Determination of Arsenic in Organic Compounds An Iodometric Semimicroprocedure HENRY A . SLOVITER, WALLACE M. MCNABB, AND E. C. WAGNER Department of Chemistry a n d Chemical Engineering, University of Pennsylvania, Philadelphia, Penna
IN
THE method describedythe substance is decomposed by action of sulfuric and nitric acids, and the arsenic is precipitated by hypophosphorous acid. The arsenic is collected on a filter and is then dissolved by action of excess bromine (from Koppeschaar’s bromate-bromide solution) in measured amount. The excess bromine is determined iodometrically. This procedure is well suited to the analysis of relatively small (semimicro) samples, for the reason that in the oxidation involved (Aso+ As6) the equivalent weight of arsenic is one iifth the atomic weight. I n familiar methods involving trivalent and pentavalent arsenic the equivalent weight is one half the atomic weight.
ride. Hypophosphorous acid is preferable also because it does not introduce the necessity for use of an acid wash liquid, as when stannous chloride is used. For these reasons reduction by hypophosphorous acid was selected for incorporation into the procedure under development. Thiele (3.5) reported the sensitivity of the hypophosphorous acid test for arsenic to be increased by presence of iodide. In trials to test this claim, solutions containing arsenious acid, sulfuric acid, and potassium iodide were treated with sodium hypophosphite and hydrochloric acid. On heating the liquid a yellowish solid volatilized from the flask, and an odor like that of hydrogen sulfide was noticed. Following this abnormal behavior the results obtained for arsenic were too high. These phenomena were observed only when reduction was attempted in the presence of iodide. An explanation has not been sought, for the reason that no interference from this cause occurs in the method to be described, the initial decomposition serving to free the mixture of any iodine present in the substance analyzed. It is suggested that the iodide (as hydriodic acid) may reduce some sulfuric acid to sulfurous acid, and that the latter may be reduced to sulfur by hypophosphorous acid, as shown by Maquenne ($8). The behavior described recalls the comment made by Andrews ( 2 ) : “At times, under conditions which were not more closely investigated, arsenic precipitated by hypophosphorous acid contains small amounts of phosphorus, whether in the free state or in combinaation with hydrogen or with arsenic it is difficult to say.” Thorough washing of the precipitated arsenic is essential. Preliminary experiments yielded high results, eventually traced to incomplete removal of hypophosphorous acid from the arsenic on the filter. It was found that if the precipitate was washed with hot recently boiled water until the washings were free of chloride, the arsenic retained no interfering substance. The use of air-free water for washing is a precaution against possible air oxidation of arsenic, a matter which has been the subject of disagreement. Thiele (55) reported air oxidation of freshly precipitated arsenic to be too rapid to permit the isolation of the element in a quantitative method. Evans (15) recommended a solution of ammonium chloride as a wash liquid which, for reasons not stated, would protect the arsenic from oxidation by air. Cooke (10) reported that oxidation of precipitated arsenic during washing with freshly boiled hot water is negligible. In the present study qualitative tests of both filtrates and washings showed presence of no appreciable amounts of arsenic which had escaped reduction by hypophosphorous acid or which had dissolved in the wash liquid as a result of oxidation on the filter.
