Dee., 1916
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
undeniable toughness of this substance, combined with its sufficiently high resiliency, were properties entirely in its favor. Considerable water is absorbed, however, when i t is placed in this liquid for a n y length of time. This property is decidedly objectionable, since t h e fiber shows a marked decrease i n its resiliency a n d toughness. I n order t o measure t h e a c t u a l q u a n t i t y of water absorbed, t h e following experiments were made with materials furnished b y t h e American Vulcanized Fiber Co. These are summed u p in Table I. T h r e e other experiments made on r a t e of absorption are summarized in Table 11. Expt. No.
1.... , . 2. . . . . 3. . . ., .
.
TABLE I1 Color Weight PERCENTAGE IICREASE and taken 1st 2nd 3rd 4th Texture Size Grams hr. hr. hr. hr. Total Hard Gray 2 X 2 Xl/az 4.235 25.92 17.24 6.42 3.52 53.10 4.082 11.14 10,20 6.53 3.23 31.13 Hard Red 2 X 2 XI,” 6.901 17.40 7.13 7.37 5.83 37.73
T h e results indicate t h a t t h e greatest p a r t of t h e water absorbed is during t h e first hour. F r o m t h e n o n t h e r e a p p e a r s t o be a s t e a d y decrease u n t i l t h e experiment was finished. T h e absorption is completed in 7 t o 8 hours, possibly. T h e t o t a l increase in absorption agrees quite well with t h a t observed in Table I , excepting t h e value obtained in E x p t . 3 ; n o reason can be given for this divergence. T h e writer decided t o t r e a t t h e fiber with various substances t o see if he could not reduce t h e degree of mater absorption, a n d b y so doing a n n u l t h e effect of this property t o such a n extent as t o make this TABLEI11 (A) Saturated with 5 per cent alum solution, then in 8 to 9 per cent ammonia solution, finally in water and then dried. (C) First saturated with water, then placed in boiled linseed oil, which is heated slowly from the ordinary temperature to 135-140’ C. for an hour or two. The fiber was then taken out and allowed to dry. (D) The fiber was placed in a nearly boiling saturated agar-agar solution for 2 or 3 hrs., then taken out and dried. (E) Saturated with 15 per cent NaOH solution for 2 or 3 hrs., then exposed to the action of carbon disulfide vapor in a tight jar for about 48 brs.; finally washed with dilute hydrochloric acid and dried. (F) Placed in about 70 per cent sulfuric acid, then washed and dried. (G) Saturated with a strong solution of zinc sulfate solution, then placed in 8-9 per cent ammonia water, washed a little and dried. (H) Saturated with a mixture of blood albumen, borax, magnesium sulfate and glycerine, then exposed t o the action of steam and dried. (I) Saturated with a hot 1.5 per cent gelatin solution, then exposed to the action of hot formaline vapor and dried. (J) T h e wet fiber was placed in hot molten paraffin, then heated to 115-120’ C. for 1 or 2 hours, then removed and allowed to cool. (K) The moist fiber placed in a mixture of Burgundy pitch, paraffin and linseed oil, which is heated to 105-110° C. for 1 or 2 hrs., then taken out and cooled. Per Time cent of Method of Water I m Expt. Treat- Color and Size in Ab- merNo. ment Texture inches sorbed sion REMARKS Hard Red l/g x 1.25 16.44 5 . 5 No change 1 A dia. hemisphere shell 41.07 6 . 0 No change 2 B Hard Gray 2.5 X 2 X’/ia 38.47 4 . 0 No change 3 Piece from Ex. 2 No change Retreated 43.05 ... No change 4 c Hard Red 2 x 2 x1/16 9.76 4 . 0 Brittle 5 Hard Gray No change 17.80 6 D 2 X 2 X1/a2 7 5.50 . . . N o change Hard Red 2 x 2 X‘/Ifi 45.45 8 E Hard Gray 2 x 2 x1/1fi 9 41.40 4 . 0 A slight in10 F 44.45 6 . 0 crease in flexi50.08 4 . 0 11 G bility appar12 H Hard Red ‘/le Irreg. 35.84 ent in shape Hard Gray 13 35.53 . . . 14 I 37.73 Brittle 8.66 . . . No change 10.88 . . . Brittle Piece from 13 Retreated 17 Piece from 15 Retreated 21.32 . . . Brittle Piece from 16 Retreated 18 16.56 . . . Brittle Hard Red 2 X 2 X1/16 17.86 . . . No change 19 K
... ...
...
