ANALYTICAL CHEMISTRY
1076 M a d , and M A r i , as well as a new “characterietic factor” defined as the density intercept MAdi. LITERATURE CITED
(1) Anderson, D. G . , and Smith, J. C., J . Inst. Petroleum, 38, 415 (1952). (2) Haaelwood, R. E.,Oura, K., and Frey, R. bI., J . Electrochem. SOC.,101, 185 (1954). (3) Hersh, R. E., Fenske, M. R., Booser, E. R., and Koch, E. F., J . Inst. Petroleum, 36, 624 (1950). (4) Karo, W., McLaughlin, R. L., and Hipsher, H. F., J . Am. Chem. Soc., 75, 3233 (1953)
(5) Lipkin, AI. R., and Martin, C. C., ANAL.CHEM.,19, 183 (1947). (6) Mair, B. J., Willingham, C. B., and Streiff, A. J., J . Research Natl. Bur. Standards. 21. 535. 565. 581 (1938). (7) Martin, C. C., and Sankin,’A.,ANAL.CHI%;.,25, 206 (1953). (8) Oura, K., Hazelwood, R. N., and Frey, R. AI., Trans. Am. Inst. E’lec. Engrs., Part 111, 72, 297 (1953). (9) Smittenberg, J., and Mulder, D., Rec. traz.. chim., 67 813 (1948) (10) Van Nes, K., and Van Westen, H. A , , “Aspects of the Constitution of Mineral Oils,” New York, Elsevier Publishing Co. Inc., 1951. RECEIVED for review June 24, 1953. Accepted January 30, 1954
Argentometric Determination of Halides Using the Dead-Stop End Point M. LOU MASTEN’ and K. G. STONE Kedzie Chemical Laboratory, Michigan State College, Fast Lansing, Mich.
T
HE dead-stop end point has been repeatedly used where a
reversible couple appears or disappears a t the equivalence point ($, dl A second type is to be found where the same substance undergoes both oxidation a t the anode and reduction a t the cathode (6, 11). Both of these types use inert electrodes, usually platinum. As has been discussed ( l a ) , the dead-stop phenomenon is based on electrochemical cell behavior with the result that the presence of a metal ion in solution should initiate electrolysis with electrodes made of the same metal. This DaDer . I discusses the use of silver electrodes and silver nitrate solution as a dead-stop technique. Some of the theoretical aspects of this system have been discussed by Bradbury ( 2 ) . The argentometric analysis of halide mixtures by potentiometric and amperometric methods has been reviewed ( 9 ) . Clippinger and Foulk ( 5 ) used the dead-stop end point for the argentometric determination of halides. Platinum electrodes were used and nitrite ion v a s required as a “depolarizer.” Salomon (IO) was the first to report a galvanometric titration using silver electrodes and silver nitrate solution to titrate chloride. This essentially was the first dead-stop technique, but the experiment was a demonstration, and the details are not clear. No work on mixtures of halides was reported. Data were given ’to show that the current increased from a very small value at the equivalence point.
then decreases, and reaches a minimum at the end point followed by an increase after the end point. In the vicinity of the end point, time for equilibration must be allowed after the addition of each drop. Usually 1 to 2 minutes are necessary. With mixed halides a slower rate of addition is required, and experience is necessary to find the correct end point which is the first permanent increase in current. After use the electrodes must be soaked in concentrated sodium thiosulfate solution and stored in water. Failure to do this results in a surfare film on the silver which must be removed by scraping a knife. RESULTS
The determination of chloride alone was done in the presence of 5 drops of concentrated nitric acid, or 10 drops of glacial acetic acid. The latter made the end point somewhat easier to detect, Since silver chloride is appreciably soluble, the end point was not sharp in any case (a). Some typical results are listed in Table I. The determination of bromide alone was done in the presence of 5 drops of concentrated nitric acid, 10 drops of glacial acetic acid, or 5 ml. of 1Jf ammonium carbonate solution as recommended by Kolthoff (8). The end points were easier to detect than those with chloride, but experience is necessary. The acetic acid medium gave the best results as shown by Table 11. The determination of iodide alone v a s tried in distilled water only, in the presence of 5 ml. of concentrated ammonium hydrox-
EXPEHIMENT4L
Apparatus. The electrical circuit described by Rernimont and Ilopkinson ( 1 4 ) is the basic requirement. The Sargent hlodel 111 polarograph or the Fisher Elecdropode may be used. In any case a galvanometer with a sensitivity of 0.01 pa. per mm. of scale is desirable. The electrodes were silver wires fused to copper leads and sealed into glass tubing with De Khotinsky cement. The seal was protected with Glyptal for water resistance, particularly in the presence of ammonium hydroxide. The use of silver foil electrodes did not improve the end point detection ( 8 ) . A stirring device is necessary, since the solution must be stirred continuously during the titration. Reagents. Standard silver nitrate solution was prepared from reagent grade silver nitrate which had been ground and dried a t 110’ C. for 2 hours. Potassium bromide used in this work was analyzed by the A.C.S. revised specificatibn method (1) and was found to contain chloride equivalent to 1 mg. of potassium chloride per gram. Sodium chloride and potassium iodide were reagent grade and were dried 2 hours a t 110’ C. General Procedure. Weigh a sample which will require 15 to 50 ml. of 0.1M silver nitrate solution into a 250-ml. beaker. Make the volume to 100 ml. with water, add 5 ml. of 2y0 dextrin solution, and add the required acid or other reagent. Start the stirrer, dip the electrodes into the solution, and apply 10 mv. between the electrodes. Add silver nitrate solution from a buret a t such a rate that drops are visible. The current first increases, 1
Present address, Diamond -4lkali Co., Painesville. Ohio.
