A
Simplified Conductometric Titration Apparatus EDMUND M. BURAS, JR.,
AND
J. DAVID REID
Southern Regional Research Laboratory, N e w Orleans, La.
extend the range in the direction of lower resistances, as this could be circumvented by titrating with more dilute solutions.
THE
advantages and disadvantages of conductometric titrations are too well known to be detailed here. It is often advantageous to titrate mixtures or turbid solutions conductometrically, and this method has been used in this laboratory for the analysis of complex mixtures encountered in research on the fireproofing of cotton cloth. However, the complicated apparatus ordinarily used discourages more general application of the method. The apparatus described by Whittemore and his coworkers (5, 4, 6) is perhaps the simplest. I n their method the voltage is adjusted t o a constant value after each addition of titrating solution and the current passing through the conductivity cell is measured. By plotting milliliters of solution added against milliamperes, a typical conductometric curve is obtained. This apparatus and procedure have been further simplified in this laboratory by the use of a constant-voltage transformer and an alternating current milliammeter of low internal resistance, as diagramed in Figure 1, considering the circuit at M completed through A . The innovation, though simple, makes the conductometric titration very easy, since it is only necessary t o read one meter, compared with the former procedure involving the adjustment of a fluctuating voltage with a potentiometer, reading this on a voltmeter, and simultaneously reading the milliammeter.
The limit of accuracy of conductometric methods, given by Kolthoff and Sandell ( 2 ) as 0.5 to 1%, is readily attained with reasonable precautions : The solution being titrated should be uniformly stirred with a glass stirrer (not a metal one), but a “whirlpool” should not be allowed, since the addition of the titrating medium will change the sha e of the vortex and cause an irregularity in the curve. The Beaker and electrodes should not be moved once the titration is begun. The two transformers (if T2is used) must be placed with their cores at right angles or sufficiently distant from each other to avoid inducing cuhent in 7‘2. The concentration of the reagent solution should be at least 10 to 20 times that of the solution t o be titrated in order to obtain rectilinear graphs.
The constant-voltage transformer, TI,is the type generally used for 8-volt lamps in such instruments as the Coleman spectrophotometer and thus is generally available. Its cost is less than that of the meter, transformer, and potentiometers it replaces. The more common 115-volt constant-voltage transformer with an auxiliary step-down transformer is equally suitable and this combination may be substituted for TI. Along with the constant-voltage transformer it may be necessary t o have an additional constant load to meet its minimum load requirement and avoid overheating. Because of war conditions, the sale of meters is restricted, and, unfortunately, an alternating current milliammeter is more rarely used than other types in a chemical laboratory. A low-range alternating current voltmeter is nearly always available, however. I n this laboratory, the 2.5-volt range of an alternating current meter of the rectifier type, 1000 ohms per volt, was used instead of a milliammeter as follows: A radio-type transformer 2’2, commonly known as an audiooutput transformer, with a; impedance ratio of about 500 to 1 (turn ratio of 22 to 1) was used as a current transformer to convert the relatively 6 g h current a t a small voltage drop to a much higher voltage, and impress this voltage on the meter. It was then possible to read the current directly from the meter in relative units. If actual values are desired, the factor may be obtained from a consideration of the meter and transformer constants, or by direct calibration. This circuit is shown in Figure 1, considering the circuit a t M completed through B. The transformer to be used as T2shouldbe selected to reflect the resistance of the meter as 5 ohms in senes with the cell-for example, to use the components cited above, a transformer matching 2500 ohms to 5 ohms was used. I n general, satisfactory results are obtained using a value within the limits of 4 to 8 ohms for the low-impedance winding. Electrodes are conveniently made by welding platinum disks to platinum wire and sealing each into appropriately shaped glass tubes as shown in Figure 1. The size of the electrodes and,the distance between them are chosen to give a convenient initial conductance. For example, in the titration of approximately 100 ml. of 0.001 N solution with 0.01 N reagents disks 1 cm: in diameterandspaced about 2.5 cm. apart were used. I n the titration of approximately 100 ml. of 0.01 N solutions with 0.1 N reagents, disks 0.3 cm. in diameter and spaced about 3 cm. apart were used. It wae found that the apparatus described was generally suitable for cell-electrolyte combinations which resulted in resistances of the order of 400 to 10,OOO ohms, It was not found necessary to
Figure I.
Diagrams of Simpl;fied Conductometric Apparatus
8-volt constant-volhgr bansformrr (or combination ol 11 5-volt cond.nt-volt.se Irandormerand 11 5. Io 8-volt step-down tranrformer)
TL
M. To be completed through eithercirculIA orE Circuit A. A , mrtrrlng cinuil using milliammrter I I , low.rrristaner 0- to 30-mllliamprrr ritematin# cunrnt metei Cinuit E . E , metering circuit using T Iand voltmeIrr Y TI,audio-output trandormrr Y , 0- to P.5.volI aitrrnating cunent mater, 1000 ohms per volt
Graphs of conductometric titrations obtainable with this a p paratus are of the same type and precision as shown by other authors ( 1 , S , 4 , 6 ) . The text by Britton ( 1 ) is particularly useful for interpretation of the graphs obtained. LITERATURE CITED
(1) Britton, H. T. S.. “Conductometric Analysis”, New York, D. Van Nostrand Co., 1934. (2) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis”, pp. 486-9, New York, Maomillan Co., 1938. (3) McEihhey, T. X., Whittemore, E. R., and Lynch, D. F. J., Paper TradeJ., 106,No. 10,3741 (March 10,1938). (4) Whittemore, E.R., Aronovsky, S. I., and Lynch, D. F. J., Ibid., 108,No. 17,33 (1939). ( 5 ) Whitternore, E. R., Reid, J. D., and Lynch, D. F. J., IND.ENQ. CHEM.,30, 1192 (1938).
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