E
’
...
I 109
material of value for a golf ball cover. T a b l e I11 is a s u m m a r y of t h e results of these later experiments. These preliminary experiments indicate t h a t t h e objectionable property of absorbing relatively large quantities of water can be done a w a y with t o a large e x t e n t , perhaps completely. T h e final mechanical difficulty of covering t h e elastic rubber core with t h e t r e a t e d fiber in such a way as t o eliminate a n y t e n d e n c y of cracking or of t h e tearing a w a y of t h e l a t t e r from t h e core, would require further experimentation t o be overcome. TULANE UNIVERSITY
OF
LOUISI.4NA, NEWORLEANS
A BOILING METHOD FOR THE DETERMINATION OF WATER-SOLUBLE ARSENIC IN LEAD ARSENATE By GEORGEP GRAYAND A W . CHRISTIE Received July 18, 1916 IKTRODUCTION
Some 3000 tons of arsenic trioxide have been a n n u a I I y produced in t h e United States during recent years a s a by-product in smelters of t h e western s t a t e s a n d am equal a m o u n t is also imported under normal conditions.’ Probably t h e largest use of this “ w h i t e arsenic” is in t h e preparation, of lead arsenate u s e d solely in t h e control of leaf-eating insects. T h i s commodity is usually supplied t o t h e consumer i n t h e form of a paste containing 40 t o j o per cent water, 1 2 t o 16 per cent arsenic pentoxide, a n d 30 t o 4 0 p e r cent lead oxide. Of recent years, however, t h e d r i e d powder is coming i n t o favor t o some extent. T h e acid a n d basic salts of lead a n d arsenic acid, a n d possibly t h e neutral salt, are believed t o comprise t h e various brands on t h e market. Most commercial lead arsenates are probably mixtures rather t h a n a n y pure salt. I n order t o compound a n arsenical which m a y b e safely applied t o growing plants without fear of defoliation a n d yet will poison t h e insects, many f a c t o r s must be t a k e n i n t o consideration. T h e most i m p o r t a n t thing t o be considered is t h a t t h e arsenical must b e as free from water-soluble arsenic as can be economically produced on a commercial scale. T h e grower has a right t o demand this protection for his orchards a n d crops for t h e reason t h a t arsenic in water-solubEe jorm is one of t h e most violent plant-poisons known. It h a s been shown t h a t certain pure salts of lead a n d arsenic acid a r e exceedingly insoluble in ~ a t e r . * ’ ~ ’Com~ mercial lead arsenates, however, usually contain a small a m o u n t of water-soluble arsenic as a n i m p u r i t y which has not been washed o u t during t h e process o f manufacture. T h e majority of lead arsenates which are being offered for sale in California a t t h e present t i m e are reasonably free f r o m impurities. Occasionally, however, a sample is received b y t h e Insecticide a n d Fungicide Laboratory of t h e University of California 1 F. I ,. Hess, “The Production of Antimony, Arsenic, Bismuth, a n d Selenium in 1912,” 1913, “The Mineral Resources of the United States,” E. S. Geological Survey. 2 R. H. Robinson and H. V. Tartar, “The Valuation of Commercial! Arsenate of Lead,” THISJOURNAL, 7 (1919, 499; “The Arsenates of Lead,” Ore. Agr. Exp. Sta., Bull. 13.9 (1915); 4 H. V. Tartar and R. H. Robinson, “The Arsenates of Lead,” J . Am. Chem. Soc., 36 (1914), 1843. J
1110
T H E JOURA’AL O F I i Y D l 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
which is found to contain sufficient water-soluble arsenic t o be a menace t o foliage. T h e Federal I n secticide ,4ct of 1 9 1 0 and t h e s t a t e insecticide laws a r e quite uniform i n restricting t h e a m o u n t of watersoluble arsenic in lead arsenate t o 0.75 per cent. A n accurate method is, therefore, necessary t o determine this solubility. An ideal method should b e r a p i d i n order t o b e a d a p t a b l e t o factory control a n d a t t h e s a m e t i m e should be a d e q u a t e for t h e examination of specimens t o determine their suitability as insecticides a n d establish their legal s t a t u s . Neither of t h e two provisional methods of the Association of Official Agricultural Chemistsl,2 nor other published. methods seem t o meet these requirements i n all respects. PROVISIONAL METHODS 05‘ TH6 A . 