Removal of Thiocyanate in the Detection of the Halides DAVID HART AND ROBERT MEYROWITZ Brooklyn College, Brooklyn, N. Y.
M
ANY methods have been proposed for the removal of
thiocyanate, which is the most serious interference in the detection of the halides. Sneed and Duschak (8) destroy the thiocyanate by igniting the silver halide precipitate and then fusing i t with a mixture of sodium carbonate and potassium carbonate. Curtman and Schneiderman (1) also destroy the thiocyanate by igniting the silver precipitate. Both Curtman and Wikoff ( 2 ) and Dobbins and Ljung (5) remove thiocyanate by precipitating it as the cuprous salt. Noyes ( 7 ) oxidizes it t i t h potassium permanganate, while MacAlpine and Soule (6) oxidize it by means of persulfate. Heisig and Heisig ( 5 ) present a modified procedure of Prescott and Johnson. Swift (9) oxidizes the thiocyanate by means of nitrate ion in a solution wherein the acidity is adjusted (using a sulfate-hydrosulfate buffer). I n a previous paper (4) the authors proposed the use of concentrated sulfuric acid to destroy the thiocyanate. Procedures for the detection of the halides involving the above methods for removing thiocyanates were found t o be very time-consuming. Therefore, the desire for a rapid method of detecting thiocyanate and the halides when as little as 1 per cent of the total concentration is present, stimulated the authors to develop a simple, rapid, and efficient procedure for the removal of thiocyanate. In this new procedure a large part of the thiocyanate is removed with lead nitrate and the remainder destroyed by boiling with sodium nitrite in dilute nitric acid solution. After the silver halide precipitate has been brought into solution by boiling with ammonia sulfide, the solution to be tested for halides is acidified and then rendered just alkaline. Lead nitrate is added to remove a large portion of the iodide and thiocyanate, since large amounts of iodide interfere with the test for thiocyanate (4). This provides a fixed concentration of anions in the form of a saturated solution of their lead salts except when the anions are present in small concentrations. The precipitate is removed by centrifuging and a portion of the centrifugate is taken for the test for thiocyanate and iodide. Ferric nitrate and carbon tetrachloride in a dilute nitric acid solution are used to detect these anions. The remainder of the centrifugate is acidified with nitric acid, sodium nitrite is added, and the solution is boiled. I n this manner iodide and thiocyanate are removed simultaneously. Potassium permanganate and carbon tetrachloride are then used to detect the bromide tn a portion of this solution.
Stir for 2 minutes, transfer to a large centrifuge tube, and centrifuge. A. DETECTIONOF THIOCYANATE. To one quarter of the centrifugate add 3 ml. of 3 M nitric acid and 5 ml. of 2 M ferric nitrate. A blood-red solution shows the presence of thiocyanate. B. DETECTION OF IODIDE. Transfer the solution of a 50-ml. glass-stoppered Erlenmeyer flask which contains 2 ml. of carbon tetrachloride. Let i t stand for 3 minutes, shaking a t 1-minute intervals. A violet-colored carbon tetrachloride layer shows the presence of iodide. C. DETECTIONOF BROMIDE.(1) Thiocyanate and Iodide Present. Under a hood transfer the remaining three quarters of the centrifugate to a beaker. Add 5 ml. of 3 M nitric acid and then 1.5 ml. of 6 M sodium nitrite dropwise and with constant stirring. Heat to boiling with constant stirring, boil for 2 minutes, cool, and flter off any precipitate. (2) Thiocyanate and Iodide Absent. Add 5 ml. of 3 M nitric acid to the remaining three quarters of the centrifugate. To two thirds of the solution resulting from either (1) or (2) add 1 ml. of 1-5 M nitric acid and then 0.02 M potassium permanganate in 1-ml. portions until the solution remains pink or until a maximum of 3 ml. of permanganate has been added. Transfer to a 50-ml. glass-stoppered Erlenmeyer flask containing 1 ml. of carbon tetrachloride and shake. A yellow or brown carbon tetrachloride layer shows the presence of bromide. D. DETECTION OF CHLORIDE. (1) Bromide Present. Dilute the remaining one third solution from C to 20 ml. and add with stirring 10 ml. of 15 M nitric acid. Heat to boiling with stirring and boil gently for a t least 4 minutes and until all the bromine has been given off. Filter off any precipitate. (2) Bromide Absent. To the remaining one-third solution from C add one tenth of its volume of 15 M nitric acid. To the solution from (1) or (2) add 3 ml. of 0.5 M silver nitrate. Let stand for a t least 5 minutes. A white precipitate shows the presence of chloride. Compare with a 1-mg. chloride control which has undergone the same procedure.
TABLE I. TESTANALYSES 7 -
CNS-
I-
Br-
C1-
CNS-
1 1 1 100
1 1 1 1 100
0 100
0 0 0
++ ++ +
M p . per 9 ml.
1 100 1 1
1
0 0 0
0 100 0 0
1 100
Using the procedure described below, only 1 ml. of 0.02 M permanganate was required to detect the bromide in all cases except where a large amount of bromide was present. In this instance the addition of potassium permanganate was discontinued after 3 ml. had been added, since sufficient bromide had already been oxidized. The remainder of the solution (after boiling with nitrite) is used for detecting the chloride with silver nitrate. If bromide is present i t is removed by boiling with concentrated nitric acid.
1
1 100
1
1 0
0
100
+ -
-
Results-
I-
Br-
C1-
+++ +++ +++ ++ ++ ++ - - + -- +- -+
Test Analyses The test analyses of Table I were carried out on 12 ml. of solution, using the above procedure.
Literature Cited Curtmaii and Schneiderman, Rec. trav. chim., 54, 158 (1935). Curtman and Wikoff, J. A n . Chem. SOC.,37, 298 (1915). Dobbins and Ljung, J. Chem. Educution, 12, 586 (1935). Hart and Meyrowitz, IND. ENG. CHEM.,Anal. Ed., 12, 318 (1940).
Heisig and Heisig, Ibid., 7, 249 (1935). MacAlpine and Soule (Prescott and Johnson), “Fundamentals of Qualitative Chemical Analysis”, pp. 466, 572, New York, D. Van Nostrand Co., 1933. Noyes, A. A., “Qualitative Chemical Analysis”, p. 151, New York, Macmillan Co., 1920. Sneed and Duschak, J. Chem. Education, 8, 1388 (1931). Swift, E. H., “System of Chemical Analysis”, pp. 453-4, New York, Prentice Hall, 1939.
Procedure Dilute the solution t o be tested (the silver sulfide has been filtered off and the sulfide and ammonia removed by boiling) to 22 ml. in a 100-ml. beaker and add 3 M nitric acid dropwise and with constant stirring until the solution is lust acid. Add 1.5 M sodium carbonate dropwise and with constant stirring until the solution is distinctly blue to nitrazine paper. Then add 12 ml. of M lead nitrate dropwise and with constant stirring.
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