R Y, M IS C E L L A N E O U S D IV IS IO N , B U R E A U OP C H E M ISTR Y , W A S H IN G T O N , D. C.
It has been fo u n d that nitrates a n d nitrites interfere in the official method o f the A ssociation o f Official A gricultural C hem ists fo r total arsenic in insecticides, causing low results. T h is is probably brought about by the fo rm a tio n o f nitrosyl chloride, which passes into the distillate, a n d there slowly oxidizes the trivalent arsenic to the pentaOalent fo rm .
T h is interference can be prevented by the application o f the method o f J a n n a sc h a n d S eidel in which hydrazine sulfate a n d sodium bromide are used as reducing agents. I n addition to giv
ing greater accuracy, this method hastens the volatilization o f the arsenic, thereby decreasing the tim e a n d the am ount o f hydrochloric acid required.
T
HE determination of arsenic by the method of reduction and distillation as arsenic trichloride has been the subject of numerous investigations. An ex
haustive review of the literature concerning this problem is given by Roark and McDonnell in a paper2 describing the application of this method to the determination of arsenic in insecticides. After testing many substances, these authors selected cuprous chloride as the most suitable reducing agent. Following their work, the Association of Official Agricultural Chemists adopted this as an “official” method,3 and it has since been used in the Bureau of Chemistry with generally satisfactory results. However, in the analysis of commercial calcium and lead arsenates it was found that sometimes duplicate determinations did not agree, and that the results were low in comparison with those obtained by the modified Gooch and Browning method.'1
It was also observed that delay between distillation and titration accentuated the difference. Impurities in the reagents were at first suspected as the cause of the trouble, but after investigation it was proved that the disturbing factor lay in the sample itself.
The decrease in the titrations with the lapse of time indi
cated oxidation in the distillate and led the writers to sus
pect the presence of an oxidizing substance in the sample.
In order to cause the effect noticed, the interfering substance or substances must either be volatile or produce volatile products which are oxidizers. These conditions, considered in connection with the process of manufacture of calcium and lead arsenates, practically limit the possible oxidizing substances to nitrates and nitrites. Qualitative tests on a sample of calcium arsenate giving discrepant results showed the presence of a considerable amount of nitrate and a trace of nitrite. Subsequent determination of the nitrogen in this sample gave results equivalent to 3.50 per cent of nitrogen pentoxide. In all cases in which trouble was experienced nitrates were found. Working upon this suggestion, experi
ments were planned to determine more definitely the action of nitrates and nitrites and to devise a modification of the method to overcome this source of error.
Standard Solutions
The following standard solutions were prepared:
a r s e n i c a c i d—A solution was prepared from solid arsenic
1 R eceiv ed O cto b er 22, 1921.
* T h i s Jo u r n a l, 8 (1916), 327.
* Assoc. Official A gr. C h em ists' M eth o d s, 2nd editio n (1920), 54.
4 D e p t, of A g ricu ltu re, Chcm. B u ll. 105 (1907), 166.
acid which had been purified by several crystallizations from water. It contained 26.05 g. of arsenic pentoxide per liter, as determined by analysis both by the official dis
tillation method3 and by the Gooch and Browning method of reduction with potassium iodide and subsequent titration with standard iodine solution.
nitrate solutions— (1) a solution containing 10.0 g.
of lead nitrate per liter; (2) a solution containing 4.954 g.
of calcium nitrate per liter.
nitrite solution—A solution containing 4.167 g. of sodium nitrite per liter (the same concentration of nitrogen as in the standard nitrate solutions).
Experimental Procedure
The analyses were made on 25-cc. samples of the arsenic pentoxide solution to which were added varying amounts of the nitrate or nitrite solutions. (The weight of calcium arsenate usually taken for analysis is 1.5 g., so that on this basis the amount of nitrate introduced with 25 cc. of the standard nitrate solutions is equivalent to about 5.4 per cent of nitrogen pentoxide in a commercial sample.) In the experimental procedure the official method was followed except as noted, and all determinations were made in dupli
cate. The reducing agents were first introduced into the distillation flasks, then aliquot portions of the arsenic pent
oxide and of the nitrate or nitrite solutions were added by means of pipets, followed by 100 cc. of concentrated hydro
chloric acid. A total of 250 cc. of hydrochloric acid was used in each determination. The distillates were made to 1 liter, and aliquot parts were titrated according to the bromate method of Gyory,6-6 as presented to the Association of Official Agricultural Chemists by the referee on insecticides and fungicides at the 1920 meeting. As the proceedings of this meeting have not yet been published, the details of the method will be quoted here:
Transfer 200-cc. aliquots of th e distillate to 500-cc. Erlen- m eycr flasks, h e a t to 90° C., an d titr a te w ith th e stan d ard p o tas
sium brom ate solution (1.688 g. potassium brom ate to 1 liter of w ater standardized against arsenious oxide in approxi
m ately 2 N HC1), using m ethyl orange as indicator. T h e indi
cato r should n o t be added until near th e end of th e titratio n , and th e solution should be ro tated continuously to avoid any local excess of th e titratin g solution. T he end of th e reaction is indicated by a change from red to colorless an d is very sharp.