0. A . C. Haywood’s method for the determination of water-soluble arsenic in commercial lead arsenates was made a provisional method of the Association of Official Agricultural Chemists in 1907~and still remains as such.4 It is given below in full in order that other methods may be more readily compared with it. The paste is prepared for analysis by drying a t from 80 to 100’ C., and grinding to a powder. Two grams of the powder are placed in a flask with 2000 cc. of COI-free water and allowed to stand I O days, shaking eight times a day at hourly intervals. A zoo to 400 cc. aliquot of the filtrate from the above digestion is used for the determination of water-soluble arsenic oxide as follows: “Add 0.5 cc. of sulfuric acid and evaporate it to a syrupy consistency; then heat on the hot plate to appearance of white fumes. Add a very small amount of water and filter off lead through the very smallest filter paper, using as little wash water as possible. Place this filtrate in an Erlenmeyer flask, and determine arsenic as described under total arsenic oxide, using the same amount of reagents and the same dilutions.” [The method for the determination of “total arsenic oxide” referred to in the above quotation is a modification of the Gooch and Browning neth hod,^ the accuracy of which is not under discussion in this paper. ] McDonnell, -4. 0. A . C. Referee on Insecticides for 1 9 1 0 , ~ conducted some experiments to determine the time necessary to dissolve the water-soluble arsenic from lead arsenate and concluded that the digestion need not exceed z to 4 days. The following year several chemists cooperated in making a comparison of the results obtained by a 1-day extraction and a Io-day extraqtioii. The data reported, however, mere rather conflicting.’ The investigation was continued by ilveritt, Referee for I 9 I z . 8 The io-day method was severely criticized in the Referee’s report, Methods which mere thought to correspond more closely- to field conditions were proposed for trial by cooperating chemists. Two grams of the paste Tverr. placed in a bottle with zoo cc. of COa-free water and shakeu in a mechanical shaker for 3 hrs. In comparison with the above method of extraction, the same quantity of sample and water were placed in a bottle : Association of Official Agricultural Chemists, Proceedings of Annual Convention, Z j t h , U. S. Dept. Xgr., Bur. Chem , D d l . 122 (1908). 2 Association of OEcial Agricultural Chemists. Proceedings of Annual Convention, 31st, Jour. A . 0.A . C., Val. I, S o . 3 , and 1, KO. 4, Part I (19 14). 3 Association of Official Agricultural Chemists, Proceedings of Annual Convention, 21th, U. S. Dept. Agr., Bur. Chem., B d . 116 < 1 9 0 2 . 4 Association of Official Agricultural Chemists, “Official and Provisional Methods of Analysis,” U. S. Dept. A g r . , Bur. Chem., Bull. 107 (reviqed), (1909). 6 lbid. 6 ilssociation of Official Agricultural Chemists, Proceedings of Annual Convention, S i t h , U. S . Dept. Agr., Bur. Chem., Bull. 137 (1910). 7 Association of Official Agricultural Chemists, Proceedings of Annual Convention, 28th, E. S . Dept. Agr., Bur, Chem., B7~11.162 (1911). 8 Association of Oficial Agricultural Chemists, Proceedings of Annual Convention, 29th, U. S. Dept. Agr., Bur. Chem., Bull. 162 (1912).
Val. 8, KO. 1 2
and shaken every half-hour during a working day. Only three chemists cooperated in this study, so that the results obtained were not very conclusive. The temperature of digestion was also studied. Very little work was done on this problem by the A . 0 . A. C. during the following year, but the opinion of the Referee is expressed in his report as follows:‘ “It is to be noted that the methods of analysis are not in question a t all. The time of digestion is pretty well established and a few degrees in temperature is not very material in view of the fact that both time of digestion and temperature are entirely arbitrary. The Referee is well aware and has pointed out the fact that what we are determining and calling soluble arsenic is largely soluble lead arsenate, but as both time and temperature of digestion are not absolute factors, it is probably a matter of small moment.”
