Derek Jaquesl
Potentiometric Determination
College of Further Education Grimsby, Lincs England
of Mixed Halides
The volumetric determination of two halides using two different indicators is a difficult exercise for students.% The presence of a third halide would present a trained analyst with a complicated and diXcult task unless he used electrochemical methods. Potentiometrically such a mixture can be readily analyzed by a single titration with silver nitrate in the presence of a silver electrode. A procedure is described to estimate a mixture of chloride, bromide, and iodide. The exercise can be carried out in 2-3 hours with an accuracy of *I/& on the iodide, *3% on the bromide, and *4y0 on the chloride. The potential of a silver electrode is given by the equation (for dilute solutions, -y can be assigned the value of unity) : E& = E*,O - (2.303 R T I F ) log
7
CA,+
If the silver half cell is connected to a standard half cell (e.g., normal calomel electrode-NCE), the measured cell emf will he a function of the silver ion concentration. NCE 1 saturated NH4NOs I Ag+ ( C A , ~1 )Ag E,,u = E,.,
for the successive titration of the three halides is suggested. Changes in volume have been neglected. I n practice the theoretical curve is not obtained for the bromide titration where some silver chloride is precipitated before all the bromide has been titrated. The similar solubility products for silver bromide and silver iodide and the very nature of the precipitate render true equilibrium a t the bromide end point unobtainable. To overcome this difficulty the weight of iodide is calculated and then the weights of chloride and bromide are obtained by the molecular weight m e t h ~ d . ~ The absolute values of the emf are not required and for this determination any quantity proportional to the emf is adequate. The NCE and the NHaNOs agaragar bridge must be prepared beforehand.= emf (voltd +0.4 +0.3 +0.2
-
+O.l
- EA. = (En., - E d ) + 0.059 log CA,+
The aqueous solution of the mixed halides is titrated with the silver nitrate. After the first addition of silver nitrate the halide solution becomes saturated with respect to silver iodide, the least soluble halide, and it precipitates. The silver concentration of the solution slowly increases as the potassium iodide concentration decreases so that the solubility product principle is satisfied. As the end point approaches, the silver concentration increases faster until a t the end point the concentration of silver equals the concentration of iodide. Further addition of silver nitrate exceeds the solubility product for silver bromide and it precipitates until a t the end point there is a sharp rise in silver concentration. Finally the chloride is titrated; after its end point, further additions of silver nitrate quickly have a snlaller effect because the emf dependency upon silver concentration is logarithmic. Figure 1 depicts the theoretical titration curve of 100 nll of N/50 KI, KBr, and KC1 respectively with N/10 AgN08 solution a t 25°C.S,4 The overall emf change
About 9 g of the halide mixture are accurately weighed out and made up to 250 ml. Twenty ml of this solution are pipetted into a 150-ml beaker and 50 ml of water added. A silver wire is sensitized by dipping it into 1: 1 nitric acid containing a little sodium nitrate, until gassing begins. The wire is then removed, washed with distilled water, and placed in the halide solution. The calomel electrode dips into the
;ent address: Chance Technical College, Smethwick 41, Stafis., England. VOGEL,A. I., "Qumtits.tive Inorganic Andy~is,"2nd ed., Longmans, Green & Co., London, 1953?p. 254. a The emf's have been calculated w ~ t hthe equation: E..u = 0.519 0.059 log CA.+
See, for example, EWING,G . W., "Instnrmentd Methods of Chemical Analysis," 2nd ed., McGraw-Hill Book Company, New York, 1960, p. 182. 6 V A. I., op. ~ cil. p. ~ 2 5 W~. ~ ' FINDLAY, A,, "Practi~d Physical Chemistry," 8th ed., Longmans, Green& Co., London 1954, pp. 251.
+
0
20 40 ml. of N f l O A ~ N O Iadded
60
80
Calculated titration curves for the precipitation reactions. 01 O Agl. S = lo-'" bb) O AgBr, S = 10-L'; sl A AgCI, S = Individual titration curves ond suggested overall titration curve. Figure 1.
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The Experiment
Volume 42, Number 8, Augurf 1965
/
429
saturated NHaNOasolution and the cell is completed by using the salt bridge. (The calomel half cell cannot he dipped directly into the silver nitrate solution because of the precipitation of silver chloride.) The balance point is found on the potentiometer, the halides are titrated by portions with silver nitrate, and the potentiometric reading is noted after each addition. A graph is plotted of potentiometric reading versus ml of silver nitrate solution. Results
I n a typical experiment 8.956 g of halide mixture were made up to 250 ml and 20 ml titrated with N/10 silver nitrate. The result is shown in Figure 2 and the table. Calculation of Halide Compositions
Calculated Actual ' Equivalent weight weight of 1 m l of Titer of of halide of halide N/10 AgN03 N/10 in 250 in 250 Halide (..Q A-~ N O I ml (z) ml (e) ..)
K1 KBr KC1
430
/
0.0166
:
19.80 39.82
Error % .
4.110 3.036
4.127 2.971
-0.4 +2.2
1.809
1.858
-2.5
Journol of Chemical Education
-2.0
1 0
10
20 30 40 50 ml. of Nil0 A ~ N O Iadded
60
70