Table I. L
Titration of Chloride with 0.1M Silver Nitrate
“01 hledium NaCl taken, g. NaCl found, g . 0.1780 0.1787 0,2553 0.2548 0,2648 0.2643 0,2295 0.2293 0.2321 0.2317 0.2330 0.2329 0 2642 0 2649 0 1881 0 1878 0 2530 0 2534
HOAc Medium NaCl taken, g. NaCl found, g. 0,2367 0.2367 0 2546 0.2545 0.2337 0.2343 0.2074 0.2071 0.248fi 0.2482
Table 11. Titration of Bromide with 0.1M Silver Nitrate HOAc Medium KBr taken, g . KBr found, g. 0.5663 0 5666 0,5307 0.5308 0.4634 0.4636 0.4396 0.4399 0.4162 0 4167 n ,5707 0.5703 0 4788 0.4779 0.4617 0 4625 0 3832 0.3830 0.3669 0 3664
“Os Medium KBr taken, g . KBr found, g. 0 3408 0.3408 0 359.5 0 3588 0 4556 0 4.547 0 3335 0 3326 ( N H r X O a hledium 0 2896 0 2883 0 33.51 0 3330 0 3218 0 3208 0 2889 0 2856 0 2934 0 2939 0 4744 0 4744
V O L U M E 26, N O . 6, J U N E 1 9 5 4 Table 111. Titration of Iodide with 0.1M Silver Nitrate Ha0 Only K I taken, g. KI found, g.
-
( N H ~ ~ C Medium OI IC1 taken, g. K I found, g.
1077 Table V. Titration of Mixed Halides with 0.1M Silver Nitrate-Acetic Acid Medium KBr, Meq.
KI. Meq. Taken 1.200 0.741 0.529 0.798 0.844 0.692 0.920 0.897 0.634 1.213
Found 1,209 0.743 0.532 0.800
0.842 0.692 0.915 0.900 0.638 1.211
Taken 1.628 1.435 0.580 1.084 1.596 1.326 1.302 1.209 1.176 2.284
Found 1.642 1.462 0.583 1.110 1.633 1.321 1.307 1,210 1.172 2.275
NaC1, Meq. Taken 1.441 1.788 3.287 2.801 1.673 3.489 3.166 1,404 2.777 1.857
Found 1.479 1.785 3.295 2.768 1.643 3.479 3.168 1,404 2.792 1.865
Total, Meq. Taken Found 4.269 4.330 3.964 3.990 4.396 4.410 4.683 4.676 4.113 4.118 5.507 5.492 5.394 5.390 3.510 3.504 4.587 4.602 5.354 5.341
Table IV. Titration of Bromide with 0.1M Silver Nitrate Acetic Acid Medium, 1.0 Gram Sodium Chloride Present KBr Taken, G. 0.5666 0.5308 0.4636 0.4399 0.4167 0,5707
KBr Found, G . 0.5671 0.5309 0.4642 0.4397 0.4171 0.5710 0
ide, 10 drops of glacial acetic acid, and 5 ml. of 1M ammonium carbonate solution (8) added to the water. Only the results in ammonium hydroxide medium were poor. The end point was easily detected, although varied current fluctuations were found in the vicinity of the end point. The results are listed in Table
l A T U R E OF T H E END POIh-T
h s was suggested in the introduction, no current was expected
to flow until a slight excess of silver ion was present. However, a current did flow which went through a maximum when about one half of the silver nitrate had been added as shown in Figure 1. I t was also observed that silver halide was adsorbed on the surface of the silver cathode during the titration and that the surface cleared in the vicinity of the end point particularly when the precipitated silver halide coagulated. One reasonable explanation for the observed current during the titration is the reduction of adsorbed silver halide at the cathode and electrolytic dissolution of the silver anode to form silver halide:
15 PO 25 ML. AG ADDED (0.1N)
10
30
35
Figure 1. Typical Titration of a Halide lllixture with Silver Nitrate in Acetic Acid Medium Cathode,