Cpon the recommendation of Averitt another provisional method was adopted by the A. 0. -4.C. at its 30th annual convention.’ The recommendation to drop the existing Io-day provisional method was not adopted. The more recent method differs from the Io-day method in the following points: ( I ) The paste is used for extraction instead of the air-dry sample. ( 2 ) The proportion of sample to water is increased. (3) The time of digestion is reduced to 24 hrs. (4) The temperature of digestion is specified a t 32 ’ C. No further work has been done by the Association in this investigation. THE COLBY METHOD
A method proposed by Colby2 for the determination of watersoluble arsenic in Paris green has been used to a considerable extent in the California Insecticide Laboratory for a similar purpose in the examination of lead arsenate. Colby weighed out a 0.j g . sample of the air-dry powder and placed it in a flask with 100 cc. of carbon-dioxide-free water, set in a warm place, and shook the flask every hour during the working day. After standing over night, the water was poured off and a Fresh roo cc. added and treated as before on the second and third day. The 300 cc. of leaching were combined, carefully filtcred, and arsenic determined as in the provisional Io-day method of the A. 0 . A. C., except that sulfuric acid was not added for the removal of the lead unless the same could be detected by a preliminary qualitative test. PROPOSED SIHORT METHOD O F CURRY AXD SMITH
Curry and Smith3 were the first to propose the weighing of the paste for analysis without previous drying. This was done with the idea that the sample in this condition would come to equilibrium with the water more quickly. Digestion was made by continuously stirring the sample with water by means of a hot air engine in a thermostat a t a temperature of 20’ C. for 18 hrs. These investigators found that higher results were obtained for water-soluble arsenic if the quantity of water per unit of sample were increased. They attributed this to the solubility of lead arsenate itself and proposed a solubility factor to be applied as a correction according to the proportion of sample to water. THE ORI3GOX STATION METHOD
Robinson and Tartar4 appear to have been the first to propose the use of hot water for the extraction of the soluble impbrities in lead arsenate. The method of these investigators, known as the “Oregon Station Method,” is in brief as follows: 1 Association of OWcial Agricultural Chemists, Proceedings of Annual Convention, 30th, Jour. .%, 0 . A. C., Vol. 1, S o s . 1 and 2 (1913). 2 G. E. Colby, “drsenical Insecticides,” Cal. Agr. Exp. S a . , Buil. 161 (1903). 8 B. E. Curry and T.0. Smith, “A Short Method for the Determination of Soluble ;Irsenic in Commercial 1,ead iirsenates,” TEXIS JOURNAL. 4 (1912), 198, 4 L O C Cil.
Dec., 1916
T H E J O U R N A L O F I N D U S T R I A L A N D E N GI ATE E R I N G C H E M I S T R Y
Four to six g. of the sample are washed on a filter paper with hot water until the washings amount to 1000 cc. Arsenic is determined in an aliquot of the washings in the same manner as in the provisional methods of the A. 0. A. C., except that the treatment with sulfuric acid and evaporation for the removal of lead is omitted, for no water-soluble lead was found in any of the samples. Attention was called to the necessity of the use of water free from carbon dioxide and ammonia, they being the first to emphasize that the water used for extraction must be ammonia-free. DISCUSSION O F THE PRESENT METHODS
As the matter now stands, there are two provisional methods for the determination of water-soluble arsenic in lead arsenate, which are recognized by the Association of Official Agricultural Chemists. The original Io-day method is time-consuming and for this reason is impracticable for factory control. The evidence produced by Robinson and Tartar‘ and confirmed by the writers, very strongly indicates that the extraction of lead arsenate by cold water does not remove all the soluble matter even if continued for a period of ten days. It has been shown by the results of chemists who have cooperated in the investigations of the A. 0. A. (!. that the time of digestion can be shortened without materially affecting the results. The more recent provisional method of the A. 0. A. C. specifies the use of the paste for digestion, shortens the time of digestion to 24 hrs., and involves the use of a thermostat to control the temperature a t 3 2 ’ C. Here again the question arises concerning the complete extraction of the soluble matters by means of tepid water. The shaking of the sample a t hourly intervals is not a serious objection, but the experience of the writers is that this detail is apt to be forgotten during attention to other duties. Colby’s method is an improvement over the Io-day method and the results obtained are practically the same by either. Curry and Smith’s method requires the use of a stirring device which is not always available. The solution of the soluble matters doubtless would be hastened if the original paste were not dried before analysis, as proposed by their method and subsequently written into the later method of the A. 0. A. C. The views of Curry and Smith concerning the correction of the results obtained by subtracting a solubility factor for lead arsenate have not been corroborated by other investigators as will be shown in a later section. Robinson and Tartar observe that the results obtained by the Oregon Station method are always higher than those obtained by the Io-day provisional method of the A. 0. A. C . It is maintained by them, however, that this is not due t o any hydrolysis of the lead arsenate nor to the solubility of lead arsenate in hot water; that the results obtained by their method are not too high, but that the results by the Io-day method are too low. In support of this, they point out the following facts: In cases where only a trace of soluble arsenic is obtained in the sample, the results are the same by either method; no soluble lead is found in the pure samples made in their laboratory nor in commercial samples ; the amount of soluble impurities is always greater by their method than by the Io-day method, indicating that the latter does not extract all of the impurities. The main points given above of Robinson and Tartar’s argument have been amply confirmed by independent investigations of this laboratory. C O N C E R S I N G T H E SOLUBILITY O F LEAD ARSENATE
Certain results obtained b y C u r r y a n d Smith’ s e e m t o indicate t h a t lead arsenate itself is sufficiently soluble in water t o give different results when differe n t volumes of water a r e used i n making t h e extract i o n of soluble arsenic. So f a r as k n o w n t o t h e writers, these results h a v e n o t been corroborated b y o t h e r investigators. T h e mass of evidence is t o t h e cont r a r y as will b e shown b y t h e following considerations:
’ LOG. cir.
