NOTE ON T H E CALIBRATION OF CONDUCTIVITY APPARATUS BY IAN WILLIAM WARK
The standard method of calibration of the bridge (see Findlay’s laboratory manual) is rather tedious. For some time past I have used a method which is much less cumbersome; indeed the whole calibration may be performed in under an hour along with the evaluation of the cell constant. The resistance box must be assumed to be correct, or else it should be calibrated previously. This is no disadvantage since a reliable resistance box is essential in conductivity work. The method recommended is to find the “cell constant,” which in the process is to be regarded rather as a calibration constant, for different positions on the bridge, setting the bridge reading successively at IO, 2 0 , 30 . 90 and adjusting the resistance to give a minimum sound. The final adjustment is, of course, made on the bridge. The “cell constant” is then plotted against the bridge reading and for any intermediate bridge position the constant to be used is read from the graph. Some idea of the variation to be expected in a bridge wire of the Kohlrausch drum type may be obtained from Table I. N/IO KC1 at 35O, whose conductivity may be taken as 15.39 X 10-3 was used in the calibration.
. .
TABLE I Resistance in
Bnd5e Readmg
Box
I500
o ,6818 0.5874
2000
0.5152
2 500
0.4594 0.4134 0.3455 0.2965
1000
3000 4000 5000
Corresponding Cell Resistance
2739 2135 2126 2124 2114 2111 2107
Cell Constant
32.92 32.85 32.72 32.68 32 . s 3 32.48 32.42
TABLEI1 Resistance in Box 5 00 450 400
380 360 3 50 330 310 270 2 50
Bridge Reading 0.4286 0.4549 0.4842 0.4972 0.5111 0.5180
j328 0,5490 0.5828 0.6014 0 .
Corresponding Cell Resistance
375.0 375.6 3i5.5 375.8 376,2 376.2 376.2 377.3 377.2 377.3
Appropriate
Cell Constant 32.62 32.66 32 . i o 32 , 7 2 3’ . i 3 32.74 32.76 32.78 32.84 32.86
Specific Conductivity 08jOO
08696 08707 ,08707 ,08700 .08joz
.OS7 0 7 ,08690 ,08707 .08j12
886
I A S WILLIAN T A R K
The results of Table I1 were obtained for one of the solutions investigated. As is usual, several minima were found corresponding with different resistances in the box. In calculating the specific conductivity, values of the cell constant corresponding to the bridge reading (read from a graph of the results in Table I) have been employed. The third column shows the variation to be expected if no calibration of the bridge has been performed, the fifth shows how the calibration correction, as applied above, reduced the variation from I part in 160 t o I part in 400. I t is of interest to compare the results for his method of calibration with those obtained by a more usual procedure, where the bridge has been calibrated by the use of two certified resistance boxes. Table I11 gives such a comparison. The fourth column shows the figures for conductivity by this method, whereas those obtained above (See Table 11) are set beside them in the final column. TABLE
Resistance in box
Bridge Reading (Corrected)
0.4294 0.4555 0.4845 0.4975 0,5111 0 .jI80
0.5327 0 . 5486 0,5822
0.6007
* Taking cell constant
111
Corresponding Cell Resistance
376.2 376,s 376.0 376.2 376.2 376.2 376.2 376.6 476. I 376.0
Corresponding Specific Conductivity* .08j02
,08696 ,0870 7
Specific conductivity by previous method
.08700 ,08696 ,08707
,08702
,08707 .08 j o o
.08;02
.08;02
.08jOZ
,08707
,08691
,08690 ,08707 ,08712
08joz
,08704 .08 7 0 7
as 32.74, the mean value over a series of readings.
I t will be seen that there is little to choose between the two methods with regard to the final results. The major divergences from the mean are evidently due rather to faulty observation than to weakness in calibration methods. The “cell constant” method outlined above has the advantage that it can be applied during the course of necessary procedure, whereas others involve additional work. I wish to thank Sir David Masson for his interest and advice concerning presentation of the method. Cnwerszty of Melbourne. August 20,1989.
