Aquas solution electrical conductivity (k) calculation
- 2. • Conductivity, k, is a measure of the ability of an aqueous solution to carry an electric current • This ability depends on the presence of ions; on their total concentration, mobility, and valence; and on the temperature of measurement • Solutions of most inorganic compounds are relatively good conductors. Conversely, molecules of organic compounds that do not dissociate in aqueous solution conduct a current very poorly
- 3. • Conductance, G, is defined as the reciprocal of resistance, R • where the unit of R is ohm and G is ohm-1 (sometimes written mho). • Conductance of a solution is measured between two spatially fixed and chemically inert electrodes.
- 4. • To avoid polarization at the electrode surfaces the conductance measurement is made with an alternating current signal • The conductance of a solution, G, is directly proportional to the electrode surface area, A, cm2 , and inversely proportional to the distance between the electrodes, L, cm. The constant of proportionality, k, such that is called “conductivity
- 5. • (k is preferred to “specific conductance”).It is a characteristic property of the solution between the electrodes. • The units of k are 1/ohm-cm or mho per centimeter. • Conductivity is customarily reported in micromhos per centimeter (µmho/cm).
- 6. • In the International System of Units (SI) the reciprocal of the ohm is the siemens (S) and conductivity is reported as milli siemens per meter (mS/m); 1 mS/m = 10 µmhos/cm and 1 µS/cm = 1 µmho/cm. To report results in SI units of mS/m divide µmhos/cm by 10.
- 7. • To compare conductivities, values of k are reported relative to electrodes with A = 1 cm2 and L = 1 cm. • Absolute conductances, Gs, of standard potassium chloride solutions between electrodes of precise geometry have been measured; the corresponding standard conductivities, ks are shown in Table 2510:I.
- 9. • The equivalent conductivity, Λ, of a solution is the conductivity per unit of concentration. As the concentration is decreased toward zero, Λ approaches a constant, designated as Λ°. With k in units of micromhos per centimeter it is necessary to convert concentration to units of equivalents per cubic centimeter; therefore: • The equivalent conductivity, Λ, of a solution is the conductivity per unit of concentration
- 10. • the units of Λ, k, and concentration are mho-cm2/ equivalent, µmho/cm, and equivalent/L, respectively. • Equivalent conductivity, Λ, values for several concentrations of KCl are listed in Table 2510:I. • In practice, solutions of KCl more dilute than 0.001M will not maintain stable conductivities because of absorption of atmospheric CO2. • Protect these dilute solutions from the atmosphere
- 11. Measurement Instrumental measurements: • In the laboratory, conductance, Gs, (or resistance) of a standard KCl solution is measured and from the corresponding conductivity, ks, (Table 2510:I) a cell constant, C, cm -1 , is calculated:
- 12. • Most conductivity meters do not display the actual solution conductance, G, or resistance, R; rather, they generally have a dial that permits the user to adjust the internal cell constant to match the conductivity, ks, of a standard. Once the cell constant has been determined, or set, the conductivity of an unknown solution
- 13. • Distilled water produced in a laboratory generally has a conductivity in the range 0.5 to 3 mhos/cm. The conductivity increases shortly after exposure to both air and the water container. • Estimate total dissolved solids (mg/L) in a sample by multiplying conductivity (in micromhos per centimeter) by an empirical factor. This factor may vary from 0.55 to 0.9, depending on the soluble components of the water and on the temperature of measurement.
- 14. • Relatively high factors may be required for saline or boiler waters, whereas lower factors may apply where considerable hydroxide or free acid is present. • Even though sample evaporation results in the change of bicarbonate to carbonate the empirical factor is derived for a comparatively constant water supply by dividing dissolved solids by conductivity
- 15. • the relative contribution of each cation and anion is calculated by multiplying equivalent conductances mho-cm2 /equivalent, by concentration in equivalents per liter and correcting units.
- 16. Table equivalent conductances for ions commonly found in natural waters
- 19. • calculate ionic strength, IS in molar units: • Calculate the monovalent ion activity coefficient, y, using the Davies equation for IS ≤ 0.5 M and for temperatures from 20 to 30°C
- 20. • In the present example IS 0.00767 M and y = 0.91. Finally, obtain the calculated value of conductivity, kcalc, from: • In the example being considered, kcalc = 578.2 x (0.91)2 478.8 µmho/cm versus the reported value as measured by the USGS of 477 µmho/cm
- 21. • If the calculated electrical conductivity (EC) is larger than the measured EC value, re-analyze the higher anion or cation analysis sum. If it is smaller, re-analyze the lower anion or cation analysis sum. • Measured EC and Ion Sums Both the anion and cation sums should be approximately 1/100 of the measured EC value. If either sum does not meet this criterion, then it is suspect; re-analyze the sample. The acceptable criteria are as follows
- 22. • Calculated TDS-to-EC Ratio If the ratio of calculated TDS to conductivity is 0.55, then the lower ion sum is suspect; re-analyze it. If the ratio is 0.7,then the higher anion or cation analysis sum is suspect; reanalyze it. If the lower anion or cation analysis sum does not change after re-analysis, a significant concentration of an unmeasured constituent (e.g., ammonia or nitrite) may be present. If poorly dissociated calcium and sulfate ions are present, then TDS may be as high as 0.8 times the EC. The acceptable criterion is as follows:
- 24. • Measured TDS-to-EC Ratio The measured TDS-to-EC ratio should be between 0.55 and 0.7. If it is outside these limits, then either measured TDS or measured conductivity is suspect; re-analyze.