1111
If p u r e l e a d arsenate is appreciably soluble i n water, lead will b e f o u n d i n the water extract thereof i n molecul a r proportion t o arsenic present. T h e ratio of AssOs t o P b O i n PbHAsO4 is I t o 1.9; t h e r a t i o is a b o u t I t o 3 i n a basic lead arsenate; therefore, if measurable quantities of arsenic were f o u n d i n t h e water extract f r o m a p u r e lead arsenate, t w o or t h r e e times this q u a n t i t y of lead would also b e found. This h a s n o t been the experience of analysts a s evidenced b y their published results, some of which mill b e reviewed below. A s a m p l e of lead arsenate was prepared b y t h e A. 0. A. C. Referee . o n Insecticides a n d s e n t t o 7 cooperating chemists f o r analysis. In t h e reports of these chemists, presented before t h e 26th a n n u a l convention of t h e Association i n 1 9 0 9 , ~it is n o t e d t h a t 5 report n o lead present i n t h e water extract, obtained b y t h e Io-day provisional m e t h o d of t h e Association, one reports 0.04 per cent, a n d one does n o t report o n lead. Robinson a n d T a r t a r 2 report n o lead f o u n d i n t h e examination of 6 samples of commercial lead arsenate. T h e y prepared specimens of b o t h acid a n d basic lead arsenates a n d deter’mined their solubility in water b y t h e Io-day method. B o t h t y p e s failed t o yield sufficient soluble arsenic f r o m 8 0 0 cc. of t h e water extract t o b e quantitatively e s t i m a t e d b y making t h e final t i t r a t i o n with N / 5 o iodine solution. M a n y samples of commercial lead arsenate h a v e been examined i n t h i s laboratory i n which neither water-soluble lead nor arsenic could b e found, except possibly i n t h e merest traces. Only i n r a r e instances is water-soluble lead detected i n a n y of t h e samples. F r o m a consideration of these d a t a , i t seems app a r e n t t h a t t h e solubility of lead arsenate itself is n o t a factor t o b e t a k e n i n t o account i n t h e determinat i o n of soluble arsenic i n Iead arsenate. REMOVAL
OF
LEAD
PR03;I
WATER
EXTRACT
BEFORE
TITRATION O F A R S E N C
It appears t h a t a considerable a m o u n t of watersoluble lead was a common i m p u r i t y i n commercial lead arsenates before t h e passage of t h e Insecticide Act of 1910. Analyses of jo samples were published, i n 1910 b y Haywood a n d SIcDonnel13 in which t h i s i m p u r i t y is reported i n a m o u n t s ranging f r o m 0.06 per cent t o 1.61 per cent. T h e removal of lead f r o m t h e water extract before t h e titration of arsenic was doubtless a necessary procedure i n t h e earlier years of lead arsenate manufacture. Very great improvements, however, h a v e been m a d e during t h e last few years i n t h e processes for its commercial manufacture. M a n y manufacturers a r e now using lead oxide a n d arsenic acid as t h e r a w materials with a small a m o u n t of acetic acid. as a catalyzer. T h e insecticide compounded i n this m a n n e r is remarkably free f r o m by-products a n d needs little or n o washing t o be salable. E v e n t h e manufacturers using t h e older process of double decomposition of lead acetate (or nitrate) a n d sodium arsenate a r e very careful t o 1 Association of Official Agricultural Chemists, Proceedings of Annual Convention, 26th, U. S. Dept. Agr., Bur. of Chem., Bull. 182 (1909). 2 L O G . cit. 3 J. K. Haywood and C. C. McDonnell, “Lead Arsenate,” U. S . Dept. Agt., Bur. of Chem., Bull. 131 (1910).