AnalyticaI Reduction of Arsenic to the Elementary State Available methods include reduction by stannous chloride and concentrated hydrochloric acid (Bettendorff reaction, 7 ) , by hypo hosphorous acid (55),by chromous sulfate (33),or by titanous chgride in the presence of Rochelle salt (22, 32). The last two were not submitted to trial, as they require inconvenient precautions and are relatively expensive. The Bettendorff reaction, familiar as a qualitative test (7, 11), was applied quantitatively by Andrews and Farr (5),who introduced tartaric acid into the reagent t o prevent contamination of the precipitated arsenic by tin. The separated arbenic is determined by dissolving in a meas red volume of standard iodine excess of which is finally titrated. This method has been used for the determination of arsenic in nonferrous alloys (do), in metallurgical agglomerates (Sf),and in steels (27,30). Reduction by hypophosphorous acid (12) was recommended by Thiele (35) for detection of arsenic, and was applied quantitatively by bngel and Bernard (13). The solution of sodium hypophosphite and hydrochloric acid used for the reduction has been called Thiele’s reagent (5, 29. 35) and also Bougault’s reagent (8). Reduction by hypophosphorous acid has been applied to the determination of arsenic in metals and ores (9, 14-17, 37). In these methods, as in those involving the Bettendorff reaction, the filtered and washed element is determined volumetrically by means of excess standard iodine solution. Excess iodine is best titrated R-ith standard arsenite solution (14) in buffered alkaline solution. Titration with thiosulfate is inadmissible whether conducted in alkaline buffered solution ( 1 , 6, 36) or in acid solution (25). In the microprocedure of Kolthoff and Amdur (23) the elementary arsenic is dissolved in standard ceric sulfate solutions, excess of which is titrated with standard arsenious acid solution. The hypophosphorous acid reagent was found by Grippa (19) t o be more sensitive than the Bettendofl-AndrewwFarr reagent. This was confirmed in the present study; comparative trials showed that hypophosphorous acid caused more complete and much more rapid separation of arsenic than did stannous chlo-
Iodometric Determination of Arsenic after Removal as the Element Unsatisfactory features of methods previously described are the relatively slow rate of solution of arsenic (especially if densely granulated) in dilute iodine solution, and the difficulty in detecting presence of undissolved particles of arsenic
516
Tune 15, 1942
ANALYTICAL EDITION
in the liquid deeply colored by iodine and containing flocculent masses of filter paper pulp. Both difficulties are excluded b y use of bromine (from Koppeschaar’s bromatebromide solution, 26) instead of iodine for bringing the arsenic into solution, an innovation b y which the procedure is much improved at this point. T o complete the analysis potassium iodide is added, the solution is buffered, and the iodine (representing excess bromine) is titrated with standard sodium arsenite solution. For the necessary buffering of the acid solution sodium carbonate was judged not desirable because of possible loss of iodine with escaping carbon dioxide. T h e use of borax led t o undesirable increase in the volume of the liquid, owing t o the relatively low solubility of this salt in cold water. Disodium phosphate, recommended b y Washburn (38)’ was found to be satisfactory in every respect. Decomposition of Organic Compounds for Analysis The use of sulfuric acid and potassium persulfate, previously found effective in the analysis of organic mercury compounds (18, $4,caused insufficiently rapid decompositions of several arsenic compounds, even in the presence of copper added as a catalyst. Comparative trials of decomposition b y concentrated sulfuric acid in conjunction with powdered potassium permanganate, potassium nitrate, or nitric acid (Neumann’s method), showed the last mentioned t o be the most rapid and satisfactory. Undecomposed nitric acid is removed by evaporating the liquid to fumes of sulfuric acid, and traces of nitrogen oxides which may persist (and which might interfere later) are eliminated b y addition of ammonium sulfate to the hot digestion mixture. During the decomposition of arsenic compounds in the presence of halogen (chlorine, bromine) no loss of arsenic occurred by escape of volatile halide.
