Coates Chemical Laboratory, Louisiana State University, Baton Rouge, Louisiana (USA).

A New Spot Test for Silver. By

Buddhadev Sen ~ and Philip W. West. (Received June 10, 1955.) The detection of traces of noble metals has always been considered an important problem in analytical chemistry. The methods employed include spot tests, formation of colored precipitates or solutions, microscopic examination of distinctive crystalline compounds, and the catalytic action of silver ion on certain reactions. Among the methods of trace analysis, the spot test technique excels many others in its simplicity and rapidity. A large number of procedures for the detection of silver, either by spot tests or by precipitation in solution, have been developed depending upon the reduction of silver ion to grey or black metallic silver by such compounds as stannous chloride 1, chloral 2, gallic acid 2, hydroquinone 2 and alkaline manganous solution s. A very sensitive reaction depending upon the reduction of silver bromide by a developer solution has been described 4. Potassium chromate in acetic acid ~, p-dimethylaminobenzylldenerhodanine6 and p-aminophenol 7 have been described as spot test reagents for silver. Feigl s used the catalytic effect of the silver ion on the reduction of mercurous chloride by phenylhydrazine as the basis for the detection of silver ion. A microscopic method of detecting silver is based on the examination of silver anthranilate precipitas 9. The present method, although a spot test, is radically different in principle from all the methods referred to above, and is essentially a demasking reaction. The reagent solution from which the test papers are prepared is an equilibrium mixture of potassium tetracyanonickelate and dimethylglyoxime, prepared by suspending excess nickeldimethyl* On leave from "The College of Engineering and Technology, Bengal" Jadavpur, Calcutta-32, India.

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Buddhadev Sen and Ph. W. West:

[Mikrochim. Acta

glyoximate in a solution of potassium cyanide, and then filtering the solution after equilibrium has been established. The following equations summarize the chemistry of the test: Ni(HD)2 ~- 4CN- ~- Ni(CN)4 z- ~- 2 H D - xg+_~ [Ag(CN)2]- ~- Ni(HD)2 (red)

where Ni(HD)2 and H D - represent nickel dimethylglyoximate and the dimethylglyoximate ion, respectively. A small amount of silver ion removes an appreciable amount of cyanide and thus shifts the above equilibrium, enabling the nickel ions to react with dimethylglyoxime and develop the characteristic red fleck. Feigll~ has described a test for the detection of silver halides in sohd residt~es which is essentially the same in principle.

Experimental. Reagents. All reagents used in the study were of A. 1~. grade. W h a t m a n No. 1 filter papers were used for preparing the test papers. Nickeldimethylglyoximate was prepared according to standard quantitative procedure.

Preparation o[ the Reagent Solution. The test papers prepared from the following solution were found to be most satisfactory in regard to sensitivity and keeping properties. Nickeldimethylglyoximate (0.5 g) was suspended in 100 ml of 0.4 per cent potassium cyanide solution. The suspension was well shaken and allowed to stand for 24 hours, after which it was filtered and the filtrate was stored in a pyrex bottle and kept away from light.

Preparation o/ the Test Paper. W h a t m a n No. 1 filter paper circles (11 cm) were dipped in the reagent solution and quickly dried under an infra red lamp and stored in stoppered bottles. The reagent papers prepared from the above reagent solution were almost white and were stable for three to four weeks.

Method o/ Detecti~y] Silver. The reagent paper was treated with a drop of the test solution which was previously adjusted to neutrality. I n the presence of silver, a distinctive red fleck was formed at the center of the spot. When the reagent paper was treated with an acidic test solution, the test sohition

1955/5--6]

/k New Spot Test for Silver.

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was immediately followed by a drop of a saturated solution of ammonium acetate, and the characteristic red color of nickel dimethylglyoximate was developed.

Inter/ering ions. Gold(III) and nickel(II) gave positive tests. With gold(HI) a reddish brown ring was ~ormed around the circumference of the spot. The reason for interference b y nickel is quite obvious, since the test itself is baseA on increasing the nickel concentration of an equilibrium system in which nickel ion is involved. The following ions gave no positive test. Lithium, sodium, potassium, thallium(I), beryllium, magnesium, calcium, strontium, barium, copper(II), cadmium, zinc, mercury(II), eobalt(II), bismuth(III), antimony(III), cerinm(III) and (IV), tin(II) and (IV), thorium, ruthenium(III), iridium(IV), platinum(IV), palladium(II), uranyl, lead(II), iron(II) and (III).

Sensitivity. Limit of identification = 0.5#g of silver. Concentration limit ~- 1 : 100000. The filtrate from silver chloride precipitate which gave no positive test of silver with chloride, gave a distinct test for silver, when a drop of the filtrate was placed upon the test paper.

Discussion. From the experimental results, it appears that the present test is one of the best methods for the detection of silver ion. The test suffers from a relatively small number of interfering ions and is highly sensitive. The procedure is the ultimate in simplicity, and the reagent papers are fairly stable when properly kept.

Acknowledgment. The financial assistance of the Mine Safety Appliances Company is gratefully acknowledged.

Summary. A new spot test for silver has been developed. Test papers are prepared by impregnating filter paper with an equilibrium mixture of potassium tetracyanoniekelate and dimethylglyoxime. As little as 0.5#g of silver can be detected. Zusammenfassung. Ein neuer Tiipfelnaehweis ftir Silber wttrde ausgearbeiteb. Man impr~gnie1~

Filtrierpapier mit einer GleiehgewiehtslSstmg yon Kaliurtmiekoltetraeyanid und Dimethylglyoxim. 0,5 #g Silber kfrmen naehgewiesen werden. Mikrochim.Acta 1955/5--6.

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Buddhadev Sen and Ph. W. West: A New Spov Tess for Silver.

R~sum@. On a mis au point tim nouvel essai ~ la touche de l'argent. Les papiers servaat aux essais sont pr@par4s par impr@gnation de papier filtre par le m@lange de t6tracyanonickelate de potassimn et de dim6thylglyoxime a y a n t la composition d'@quilibre. On peut ainsi ddtecter des traces d'argent de 0,5 #g.

References, 1 N. A. N . D. a N. A. 4 p.E.

Tananaef], Z. anorg. Chem. 140, 320 (1924). Costeanu, Mikrochem. 26, 170 (1939). Tananae]], Z. anorg. Chem. 170, 120 (1928). Wenger and R . Duckert, Reagents for Quahtative

Inorganic Analysis. New York: Elsevier. 1948. p. 3. 5 F._Feigl, Spot Tests. ~qew York: Nordman. 1937. p. 13. 6 F . Feig[, Spot Tests, 4th Ed., Vol. I. New York: Elsevier. 1954. p. 56. 7 E . A . Kocsis, Mikrochem. 29, 166 (1941). s F . Feigl, lY[ikrochem. 10, 305 (1931). 9 0 . G. Sheintsis, J. Gem Chem. (U. S. S. R.) 8, 596 (1938). lo F.t~eigl, l~/Iikrochem. 20, 198 (1936).