XI12
T H E J O U R N A L O F I Y D U S T R I A L A N D ENGI-lTEERIiVG C H E M I S T R Y
wash out a n y excess of t h e arsenical salt a n d soluble by-products in order t o comply with t h e National a n d s t a t e insecticide laws. By either process, i t is customa r y t o use t h e arsenical ingredient in excess. It is, therefore, quite improbable t h a t more t h a n a trace of water-soluble lead will be found in a n y lead arsenate of modern manufacture. As evidences of these improvements, t h e following m a y be cited: I n 1 9 1 2 , Holland a n d Reed’ determined watersoluble lead in five samples with t h e following results: none, 0.01 per cent! none: 0.14 per cent, a n d 0.06 per cent. As previously noted, Robinson and T a r t a r failed t o detect lead in t h e water extract from six samples. Curry and Smith2 considered i t unnecessary t o remove the small a m o u n t of lead in t h e water extract from certain samples which mere being studied. This laboratory frequently makes a qualitative test for water-soluble lead, b u t very seldom is a n y found. Both provisional methods of t h e 0. -1.C. provide for t h e removal of lead from t h e water extract before the reduction and titration of t h e arsenic therein. This procedure m a y be sometimes desirable, b u t in t h e opinion of t h e writers m a y be usually omitted in t h e examination of a product of modern manufacture. A$.
THE PROPOSED METHOD
T h e senior writer became engaged in the work of t h e Insecticide a n d Fungicide Laboratory a number of years ago a n d was very soon impressed with t h e desirability of developing a more satisfactory method for t h e determination of water-soluble arsenic in lead arsenate. ,4 critical s t u d y was m a d e of t h e published methods a n d from time t o time, as opportunity afforded, practically all of t h e m were tried. Yone seemed t o be entirely satisfactory, so new methods were given a trial. I t soon became evident t h a t neither t h e acid nor t h e basic t y p e of lead arsenate is appreciably soluble in boiling water, nor is either t y p e decomposed by prolonged boiling. I t also seemed entirely feasible t o extract completely t h e soluble matters b y boiling in water from j t o I O min. Digestion on t h e s t e a m b a t h also seemed permissible after t h e manner of digestion of a barium sulfate precipitate in order t o clear up t h e liquid a n d facilitate filtration. This preliminary work was so encouraging t h a t t h e writers felt warranted in making a thorough test of a t e n t a t i v e method which had been outlined. I t is especially gratifying t o note t h a t independent investigations of Robinson and T a r t a r 2 have produced a method t h a t is based u p o n t h e same principle as t h e one proposed b y t h e writers; namely, t h e insolubility of lead arsenate in hot water. T h e detail of t h e m e t h o d finally decided u p o n is as follows: PRocEDcRE-Tveigh 0 . j g. of t h e d r y powder a n d place in a j o o cc. Erlenmeyer flask. Add zoo cc. of distilled water a n d boil briskly for I O min. (If t h e liquid is cloudy or t h e sample does not settle readily, digest o n t h e s t e a m b a t h a n hour or two, or until t h e supernatant liquid is clear.) Filter through 1 E. €3. Holland and J. C. Reed, “The Chemistry of Arsenicai Insecticides,” Mass. Agr. Exp. Sta., 24th Annuol Report (1912). 2 LOC. Lit.