Analytical Procedure APPARATUS. The all-glass apparatus recommended (34) for use in the determination of mercury in organic compounds is suitable also for the analysis of organic arsenic compounds as described below. DECISORMAL SODIUM ARSESITE~ O L U T I O S . Dissolve rapidly about 5 grams of pure arsenic trioxide (accurately weighed) in water containing 10 grams of sodium hydroxide, and at once add dilute sulfuric acid until the solution is just acid to litmus ( 2 4 ) . If the solution is diluted to a liter in a calibrated flask the normality may be calculated. An approximately decinormal solution may be standardized as follows, using potassium iodate as a primary standard: Transfer to a 250-ml. glass-stoppered Erlenmeyer flask 25 ml. of 0.1 A potassium iodate solution, or an accurately weighed portion (about 0.1 gram) of the pure salt, and add water to 50 ml. Add 5 ml. of 20 per cent potassium iodide solution and 2 ml. of 6 IT sulfuric or hydrochloric acid, stopper the flask, mix the contents, and allow to stand one minute. Remove the stopper, rinse it with water, add 30 ml. of phosphate solution (350 grams of Sa2HP04.12Hz0 per liter of water), and titrate the liberated iodine with the sodium arsenite solution, adding 3 ml. of 0.5 per cent soluble starch solution just before reaching the end point. PROCEDURE. Veigh the sample (of size to yield approximately 15 mg. of arsenic) on a piece of cigaret paper or in other convenient manner and transfer to the flask. Introduce 2 ml. of concentrated sulfuric acid and warm the mixture gently, while agitating the flask, until the cigaret paper is consumed and the sample is completely dispersed in the acid. Support the flask in an inclined position in the hood and from a pipet introduce in single drops 1 ml. of concentrated nitric acid. M‘hen the initial vigorous reaction has subsided, apply to the lower tip of the flask the small flame of a “micro” burner. Regulate the heating so that the nitric acid slowly evaporates but without active ebullition of the liquid. When the removal of nitric acid is complete and fumes of sulfuric acid appear, the digestion mixture is usually not
517
TABLE I. DETERMINATION OF ARSENICIN PUREARSENIC TRIOXIDE (By method described with omission of decomposition procedure) Arsenic Taken Arsenic Found Error Error IIg. Ng. Ng. % 25.52 25.28 -0.26 -0.9 25 52 25.25 -0.27 -1.0 25,52 25.84 +0.32 +1.2 10.23 10.31 +0.08 +0.8 10.23 10.27 +0.04 CO.4 10.23 10.22 -0.01 -0.1
10.23
10.23
*o.oo
*o.o
darker than a light straw color. If the liquid is dark in color a t this point, allow the flask and contents to cool somewhat, and repeat the treatment with 1 ml. of concentrated nitric acid. Remove excess nitric acid as before and when light fumes of sulfuric acid appear allow the mixture to cool slightly and add cautiously 1 gram of powdered ammonium sulfate. When gas evolution ceases, rotate the flask and heat gently. The liquid should be colorless while still hot; a light yellow color indicates presence of nitrogen oxides. The decomposition ordinarily requires 15 to 30 minutes. Allow the flask to cool, wash the mouth and walls with water, and mix the contents (volume about 15 ml.), Add 3 grams of sodium hypophosphite (KaH2POz.H10) and at once agitate the flask so as to dissolve the salt rapidly. If the sodium hypophosphite is allowed to stand in the liquid it quickly forms a hard cake and then dissolves slowly. Dilute the mixture to about 30 ml., add 35 ml. of concentrated hydrochloric acid, and mix. Ignore any solid which separates at this point, as it redissolves during the subsequent heating. Attach the condenser and heat the flask gently with a small flame. Precipitation of elementary arsenic begins in about 2 minutes and is apparently complete in about 5 minutes, as is indicated by the fact that no further darkening of the contents of the flask occurs. Regulate the heating so that the liquid just begins to boil after about 