Page 45 - Geochemistry of Oil Field Waters

34 ANALYSIS OF OILFIELD WATERS

electrodes; water bath, complete with stirrer, thermostat, and thermometer,
with 0.loC graduations; source of alternating current, 25- to 60-cycle a.c.
galvanometer, and an appropriate isolating transformer.
Selection of the cell constant is limited by the accuracy and sensitivity of
the bridge when measuring very high and very low resistivities. Also, current
frequency should not be excessively high since a.c. resistance is a complex
function of frequency; e.g., at frequencies necessary to avoid polarization,
the differences between a.c. resistance and d.c. resistance may be appreciable
unless the cell has been carefully designed to minimize this difference. In
essence, the ideal single apparatus for measurement of resistivity throughout
a wide range necessarily incorporates compromises between low input
voitage, high cell constant, high current frequency, and accuracy and sensi-
tivity of the bridge.

Cell preparation

To prepare the cell, clean it with chromic-sulfuric acid solution and rinse
thoroughly with water. Immerse the cell or fill it, depending upon whether a
dip or pipet cell is used, in the platinizing solution. Connect the electrodes of
the cell to three dry cells (1-1/2 V each) in parallel through a limiting
resistance of approximately 1,000 52. Reverse the direction of the current
once a minute for 6 minutes or until the shiny platinum surface is covered
with a dense black coating. Repeat the electrolytic process using 10%
sulfuric acid solution to remove chlorine. Remove the electrodes, rinse with
distilled water, and store in distilled water.
Note: new cells should be cleaned and platinized before use. They should
be cleaned and replatinized whenever the readings become erratic or when
the platinum black flakes off.

Cell resistance

To determine the cell resistance using the standard potassium chloride
solutions, adjust the temperature of each potassium chloride solution to
exactly 25OC and obtain a reading in ohms for each solution with the cell.
Calculate the cell constant using the following formula:

C = RKCl x specific conductance of standard KC1 solution
where RKCl = reading obtained in ohms for standard KC1 solution.
Note: the specific conductivities of the standard KC1 solutions at 25°C are
as follows (Hodgman et al., 1962, p. 2690):
1.OON KCl = 0.11173 mho/cm
0.1ON KC1 = 0.012886 mho/cm
0.01N KCl = 0.0014114 mho/cm