Vol. 8 , S O .1 2
NO.j 9 0 S. & S. filter paper; wash with a small q u a n t i t y of hot water, receiving t h e filtrate in a j o o cc. Erlenmeyer flask. Add I g. of potassium iodide and 4 cc. of concentrated sulfuric acid a n d boil until t h e volume is reduced t o about 40 cc. Dilute t o about 2 0 0 cc. and discolor a n y free iodine remaining b y means of approximately N / z o sodium thiosulfate. Add methyl orange a n d nearly neutralize with a concentrated solution of sodium hydroxide. Add sodium bicarbonate until alkaline and then a n excess. T i t r a t e with N/zo iodine solution, using starch paste as indicator. DISCUSSIOX O F THE PROPOSED METHOD
T h e air-dry sample is preferred t o t h e paste for analysis as it is thought t h a t the d r y salt more nearly represents the condition of t h e insecticide after application t o foliage and exposure t o weather. I t does not seem necessary t h a t t h e extractive water be free from carbon dioxide for t h e reason t h a t arsenic acid is more highly ionized in aqueous solution t h a n carbonic acid and t h e latter would not be expected t o liberate free arsenic acid from a n arsenate. Haywood and McDonnelll concluded t h a t lead arsenate i s s l i g h t l y less soluble in distilled water satzirated with carbon dioxide t h a n in distilled mater free from carbon dioxide. No difference in t h e results was observed by t h e writers whether boiled or unboiled distilled water was used for making t h e extraction. T h e influence of ammonia if present in t h e extraction water was not investigated by t h e viriters. Robinson a n d T a r t a r , ’ however, make t h e statement t h a t : “ I t is imperative t h a t no carbon dioxide nor ammonia be present in t h e water as t h e y m a y react with the lead arsenate, liberating arsenic in soluble form.” It is a well kno.ir-n fact t h a t t h e acid t y p e a n d m o s t commercial lead arsenates are partially soluble i n ammonia. I t , therefore, seems reasonable t o conclude t h a t t h e extractive water should be ammonia-free. T h e writers are not convinced, however, t h a t the absence of carbon dioxide therefrom is essent,ial. Both of these points need further s t u d y . Experiments by this laboratory have shon-n t h a t t h e soluble matters of lead arsenate are completely extracted from lead arsenate b y boiling in water 5 or I O min., n-hile a complete extraction of soluble matters is almost impossible by digesting in cold o r tepid water. Moreover, as previously pointed o u t , there appears t o be no solution or hydrolysis of lead arsenate even b y prolonged boiling or by digestion o n t h e s t e a m b a t h for several hours. If t h e sample settles out clear after boiling, filtration m a y be made a t once. M a n y samples, however, have a tendency t o remain in suspension. Boiling a n d digestion on t h e steam b a t h will clear up most. samples which are difficult t o filter otherwise. Samples containing soap or other organic matter are occasionally encountered which are still obstinate after t h e a b o v e t r e a t m e n t , b u t a clear filtrate is usually obtained b y running through t h e filter several times. I n all. cases t h e filtration is greatly facilitated by boil& and, digestion. 1
Lac.
cif.
Dec., 1916
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
T h e omission of t h e procedure for t h e removal of lead i n t h e water extract h a s been discussed i n a previous section. It is found t h a t t h i s s t e p is not usually necessary i n t h e examination of lead arsenates of modern manufacture. T h e evaporation of zoo cc. of t h e filtrate i n t h e presence of 0 . j cc. of sulfuric acid t o a s y r u p y consistency is a time-consuming operation; t h e heating of this small q u a n t i t y of “syrup” t o t h e appearance of white fumes of sulfuric acid is n o t without danger of volatilizing some of t h e arsenic, especially since t h e s y r u p y residue is distributed over t h e surface of a large evaporating dish. If t h e r e is a n y d o u b t a b o u t t h e advisability of omitting this p a r t of t h e customary procedure, a qualitative test for lead can be easily made of t h e filtrate a n d t h e s t e p o m i t t e d or not according t o t h e test. T h e remainder of t h e procedure is sufficiently familiar n o t t o require discussion. EXPERIMENTAL DATA
Ten samples were selected representing t h e products of several manufacturers a n d showing a wide range i n composition, including b o t h acid a n d basic types. Some contained only traces of water-soluble arsenic a n d others h a d considerably more t h a n t h e limit fixed b y law, Some contained organic m a t t e r . It is t h o u g h t t h a t practically all t y p e s of lead arsenate which a r e o n sale were represented in t h e samples analyzed. It did not seem worth while t o make a comparison of all of t h e methods which h a v e been proposed, b u t water-soluble arsenic was determined b y the Io-day provisional method of t h e A. 0. A. C.’ a n d b y t h e p r o posed boiling method. T h e results are given i n Table I. TABLE I-PER
CENT OF WATER-SOLUBLE ARSENIC CALCULATED AS ON 50 PER CENT WATERBASIS
AS205
The samples analyzed contained various amounts of water ranging from 21 t o 57 per cent. The maximum amount ermitted by Federal and state laws is 50 per cent. In order to make tge results comparable and to show more readily the legal status of the samples by both methods, the results were calculated to a 50 per cent water basis. A. 0. A. C. Proposed 10-day Boiling Method Method Remarks (Duplicates) (Duplicates) Sample . . A . . . . . . 0.25 0 . 2 5 0 . 3 6 0 . 3 9 Acid type 2 . 1 7 2 . 1 4 Acid type (contains much B . . . . . . 1.26 1.26 organic matter) 0.29 0.29 0 . 4 6 0 . 4 4 Acid t v D e . 0.07 0.07 0.06 0.06 D ,,. 0.45 0.45 E. . . . 0 . 3 6 0 . 4 0 0.07 0 . 0 7 0.03 0.03 F 0.19 0 . 1 6 Acjd type G. . 0 . 1 8 0.15 0 . 0 8 0.08 0 . 0 9 0 . 0 8 Acid type H 1 . 1 5 1 . 1 2 Acid type I. , . 0 . 9 5 0.91 0.07 0 . 0 7 0.01 0.01 Basic type J , .
