Rapid Consta nt-Current Potenti rimetric Titration of Chloride Ion Use of Polarizable Platinum Electrodes R. W. FREEDMAN Consolidation Coal Co., Research and Development Division, Library, Pa.
F A simple, rapid procedure was needed for titration of chloride ion, and a constant-current potentiometric titration using polarizable platinum electrodes filled the requirements. Interpretation of tabulated results by direct observation is more rapid and practical than plotting if the derivative method i s used. Low concentrations of chloride ion can be titrated with silver ion in either aqueous or nonaqueous media using the same apparatus. This method has been applied for the direct titration of tars and other materials containing chlorine in the form of chloride ion.
involving silver chloride reference electrodes were not readily applicable t o nonaqueous solutions. Reilley et al. (9) carried out redox titrations in aqueous media employing an essentially constant-current system. They used a 22megohm resistor in series rvith a 45-volt battery to supply a current of 2 pa. through the electrodes. The constant-current titration method described by Reilley and coworkers is adaptable for use in nonaqueous media as well as in aqueous solution, which provides flexibility, speed, and simplicity in its use for control analysis.
EXPERIMENTAL APPARATUS
A
was sought for electrometric. titration of chloride ion in both aqueous and nonaqueous media which iyould allow both watersoluble and oil-soluble organic materials to be titrated directly for chloride ion without preliminary treatment. The usual potentiometric methods RAPID METHOD
Table I. Titration of 2 MI. of 0.05M Potassium Chloride with 0.1 OON Silver Nitrate
Volume, Voltage, AV/AX, RIl. Rlv. Mv. per M1. In Aqueous Solution 66 30 140 120 80 160 180 540" 300 200
959 926 923 909 897 893 885 876 849 834 824
0 0.5 0.6 0.8 0.85 0.90 0.95 1.00 1.05 1.10
1.15
In Organic Electrolyte 0.5 0.8 0.9 0.95 1.00 1.05 1.10 1.15
892 88 1 861 847 802 647 452 423
End point can be taken as 1.025 ml.
* End point.
(1
37 200 280 900 3100 3900s 580
214
ANALYTICAL CHEMISTRY
supporting electrolyte and titrated, adding large initial increments followed by small increments in the vicinity of the end point. The volume, voltage, and ratio of incremental voltage, AV, to incremental volume, AX, are tabulated. The end point is indicated by a maximum value of the ratio AV/AX. Graphic representation can be omitted for the sake of rapidity. Nonaaueous Solutions. The measured s a i p l e is added to 100 ml. of organic electrolyte and titrated as in the aqueous method. Blanks are usually very small.
The simple circuit of Reilley et al. ( 2 ) was used. A platinum electrode pair similar t o that described previously ( I ) was inserted through a drilled hole in the cover of an 8-ounce polyethylene drinking glass which was painted to exclude light. The 10-ml. buret fitted loosely into a small hole in the polyethylene cover. Magnetic stirring was employed. REAGENTS
Silver nitrate, 0.1N. Potassium chloride, 0.5M and 0 .O5 M . Organic electrolyte ( I ) consisting of 600 ml. of benzene, 400 ml. of methanol, and 6.7 ml: of concentrated nitric acid. Supporting electrolyte, stock solution, potassium sulfate 0.5M. The C.P. crystals are ground in a mortar and dissolved in boiling water. PROCEDURE
Aqueous Solutions. A blank titration is made by adding 10 ml. of supporting electrolyte stock solution to 100 ml. of distilled water and titrating by dropwise addition of silver nitrate. A sharp drop in potential occurs at the end point. (Reagent consumption is usually less than 0.10 ml.) A measured or weighed amount of sample, sufficient to consume 1 t o 5 ml. of silver nitrate, is added to 100 ml. of distilled water plus 10 ml. of
Results for titration of a known amount of potassium chloride in aqueous solution are given in Table I. A known amount of potassium chloride gives stoichiometric results in nonaqueous solution. DISCUSSION
The dead-stop end point of the chloride precipitation is essentially nonreversible ( 2 ) . A rapid decrease in voltage drop across the electrodes results from cathode depolarization in the presence of excess silver ion. This type of titration is, of course, not limited to chloride ion but will work n4th any ion yielding a silver salt of low solubility under the conditions of the experiment. The derivative method of end point location ( 2 ) gives much sharper breaks than can be obtained by volume-voltage plotting of potentiometric curves. In addition, actual plotting is unnecessary, as the maximum deriyative is readily observed by simple tabulation. This method has been applied to the direct titration of tars and pitches after dilution Kith alcohol. Thiols, if present, giye a second break. LITERATURE CITED
(11 . , Freedman, R. W., ANAL. CHEY. 28, 247 (1956). ' (2) Reilley, C. N., Cooke, W. D., Furman, N. H., Ibid., 23, 1223 (1951).
RECEIVED for review March 14, 1958. Accepted October 1, 1958.