10 minutes, and then continue gentle boiling for 2 minutes. Wash the inner wall of the condenser by water introduced at the top, and again boil the liquid gently for one minute. Prolonged boiling is to be avoided, as it was found to yield an arsenic precipitate of such character that effective washing was difficult. Allow the flask and its contents to cool somewhat, rinse the condenser, and t,hen withdraw it and set it aside. Decant t,he solution through a retentive 11-em. filter paper (Whatman KO.44 was used) retaining in the flask as much of the precipitate as possible. Wash the precipitate in the flask three or four times by decantation with freshly boiled hot water. Finally, wash the filter with hot water until the washings show no more than a trace of chloride when tested with silver nitrate solution. Transfer the filter paper and contained precipitate to the flask after using the folded filter to pick up any of the arsenic which may have crept onto the funnel. Wipe the flange and lip of the flask with a fragment of moistened filter paper and add this t o the contents of the flask. Introduce about 25 ml. of water, and then add from a pipet 15 ml. of Koppeschaar’s solution (2.8 grams of potassium bromate and 50 grams of potassium bromide per liter). While holding the flask quiet add rapidly 2 ml. of 6 A‘ hydrochloric acid and at once insert the ground-glass stopper. Shake the flask vigorously until the filter paper is completely disintegrated and no dark particles of arsenic are visible; this should require less than one minute. Immerse the flask in ice water to produce subnormal pressure within, and after a minute or two pour 5 ml. of 20 per cent potassium iodide into the gutter of the flask, and loosen the stopper slightly to allow the potassium iodide to pass into the flask. Shake the tightly stoppered flask to absorb in the liquid, the vapors of bromine, then remove the stopper and rinse it and the mouth of the flask with water. Add to the flask 40 ml. of phosphate buffer solution (350 grams of crystallized disodium hydrogen phosphate, ?riazHPOi.l2H10, per liter of water) and mix the contents well. Titrate the iodine with the 0.1 N sodium arsenite solution. When the iodine color is faint, stopper the flask and shake well to dislodge traces of iodine adherent to the paper. Then add 3 ml. of 0.5 per cent solution of soluble starch and complete the titration. Determine the effective strength of the bromate-bromide solution in a blank titration conducted exactly as described above, introducing a filter paper to compensate any effect due to the filter paper present in the analysis. Calculate the arsenic present from tne dilierence between the two titrations (1 nd. of 0.1000 .v arsenic represents 1.1982 mg. of arsenic).
518
INDUSTRIAL AND ENGINEERING CHEMISTRY
Experimental Results TABLE
The accuracy of the procedure described, exclusive of the decomposition of organic material, was tested b y determinations of arsenic in aliquot portions of a solution prepared from Bureau of Standards arsenic trioxide in the same manner as was the standard arsenite solution. The results, shown in Table I, indicate that accuracy is higher when relatively small quantities of arsenic are present. I n this case also the small size of the precipitate of arsenic shortens the operation of washing. I n order to determine whether or not arsenic is lost during decomposition of organic matter, samples of Bureau of Standards arsenic trioxide mere weighed on cigaret paper and carried through all the steps of the procedure described above. The results shown in Table I1 indicate loss of arsenic during the procedure to be relatively small.
TABLE 11. DETERMIXATIOX O F ARSENCI N ARSESICTRIOXIDE BY
COMPLETE
PROCEDURE
Arsenic Taken
Arsenic Found
Ng.
Mg.
Mg.
13.92 13.70
13.82 13.66
-0.10 -0.04
Vol. 14, No. 6
111. DETERlfISATIOX
Compound
O F ARSESICI N POUNDS
Seutralization Equivalent CalcuFound lated Sample Mg.