111. Since the previous results indicated that the acetic acid medium way the only one in which all three halides gave good results, a check was made to see if bromide could be determined in the presence of chloride in this medium. For this purpose 1 gram of sodium chloride and 10 drops of glacial acetic acid were added to the medium. In order to minimize coprecipitation the reagent must be added slowly, and time for equilibration must be allowed near the end point. The current break is easily detected after a few trial observations. Some results are shown in Table IV. Since the determination of bromide in the presence of chloride was successful, the determination of iodide, bromide, and chloride in a mixture of all three was tried. For this determination about 0.4 gram of each halide was taken, and 10 drops of glacial acetic acid were added to the solution. By experiment it was found that 10 mi. of 2% dextrin solution gave better results than the 5 ml. previously used. Also by experiment it was found that the titration must be done almost dropwise to minimize coprecipitation and to make the end point detection possible. The end point breaks were not abrupt, and care was necessary to detect the minima. Some results are shown in Table V. Ammonium carbonate, ammonium hydrouide, and mixed media which had been recommended (8, 9) gave lees satisfactory results than the acetic acid medium. -4typical titration plot is shown in Figure 1.
5
Anode.
+ e Ag + S+4g - e + S- AgS AgX
-+
-+
The adsorption of silver halide would limit the current up to the mid-point of the titration, and the concentration of halide ion would limit the current in the last half. The concentration of silver ion would limit the current beyond the equivalence point. CONCLUSIOh
In a medium containing 10 drops of glacial acetic acid per 100 ml. of water, iodide, bromide, and chloride can be titrated in succession with silver nitrate solution, using silver electrodes with 10 mv. applied, by observing the dead-stop end points. Time for equilibration is necessary. LITERATURE CITED
(1) (2) (3) (4) (5) (6)
SOCIETY,Washington, D. C., "Reagent Chemicals," p. 246, 1951; ANAL.CHEM.,25, 379 (1953). Bradbury, J. H., Trans. Faradag SOC., 49,304-13 (1953). Buck, R. P., Farrington, P. S., and Swift, E. H., A N ~ LCHEM., . 24, 1195-7 (1952) and earlier papers. Carson, W.N., Jr., d x . 4 ~CHEM., . 22, 1565-8 (1950). Clippinper, D. R., and Foulk, C. W.,IND.ENG.'CHEY.,ANAL. ED., 11, 216-18 (1939). Ferrero, P., and Brehain, J., Indzcstrie chim. belye, 16, 103-8
= ~ Y E R I C A N CHEMIClL
(1951).
Gale, R. H., and Mosher, E . , ANAL.CHEM.,22, 942-4 (1950). Kolthoff, I. M,, 2. anal. Chem., 70,395-7 (1927). Laitinen, H. d.,Jennings, W. P., and Parks, T. D., IND.ENG. CHEY.,ASAI.. ED.,18, 355-8, 358-9 (1946). (10) Salomon, E., Z . Elektrochem., 4, 71-3 (1897). (11) Scholten, H. G., and Stone, K. G., ~ ~ N ACHEM.. I.. 24, 749-50 (7) (8) (9)
(1952). (12) (13) (14)
Stone, K. G., and Scholten, H. G., Ibid., 24, 671-4 (1952). Swinehart, D. F., Ibid., 23, 380-1 (1951). Wernimont, G., and Hopkinson, F. J., IND.ENG.CHEM.,A 4 ~ ~ ~ , . ED.,12, 308-10 (1940).
RECEIVED for review November 20, 1963. Accepted March 22, 1954. Presented before the Division of Analytical Chemistry at the 124th M e e t CHEMICAL SOCIETY, Chicago, Ill. Abstracted from ing of the BMERICAN the thesis for the degree master of science submitted by M. Lou Masten. December 1962.
