Volume 15, Number 3, Sept 1993, pp.25-28

Conductivity Measurements: A Discussion and Comparison of Two Instruments Used to Follow the Removal of Soluble Salts from Ceramics

Introduction

Porous ceramics buried in the earth for long periods of time absorb groundwater, which often contains soluble salts. These salts are commonly forms of chlorides, sulfates, and nitrates. Once the ceramics are removed from the soil and dried, the salts crystallize. In low-fired ceramics, the pressure caused by crystallization of the salts damages the artifact, causing disintegration of the structure, such as powdering and spalling. If the ceramics are exposed to fluctuations in temperature and humidity, the salts will alternately crystallize and hydrate, resulting in ongoing damage as they migrate toward the surface.

A standard treatment for removal of the soluble salts is soaking in distilled or deionized water. The soaking water is changed repeatedly. The process is followed by monitoring the soluble salt concentration of the wash water after the item has been soaking in it for some time (days or weeks). When the wash water no longer picks up soluble salts, it is assumed that most of the soluble salts have been removed.

Measuring the Conductivity of the Wash Water

Salt concentration in the wash water is determined by measuring the conductivity (or conductance) of the water with a conductivity meter. These instruments are based on the fact that pure water (containing no dissolved ionic salts) has a very low conductivity; as soluble salts are added, the water starts to conduct current. Conductance is a direct function of the ion (salt) concentration.

The resistance that an electrolyte solution offers to the flow of current is described by Ohm's law:

                             V=RI

where V=voltage, R=resistance, and I=current.

Resistance (R) is measured in ohms, and the reciprocal of resistance, conductance (L), is measured in mhos or siemens. At a constant voltage (V), conductance (L) is proportional to the current (I).

                             R=V/I
                          L=1/R=I/V

The size of the electrodes and the distance between them will determine the actual values measured, and therefore will differ from instrument to instrument. The manufacturers determine these and incorporate them into the design and scales of their instruments. Theoretically, all instruments will measure the same conductance for any given solution.

The Arizona State Museum Conservation Lab undertook a project to determine (1) the relationship between conductance and ion concentration, and (2) the variability among various conductivity measuring devices. We used our standard instrument, the Lab-Line Portable Electro Mho Meter (MC-3)^TM or conductivity bridge and compared it with Omega Conductivitystik^TM CDH-2x and CDH-3x conductivity testers--small, relatively inexpensive, and easily portable testing devices that would be more convenient than a conductivity meter for many situations, including archaeological field laboratories. (See Appendix for testing instrument specifications and sources.)

The Relationship between Conductance and Ion Concentration

Conductivity Testers Compared

In order to obtain data on the relationship between conductance and ion concentration, and to test our instruments in a range of concentrations, we prepared stock solutions of two salts, potassium sulfate (K₂SO₄) and sodium nitrate (NaNO₃). K₂SO₄ represents a salt with a monovalent cation and a divalent anion (2/1). NaNO₃ represents a 1/1 salt. These are common types of salts found in archaeological contexts. Starting with a concentrated solution, the solutions were diluted repeatedly with deionized water and measured with all the instruments before the next dilution. We found that both salts had equal conductivities at the same concentrations in equivalents/liter. See Table 1 (below, at end of article) and Figures 1 and 2

We tested the conductivity-measuring instruments over a very wide range of salt concentration; from 0.00005 to 0.01 equivalents/liter of dissolved salts. The conductivity values ranged from 8 to 10,000 microsiemens. In order to plot these wide ranges we used log/log plots. We found on such a scale the relationship between concentration and conductivity is linear.

We also found that all the conductivity measurement instruments gave very similar values (see Figures 3 and 4 ), and we concluded that both of the Conductivitystiks are good enough to monitor salt removal from a ceramic being washed.

Additionally, we tested the use of a Conductivitystik that measures salt concentrations in parts per million (ppm), Conductivitystik CDH-1x (specifications in Appendix). The values read on the test instrument were compared with the amounts known to have been added to the water to prepare the solutions. These were calculated by multiplying the concentrations in equivalents/liter by the equivalent weights. The results, shown in Figures 5 and 6, show a slope of one, but a slight offset of the 0 value. Again, this should not cause any serious problems for the intended use of this instrument.

Conclusion

Without endorsing or recommending any particular instrument or vendor, we can conclude that all the instruments tested and compared in our work are satisfactory for monitoring wash water during the removal of soluble salts from ceramics. The Conductivitystiks are more portable and far less expensive than the Electro-Mho meter, but have a more limited range. For many situations, two Conductivitystiks would probably be needed, and even then they cannot measure the lowest value available from the Electro-Mho meter (< 0.2 microsiemens). This lack of sensitivity at low ion concentration may cause a problem when trying to determine the suitability of the water being used for soaking, since its conductivity should be below the lowest value that can be read on the CDH-2x instrument (10 microsiemens). Since we generally stop soaking when we reach values near 30 microsiemens, the most serious consequence would be that the time to reach this value would increase.

Appendix

Lab-Line No. 11025 Portable Electro Mho Meter (MC-3)^TM
Specications

CDH-1X Conductivitystik^TM

The Omega CDH-1x Conductivitystik is an economical, portable handheld unit. The CDH-1x features large display numerics for easy reading, automatic temperature compensation, and a calibration trimmer. All readings are referenced to 25° C using a 2% per degree C temperature coefficient. The CDH-1x reads out in ppm of total dissolved solids (TDS). The display value must be multiplied by 10 as indicated below the LCD display.

Specifications

CDH-2X Conductivitystik

The Omega CDH-2x Conductivitystik is an economical, portable handheld unit. The CDH-2x features large display numerics for easy reading, automatic temperature compensation, and a calibration trimmer. All readings are referenced to 25° C using a 2% per degree C temperature coefficient. The CDH-2x reads out in microsiemens. The display value must be multiplied by 10 as indicated below the LCD display.

Specifications

CDH-3X Conductivitystik

The Omega CDH-3x Conductivitystik is an economical, portable handheld unit. The CDH-3x features large display numerics for easy reading, automatic temperature compensation, and a calibration trimmer. All readings are referenced to 25° C using a 2% per °ree C temperature coefficient. The CDH-3x reads out in microsiemens.

Specifications

1. "equs/l" is equivalents/liter, which actually counts the number of negative or positive charges per liter of solution. It consists of the grams of material divided by the equivalent weight of the substance in a liter of the solution.

2. "log equs/l" is the logarithm of the values in column 1.

3. "Lab-Line siemens siemens" stands for the conductivity measured by the Lab-Line Mhometer, in microsiemens.

4. "Lab-Line log siemens siemens" is the logarithm of the values in column 3.

5. "CDH-2x siemens siemens" is the conductivity of the solution in microsiemens measured by the Conductivitystik CDH-2x.

6. "CDH-2x log siemens siemens" is the logarithm of the values in column 5.

7. "CDH-3x siemens siemens" is the conductivity of the solution in microsiemens measured by the Conductivitystik CDH-3x.

8. "CDH-2x log siemens siemens" is the logarithm of the values in column 7.

9. "CDH-1x ppm" is the concentration of the salt in parts per million, or milligrams of salt in a liter of solution. It is the weight of salt in solution (rather than the number of equivalents, as in column 1) measured by the Conductivitystik CDH-1x, which measures in ppm.

10. "Actual ppm" is the concentration by weight in ppm as determined from the actual amount weighed out and the dilutions carried out.

11. "CDH-1x log ppm" is the logarithm of the values in column 9.

12. "Actual log ppm" is the logarithm of the values in column 10.