Diphenylarsinic acid, 261 . O Eastman o N o . 2208, 262.5 m. p . 174 AV. 261.8
34.27 31.74 261.0 33.43 45.72
% -0.7 -0.3
Analytical Results for Organic Compounds of Arsenic Results obtained in the analyses of organic arsenic compounds are presented in Table 111. The compounds (except neoarsphenamine) were obtained from the Eastman Kodak Company, and were purified by crystallization prior t o use. Neoarsphenamine was obtained from the Mallinckrodt Chemical Works; samples were withdrawn from freshly opened ampoules and weighed rapidly. Most of theJe compounds are arsonic or arsinic acids, some of which do not have well-defined melting points. In such cases the purity of each compound was judged from the determined neutralization equivalent. The monobasic disubstituted arsinic acids can readily be titrated with standard alkali, using phenolphthalein as indicator. The dibasic primary arsonic acids cannot be titrated to a sharp end point corresponding to either one or both acid hydrogens by the usual procedures. King and Rutterford (21) found that the neutralization equivalents of primary arsonic acids can be determined by titrating them with standard alkali, using thymolphthalein indicator in a solution which is half-saturated with sodium chloride. This procedure was used for determining the neutralization equivalents of the primary arsonic acids analyzed. The sample (about 0.2 gram) is transferred to a 50-ml. flask, two drops of 0.1 per cent thymolphthalein in alcohol are added, and then 0.1 iV sodium hydroxide (standardized against Bureau of Standards acid potassium phthalate) is run in until all the sample is dissolved and the solution assumes a definite blue color. The volume of the contents of the flask is approximately doubled by addition of saturated sodium chloride, neutral to thymolphthalein, and the bleached solution is titrated with the alkali to a definite blue end point. The neutralization equivalents determined in this way are shoJ+n in Table 111. T h a t the method for arsenic described is applicable to halogen-containing compounds was shown by analysis of organic arsenic compounds in the presence of added organic compounds containing chlorine or iodine (Table 111). The results show that the presence of halogens causes no loss of arsenic during decomposition and does not interfere with the subsequent procedure. The results in Table I11 show the method to be satisfactory with respect to accuracy and precision. The time required for a single analysis is about 2 hours. If several flasks are available, time can be saved by running analyses on a n overlapping schedule.
Arsenic Found
Arsenic Calculated
70
%
28.46 28.56 28.85 28.48 Av. 28.59 37.22 37.13 4 v . 37.18 30.65 30.46 Av. 30.56 34.59 34.58 Av. 34.58 28.70 28.61 Av. 28.66 28.54 28.50 Av. 28.52 34.24 34.30 Av. 34.27
28.60 41.94 100.5 102.1 43.13 Av. 101.3 101.0 37.10 rn - Xitrophenylarsonic 123.1 41.34 acid, Eastman KO. 122.9 47.48 Av. 123.0 123.4 30.31 4469 Arsanilic acid, East107.0 47.56 man No. 1369 107.3 47.30 Av. 107.2 108.4 34.53 Diphenylarsinic acid (261.8) 261.9 52.71 with 50 mg. of i3do50.64 benzoic acid 28.60 Diphenylarsinic acid (261.8) 261.9 50.80 with 50 mg. of chloro51.14 benzoic acid 28.60 p Hydroxyphenyl42.50 arsonic acida, East42.77 ma; No. 4836, m. p. 34.37 171 n-Butylarsonic acid, 93.3 40.43 40.65 Eastman No. 1892, 92.9 41.22 40.81 m. p. 160' Av. 93,lb 91.0 Av. 40.73 41.18b p-Benzarsonic acid, 82,6 53.62 29.90 Eastman X o . 2082 82.6 55.25 30.00 Av. 82.6C 82.0 Av. 29.95 30.46c Seoarsphenamine, U. 41.21 19.62 19.37d S. P. XI, liallinc46.20 19.78 19.53d krudt No. 3676 A v . 