.. c.. . . .. . .. .. . . . ......... . .. .. . . . . . .. .. .. .. . ..
DISCUSSION OF RESULTS
T h e results obtained b y t h e proposed method a r e higher t h a n those obtained b y t h e Io-day method except where t h e r e i s no soluble arsenic present or only a trace. I t is a d m i t t e d t h a t t h e boiling method is a severe test. All t h e samples listed i n T a b l e I, however, which d o n o t pass b y t h e boiling method do n o t pass b y t h e I o - d a y provisional method. Examination of some 30 samples, t h e products of different manufacturers, collected b y t h e inspectors of this l a b o r a t o r y revealed t h e f a c t t h a t only 3 samples would n o t pass inspection even b y t h e more severe t r e a t m e n t of t h e method under discussion. T h e manufacturers a r e able t o a n d d o produce lead arsenate which will s t a n d this t e s t ; if n o t , carelessness i n t h e manufacturing process is indicated. 1
LOC.cit.
1113
Some of t h e advantages claimed for t h e method are: ( I ) Rapidity and accuracy: This should appeal t o t h e f a c t o r y m a n as a quick means of laboratory control for his product. Assuming t h e proof t o be sufficient t h a t lead arsenate is n o t appreciably dissolved b y boiling water, t h e extraction of t h e soluble m a t t e r by t h e proposed method is logical a n d is t h e most rapid manner of complete extraction. ( 2 ) A clear filtrate is readily obtained from most samples. (3) T h e procedure for t h e removal of lead m a y usually be o m i t t e d , t h u s avoiding t h e possibility of volatilizing some of t h e arsenic a n d saving several hours time. SUNMARY
I t is highly desirable t h a t t h e methods in use b y t h e officials of control laboratories should be suitable for factory control. Accuracy is t h e prime essential i n t h e first case; b o t h accuracy a n d rapidity a r e desirable i n t h e latter. This desideratum is not fully realized in either of t h e t w o provisional methods of t h e Association of Official Agricultural Chemists, nor in other published methods for t h e determination of water-soluble arsenic in commercial lead arsenate. It is shown t h a t t h e solubility of lead arsenate itself is n o t a factor t o be t a k e n i n t o account i n t h e determination of water-soluble arsenic. T h e procedure for t h e removal of lead f r o m t h e water extract before t h e t i t r a t i o n of soluble arsenic may be usually omitted. T h e soluble m a t t e r s are completely extracted from commercial lead arsenates b y boiling in water 5 or l o min., while a complete extraction is almost impossible b y digesting in cold or tepid water. There appears t o be no appreciable solution or hydrolysis of lead arsenate even by prolonged boiling or b y digestion on t h e s t e a m b a t h for several hours. A method for t h e determination of water-soluble arsenic in commercial lead arsenates is s u b m i t t e d b y t h e writers for criticism. T h e method is t h e result of t h e conclusions which have been s t a t e d above. T h e t e s t is a d m i t t e d t o be a severe one, b u t t h e manufacturers a r e producing lead arsenates which a r e easily able t o s t a n d t h e test. T h e results obtained b y t h e boiling method are higher t h a n those obtained b y t h e Io-day provisional method of t h e A. 0. A. C., b u t are t h o u g h t t o be more nearly correct t h a n t h e latter. Samples which a r e shown by t h e boiling method t o contain t o o much water-soluble arsenic t o be legally salable will very seldom pass inspection b y either method. T h e procedure, as outlined, is shorter t h a n a n y heretofore published, consuming n o t more t h a n 3 t o 4 hours’ t i m e a f t e r t h e sample is sufficiently dried t o powder readily. I t yields accurate results a n d does n o t require t h e use of expensive a p p a r a t u s . I t should, therefore, be a d e q u a t e t o t h e needs of b o t h t h e official chemist a n d t o t h e factory chemist. INSECTICIDE AND FUNGICIDE LABORATORY AGRICULTURA~ EXPERIMENT STATION UNIVERSITY OF CALIFORNIA, BERKELEY