The first experiments showed conclusively that both nitrates and nitrites had a marked effect on the results.
When 0.25 g. of lead nitrate was used the distillate in the receiving flask was decidedly yellow and even 0.05 g. caused a slight yellow coloration. Nitrites produced the same effect. This color disappeared when the distillates were made to volume in a liter flask. On heating, the distillate slowly destroyed the color of methyl orange, but the fading was not sufficiently rapid to interfere with the titration.
Aliquot portions titrated at successive intervals showed a steady decline in the arsenic value.
* Z . anal. Ch»m., 32 (1893), 415.
• Enough experim ents w ere m ad e to show t h a t th e b ro m a te m ethod gave th e sam e resu lts as th e iodine titr a tio n m e th o d , an d i t w as used be
cause of its d irectness a n d g reater speed.
T a b l e I — E f f e c t o p N i t r a t e s a n d N i t r i t e s o n D e t e r m i n a t i o n o f A r s e n i c b y D i s t i l l a t i o n (A rsenic p e n to x id e ta k e n in each ex p erim e n t 0.1303 g.)
-Red u cin o Ag e n ts . L ead C alcium Sodium Ar s e n ic Pe n t o x id e Fo u n d, Grams -—Pr o p e r t ie sof Dis t il l a t e- , rhPnrWV . . . . X 'V a te £ ,trite T itra te d T itra te d T itra te d T itra te d T itra te d T i tr a te d B le S k ta z C hlo rid e A d d itio n a l T a k e n T a k e n T a k e n Im m e- a fte r a fte r a fte r a f te r a fte r A ction m M eth v l G ram s G ra m s G ra m s G ra m s G ra m s d ia te ly 4 H rs. 1 D a y 2 D a y s 3 D a y s (*) D ay s C o lo r O ran g e i t 90» C
? ... n - i . • • • 0 .1 3 0 3 --- 0 .1 3 0 3 (13) N o n e N o n e
• • • --- 0 .1 2 7 9 0 .1 2 6 8 --- 0 .1 2 5 8 (1 0 ) L is h t yellow M ark ed
fi o ’ i ’ 4 n ’ l^ n ? n ' i i i n n 'i i n n n '} ? n n --- 0 .1 0 5 1 (1 0 ) D ecided yellow M ark ed
5 ' ... ** 0 .1 - 4 . * * S ’ J o - i 0 .1 2 7 0 0 -J 2 0 0 0 .1 1 6 9 • ... D ecided yellow M ark ed n o - *’ * 0 .1 0 4 0 . 1 -D/ . . . . 0 .1 0 7 5 . . . . 0 .0 9 5 0 0 .0 9 2 0 (5) D ecided yellow M ark ed
5 F e S O « io 0 ^ 5 n ’ i a o ? * * '* 0 .1 1 8 6 (IS) D ecided yellow M ark ed
I ' l r I t ° : ik ::: 0 : 1 3 0 3 0:i29i 8: Hie o:if35 :::: ° ! ™ 3 £ £ S
5 N a l i r l '* o ’ i2 4 " 0 1303 0 1277 0 1195 o 'l l f i q --- 0 .1 0 3 4 (IS) D ecided yellow M ark ed
5 AnUine oil 1 ( c c ) 0 25 n i n S i™ ? 0 ' U 6 9 ... D ecided yellow M a rk e d
5 A niline oil 5 (cc ) 0 25 " o ' n 2? --- n ' i 907 ... D ecided yello w M a rk e d f 5 A niline oil 5 (cc j o ' i l k n m " " n ' H t l ... D ecided yellow M a rk e d f
5 N H*C1 ° o ' « " 2 'T o t J ° - 1329 ... D ecided yellow S lig h tf
5 N H ,C 1 2 + N a B r l o . h i ! ] 0 1291 ... 0 1103 " " o ' i m i $ y e n ° W i f “ & } H y d ra z in e su lfa te 2 0 .2 5 . . 0 1303 " " 0 'i 3 0 3 0 1303 " " n m f « D ecided yellow M ark ed H y d ra z in e su lfa te 2 . . i " 0 ¿04 0 1303 0 1303 o 'i i n - ? n r m l L ig h t b row n N o n e 5 H y d ra z in e su lfa te 3 0 .1 2 4 . . . 0 .1 3 0 3 0 .i 3 0 3 o !l3 0 3 0 ^ 3 0 3 . . . . C N o n e N oSe
3 H y d ra z in e su lfa te 1 0 . 2.j . . . o 1303 0 1300 0 12Q-"> n i o A i' £1?“
° - 124 • •• 0 - « 0 3 0 .1 3 0 1 0 j a « 8 v O : i » 8 ! . . . ° ' 1291. N o n e i g j
5 H y i i £ ? “ S l f a t e l ° 25 ° 12" ... 0 1277 0 ' 1277 0 .1 8 0 6 (1 0 ) N o n e S lig h t
5 H y d ra z in e su lfa te 1 ° ' 25 ... ° ' 1303 ... 0 1303 0 1303 ° - 1 3 0 3 (1 0 ) N o n e N o n e