19.7OAv.19.45d 0 Neutralization equivalent could not be determined, probably because dissociation constant for phenolic hydrogen is close t o t h a t of indicator ( 2 1 ) . b Neutralization equivalent found indicates a purity of 97.77' if impurity is a n inert substance arsenic content should be 40.3%. I q j m p u r i t y is an acid or a base, whether arsenical or not, it will have a different effect upon neutralization equivalent t h a n upon arsenic content. c Neutralization equivalent found indicates a purity of 99.370; if impurity is a n inert substance arsenic content should be 30.2%; see b . d Values represent analyses made by a n umpire method, involving distillation of arsenic trichloride and titration with potassium bromate standardized against arsenic trioxide. Procedure is essentially t h a t of Association of Official Agricultural Chemists for determination of arsenic in iron-arsenic tablets (4). Phenylarsonic acid, Eastman No.2020
-
-Error-
ORGAXIC CoM-
Summary
A semimicromethod is described for the determination of arsenic in organic compounds. It involves decomposition of the sample by action of hot concentrated sulfuric and nitric acids, precipitation of elementary arsenic by hypophosphorous acid, solution of the filtered and washed arsenic in excess bromine (acidified Koppeschaar solution) , and iodometric determination of excess halogen by titration with standard sodium arsenite in a solution buffered with disodium phosphate. Results indicate the accuracy and range of applicability of the method. The procedure is convenient, moderately rapid, and applicable in presence of hahgens. Acknowledgment Specimens of neoarsphenamine were provided through t h e kindness of V. H. Wallingford, Mallinckrodt Chemical Rorks, St. Louis, Mo.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9)
Abel, Z . anorg. Chem., 74, 395 (1912). Andrews, Chem.-Ztg., 38,295 (1914). Andrews and Farr, Z . anorg. Chem., 62, 123 (1909). Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., p. 615 (1940). Babich, Khim. Farm. Prom., 5, 27 (1934). Batey, Analyst, 36, 132 (1911). Bettendorff, 2.Chem., 5,492 (1870). Bougault, J . pharm. chirn., 1909, 13. Brandt, Chem.-Ztg.. 37, 1445, 1471, 1496 (1913); 38,461 (1914).
ANALYTICAL EDITION
June 15, 1942
Cooke, Proc. Chem. SOC.,19, 243 (1903). Durrant, J . Chem. Soc., 115, 134 (1919). Engel, Compt. rend., 96, 497 (1883). Engel and Bernard, Ibid., 122, 390 (1896). Evans, Analust, 52, 565 (1927). Ibid., 54, 523 (1929). Ibid., 57, 492 (1932). Fainberg and Gintrburg, Zavodskaya Lab., 7, 23 (1938). Fenimore and Wagner, J . Am. Chem. SOC.,53, 2472 (1931). Grippa, Ann. chim. applicata, 20, 249 (1930). Ibbotson and Aitchison, “.4nalysis of Non-Ferrous Alloys”, p. 124, London, Longmans, Green and Co., 1915. King and Rutterford, J . Chem. SOC.,1930, 2138. Knecht and Hibbert, “New Reduction Methods in Volurnetric Analysis”, p. 6 , 2nd ed., London, Longmans, Green and Co., 1925. Kolthoff and Amdur, IND. ENG.CHEM.,-4s.~~. ED., 12, 177 (1940). Kolthoff and Furman, “Volumetric iinalysis”, Vol. I, p. 235, New York, John Wiley & Sons, 1929. Ibid.,Vol. 11,pp. 356, 408.
(26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38)
519
Koppeschaar, 2. anal. Chem., 15, 233 (1876). Lombardo, Met. ital., 29, 1 (1937). Maquenne, Bull. soc. chim., (3) 3, 401 (1890). Matthes, Pharm. Ztg., 71, 1508 (1926). Masretti and Agostini, Gam. chim. ital., 53, 257 (1923). Miloslavski, Lyubimova, and Belogorskaya, Zavodskaya Lab., 6, 1184 (1937). Oliverio, Ann. chim. applicata, 21, 211 (1931). Shat’ko, Zavodskaya Lab., 7 , 4 1 2 (1935). Sloviter, McNabb, and Wagner, ISD. ENG.CHEN.,ANAL.ED., 13, 890 (1941). Thiele, A n n . , 265, 55 (1891). Topf, 2. anal. Chem., 26, 184 (1887). Tsyvina and Dobkina, Zavodskaya Lab., 7 , 1116 (1938). Washburn, J . Am. Chem. SOC.,3 0 , 4 4 (1903).
