3-3-2 THE PLATINUM CELL
4.2 CONDUCTIVITY
Conductivity results are shown graphically in
Figs.,4.5 - 4.9 on the phase diagrams, Figs. 4.27 - 4.46 and in tables 4.1 - 4.9.
4.2.1 THE CELL CONSTANTS
n +
The cell constant of the copper cell was o.3~
0.2 m ^ at 22°C. The cell constant of the Mullard cell was 15 1a*'( error < lm ^) at 25°C.
4.2.1. Cont'd.
The cell constants measured for the first platinum cell at 25•0- 0.2°C were as follows:
6.92 m"1
6.83
m"1
7.15- 0.04 nT1
The first two Values refer to a small number of measure
ments made on two days with the newcell. After this one platinum plate dropped off due to inadequate solder
ing; its replacement resulted in a slightly different cell constant.
The cell constants measured for the second platnum cell at 25.0- 0.2°C were:
7.62 i 0.04 m-1
7.69 - 0.04 m”1
The difference arose because the tube in which the cell constant was measured was changed correction factors
(cell constant for cell in tube y=cell constant measured
in tube x multiplied by the correction factor) were determined for the different tubes and varied between
0.995 and 1.035. These correction factors were different
for the two cells.
The variation of the cell constants with temperature was estimated to be 0.003m ^ °C ^ , the cell constants
increasing with increasing temperature. Values of the conductivity of potassium chloride solution at varying
temperature,obtained from The International Critical Tables ,were used.
4.2.2 SOURCES OF ERROR
Except at very low conductivities instrumental error was outweighed by other factors. Below lOnS conductance readings could only be obtained to two figures on the Wayne Kerr conductance bridge. Below InS only one approximate figure was obtained which ser ved only as an indication of order of magnitude.
Errors arose due to changes in sample composition: part of the volatile hydrocarbon component could be lost
at higher temperatures (above™ 50°C); part of the water
component could be lost at the highest temperatures in
the binary systems (above^ 80°C); the sample could
absorb water particularly at higher temperatures, because of the humid atmosphere above the water baths used.
Although the stoppered tubes reduced these problems they were opened and closed several times in the course of measurements so that these changes could occur. Slight changes in the water content of samples containing only a small proportion of water could cause significant changes in conductivity; this was particularlyso for the pure surfactants which tended to absorb water.
For certain samples, in the C^E^-HgO-hydrocarbon systems, very small temperature variations (<0.1°C) caused large changes in conductivity (e.g. sample number 7). Some of these samples were studied in detail in a thermostatted water bath with a high degree of temperature stability (temperature change on 0.1°C thermometer too
4.2.2. Cont'd.
small to observe over 4hr.). Figs. 4.42 and 4.43 show
these results. Others on the edges of the € and % -
regions were not studied in detail and the conductivities quoted could have been considerably different at
temperatures only very slightly different.
The conductivities of samples in the Brij-Ho0 and
Brij-H^O-hexane systems were measured with the copper cell. Measurements in the ternary system at
10%
water content were repeated with the platinum cell. The results from measurements with the copper cell are subject to rather more error due to the design of the cell (section 3 -3 «l)•
To exclude the possibility of changes in conductivity
being due to the presence of sodium chloride, the con ductivities of the following samples were also measured
in the absence of sodium chloride (the samples were made
up with distilled deionised water):
maximum IC minimum K
sample number 6 8.0 ^tSm ^(133) 2.7/<-Sm ^ (21.3) sample number 11 1.2 mSm ^(20.9) 0.38/^Sm ^ (O.5 8 ) 13.0% C_0E, - 31*