„ . . < : ,,'): N a B r l •• ° ' 124 •• 0 1303 0 1 3 0 3 0 1 3 0 3 0 1303 ... N o n e N o n e H y d ra z w e S u lfate
2 ... n '? ? 2 ’ --- 0 .1 2 1 2 0 .1 2 1 2 --- 0 .1 2 1 2 (1 8 ) L ig h t b ro w n N o n e
3 N a l i r l ' *' * * o V ° : i 2 i - » • ' « « " S O .O sSI i ” : ° - 1009.(1S) & £ b r0 " £ “ e
I I ; ::: 8:il8l 0:1 3 6 3 8:1181 8:il8i :::: °-1303 <ls> ¥ £ b™ a g ™
1 o : i 2 4 : : : 8 : i i 8i 0 » 0 :1 3 0 3 8 : i i 8l ° : ! 303 ° - ! 3? 3 .(18) g ™
* StrorljT odoV of"uniUnt- 1 ^ " C “ in d ic a te th e n u m b e r of d a y s h a v in g elap sed a t th e tim e of th e la st titra tio n .
Re d u c i n g Ag e n t s
Having demonstrated that nitrates and nitrites cause low results, the next step was to find a method of reduction whereby the arsenic could be accurately determined in the presence of these substances. Experiments were made with various reducing agents and combinations thereof, using in every ease 25 cc. each of the standard arsenic solution and nitrate or nitrite solution. The use of a larger quantity of cuprous chloride was first tried, but even 15 g. of this material made practically no change in the results. Ferrous sulfate in combination with cuprous chloride gave distillates which were decidedly yellow, oxidized methyl orange very rapidly, and showed a more rapid decline in the arsenic value than when cuprous chloride was used alone. Rohmer7 noticed that hydrobromic acid accelerated the reducing action ol sulfur dioxide, and attributed to it a catalytic effect. 1 his suggested the use of sodium bromide in com
bination with cuprous chloride. Distillations made in this way, however, showed only a slight improvement over those made with cuprous chloridc alone.
Jannaseh and Seidel* found that hydrobromic acid to
gether with a salt of hydrazine gave excellent results in the reduction of ursenio. Since hydrazine is such a strong re
ducing agent it seemed likely that it might destroy the nitrates as well as reduce the arsenic. It was used at first alono and then in combination with cuprous chloride and with sodium bromide. The use of hydrazine sulfate alone eliminated the yellow color and prevented the gradual oxidation of the distillate, but the arsenic was not completely reduced in tho time required to obtain 250 ce, of distillate (30 to 40 min.), even when as much as 4 g. of hydrazine sulfate were employed. (This is in accord with the results of Jannaseh and Seidel, who found that several hours were required to secure complete reduction by hydrazine sulfate alone.) When the distillations were made with 5 g. of cuprous chloride and L g. of hydrazine sulfate tho arsenic was completely reduced, but tho oxidation of the distillate*was not
eliini-> fttr., Si (1001). 33,
• I b i d ,, « (1910), 1218; J . prakt. C htm ., »1 (101S). 133.
nated. When, however, the hydrazine sulfate was increased to 2 g. the oxidation of the distillate was completely pre
vented. Jannaseh and Seidfel found that the addition of a bromide to the hydrazine sulfate greatly hastened the reduction of the arsenic, and the writers have found that sodium bromide and hydrazine sulfate when added to the cuprous chloride also completely destroy nitrates. Later experiments showed that equally as good results were ob
tained without the use of cuprous chloride. One gram of hydrazine sulfate and 1 g, of sodium bromide proved suffi
cient where the quantity of arsenic pentoxide present did not exceed 0.75 g.