THISpaper represents the second portion of the oompleted dissertation of Henry A. Sloviter in partial satisfaction of the requirements for the Ph.D. degree a t t h e University of Pennsylvania, 1942. The first portion was published in an earlier issue [IND. ENQ.CHEM, ANAL.ED.,13, 890 (1941)].
A New Procedure for Detecting Acidity’ FRITZ FEIGL AND PAUL0 E. BARBOSA Laboratorio Central da ProduGZo Mineral, Ministerio da Agricultura, Rio de Janeiro, Brazil
A
R E C E N T paper ( I ) from this laboratory described in detail a sensitive procedure for detecting “basicit.y” in both soluble and slightly soluble materials, such as hydroxides, oxides, phosphates, fluorides, organic bases, and the salts of weak organic acids. The reagents for this delicate test are prepared by treating neutral solutions of copper or nickel sulfat.e, chloride, or nitrate with a deficit of dimethylglyoxime, thionalide (P-aminonaphthalide), or cupron (benzoinosime) and then filtering. A typical instance, nickel solution treated with dimethylglyoxime (= DH,), car1 be represented :
Ni++
+ 2DHz = Si(DH)a + 2H’
The filtrates from these incomplete precipitations are slightly acid equilibrium solutions; each has a characteristic pH value. If a solution of this type is brought into contact with a material that decreases the effective hydrogen-ion concentration of the reagent, a highly visible precipitate of the inner-complex salt is produced a t once: nickel dirnethylglyoxime (red), nickel thionalide (black-brown), or copper benzoinoxime (green). This procedure has proved useful in a variety of practical cases, and has also found microchemical application in connection with spot tests. The successful analytical use of a procedure based on disturbing an established equilibrium in solutions of complex salts naturally led to attempts to extend this principle to other systems of this nature. Such solutions always present an equilibrium between the complex compound (or its ions) and the materials that have united t,o form the complex. The stability of the complex (or its ions) determines the position of the equilibrium. Equilibrium solutions of silver-ammine chromate were studied with a view to applying them in this new type of test,. I t was found that they can be used to detect materials that consume acid, ammonia, or ethylenediamine. Silver chromate dissolves readily in excess ammonia water. The yelloa solution (Cr04-- ion) contains the complex salt [-lg(SH8)2]2Cr04. Therefore, if silver chromate is digested with a quantity of ammonia w t e r insufficient to bring about
’ Translated b y
Ralph E. Oesper, University of Cincinnati.
complete solution, there will be obtained, on filtration, a yellow solution that smells of ammonia. I n this there have been established the equilibria Ag,Cr04
+ 4SH3@ [Ag(NH3)2]2Cr04Ft 2Ag(NH&+ + Cr04--
and any removal of ammonia will lead to the precipitation of the highly colored silver chromate. Consequently, this type of ammoniacal solution of silver chromate can serve to reveal the presence of materials that consume ammonia by uniting with it to form an ammonium salt, react with it to form an ammoniate, or consume ammonia in the precipitation of a hydroxide. The ammoniacal solution of silver chromate has the practical defect that it loses ammonia unless the reagent is stored in a tightly stoppered vessel. This loss results in the formation of a precipitate, even in the absence of nmmonia-consuming materials. If a drop of the solution is placed on filter paper a brown spot of silver chromate appears in a few minutes. On a spot plate the solution decomposes more slowly, and tiny crystals of black-brown silver chromate separate on the surface of the drop. Sonetheless, the ammoniacal solution can be used for the present purpose because the silver chromate produced by the slow breakdown of the solution differs in form and color from that arising immediately as the result of chemical action. A stable silver chromate equilibrium solution can be prepared by dissolving the salt in aqueous ethylenediamine ( = e n ) . Since this base boils a t 118”C., its vapor pressure in a silver-en chromate solution is so slight that the reagent can even be kept a t mater-bath temperature for as long as 5 minutes without danger of decomposition. The silver-en chromate is far more stable than the analogous silver ammonia chromate because coordination of the en-molecule on the silver ion produces a five-membered ring