The investigation included some attempts to find a satis
factory substance to destroy the nitrates, which would be cheaper than the hydrazine sulfate. A m m onium chloride and aniline, owing to their known action on nitric acid and nitrites, respectively, were tried but proved to be of no value.
Tho conditions to be fulfilled in this distillation are so cir
cumscribed that the list of possible substances as reducing agents is limited.
R e s u l t s ov E x p e r i m e n t s
Tho results given in Table I show conclusively that, in tho presence of nitrates (or nitrites), the distillation method for arsenic as ordinarily carried out, using cuprous chloride or cuprous chloridc and ferrous sulfate as the reducing agents, gives low results. This is due to some volatile substance, possibly nitrosyl chloride, carried over in the distillate that oxidizes the arsenio trioxide. The extent of this oxidation depends largely upon the length of time tho distillate stands before titration. Where a number of distillations are being carried on at the same time, it is impossible to titrate all of them immediately and the error caused by this oxidation may be a very material one. This error may be avoided by the use of hydrazine sulfate as the reducing agent, or preferably hydrazine sulfate and sodium bromide, in which case tho reduction takes place much more rapidly. The nitrates and nitrites are probably reduced to nitrogen by the hydrazine sulfate.
Mar., 1922 T H E JO U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M IS T R Y 209
Standard Arsenic Trioxide Solution—Dissolve 2 g. of pure arsenic trioxide in a beaker by boiling with about 150 to 200 cc. of water containing 10 cc. of concentrated sulfuric acid, cool, transfer to a 500-cc. graduated flask, and dilute to the mark.
Standard Iodine Solution—Prepare an approximately 0.05 N solution as follows: Mix intimately 6.35 g. of pure iodine iodine solution from a buret, shaking the flask continuously, until the yellow color disappears slowly from the solution,
Standard Bromate Solution—Dissolve 1.688 g. of pure potassium bromate in water and dilute to 1 liter. Stand
ardize as follows: Pipet 25 cc. of the arsenic trioxide solution into an Erlenmeyer flask, add 15 cc. of concentrated hydro
Hydrazine Sulfate and Sodium Bromide Solution Dissolve 20 g. of hydrazine sulfate and 20 g. of sodium bromide in 1 liter of dilute (1:4) hydrochloric acid.
d e t e r m i n a t i o n—Weigh accurately an amount of the sample containing the equivalent of not more than 0.6 g.
of arsenic pentoxide and transfer to a distilling flask. Add 50 cc. of the solution of hydrazine sulfate and sodium bromide and close the flask with a stopper through which passes the
centrated hydrochloric acid by means of the dropping funnel and distil until the volume in the distilling flask is reduced to about 40 cc.;add an additional 50 cc. of concentrated hydrochloric acid and continue the distillation until the con
tents of the flask are again reduced to about 40 cc. Wash down the condenser, transfer the contents of the receiving flasks to a 1-liter graduated flask, make to volume, and mix thoroughly. Pipet a 200-cc. aliquot to a 500-cc. Erlen
meyer flask, nearly neutralize with sodium hydroxide, finish the neutralization with sodium bicarbonate, add 4 to 5 g.
in excess and titrate with standard iodine solution, using starch solution as indicator; or to the 200-cc. aliquot add 10 cc. of concentrated hydrochloric acid and titrate with stand
ard bromate solution, as previously described in this paper.
With a sample equivalent to 0.6 g. of arsenic pentoxide, practically all of the arsenic is distilled over in the first 50 cc. of the distillate, which is a much more rapid volatilization than that w-hich occurs in the official distillation method, A total distillate of 150 cc. is sufficient to insure complete distillation of the arsenic. The method has been applied to the analysis of commercial samples of calcium arsenate, lead arsenate, and other arsenical insecticides containing nitrates, with most satisfactory results in all cases. Table II shows the results obtained by the “official” and by the modified method on commercial samples of lead and calcium arsenates containing nitrates. In the case of the calcium when the titrations are made immediately after distillation, in some instances the official method gives lower results, and when the titration is delayed for several hours the results by the official method are lower in every case. The maximum difference after 9 days amounts to more than 5 per cent of
T H E J O U R N A L OF I N D U S T R I A L A N D E N G IN E E R IN G C H E M I S T R Y Vol. 14, No. 3