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Drinking Water Testing, Conductivity, and Total Dissolved Solids

Conductivity and Water Quality

Chart of conductivity and water quality total dissolved solids

Conductivity is a measurement of the ability of an aqueous solution to carry an electrical current. An ion is an atom of an element that has gained or lost an electron which will create a negative or positive state. For example, sodium chloride (table salt) consists of sodium ions (Na+) and chloride ions (Cl-) held together in a crystal. In water it breaks apart into an aqueous solution of sodium and chloride ions. This solution will conduct an electrical current. An equation which shows this is:

Na (atom) + Cl (atom) Na⁺ Cl - (ionic crystal)

Na⁺Cl⁻ ( in a water solution) = Na⁺ (ion) + Cl⁻ (ion)

There are several factors that determine the degree to which water will carry an electrical current. These include:

1 The concentration or number of ions and the type of ions.

2 Mobility of the ion.

3 Oxidation state (valence) and Solubility in Water  (Salts must disassociate).

4 Temperature of the water.

Table 1 | KCL Solution - Conductivity vs. Total Dissolved Solids.

KCl Potassium Chloride Solution Equivalent
Molar KCL Solution
0.0001 7.9 148.9 14.9
0.0005 39.3 147.7 73.9
0.001 79 146.9 146.9
0.005 393 143.6 717.5
0.01 785 141.2 1412
0.02 1571 138.2 2765
0.05 3927 133.3 6667
0.1 7854 128.9 12890
0.2 15708 124 24800
0.5 39271 117.3 58670
1 78541 111.9 111900

Resistance, which is an electrical measurement expressed in ohms, is the opposite of conductivity. Conductivity is then expressed in reciprocal ohms. A more convenient unit of measurement in the chemical analysis of water is micromhos (µmhos). The specific conductance or conductivity measurement is related to ionic strength and does not tell us what specific ions are present or their specific concentration, see Table 2.

Methodology: The specific conductance of a sample is measured by a self-contained conductivity electrode.

Environmental Impact: Conductivity is a measurement used in a number of applications related to water quality. These are as follows:

1 Determining mineralization: this is commonly called total dissolved solids (TDS). Total dissolved solids information is used to determine the overall ionic effect in a water source. Certain physiological effects on plants and animals are often affected by the number of available ions in the water.  Conductivity is commonly used to estimate the total dissolved solids of a solution. This is done by measuring the conductivity of the solution and then either programming the tool to calculate the estimated total dissolved solids by using a "fudge factor" or common ratio of total dissolved solids to conductivity.

Total Dissolved Solids (mg/L) = Measured Conductivity (µohms/cm) * Factor

2 Noting variation or changes in natural water and wastewater quickly.

3 Estimating the sample size necessary for other chemical analyses.

4 Determining amounts of chemical reagents or treatment chemicals to be added to a water sample.

Elevated dissolved solids can cause "mineral tastes" in drinking water. Corrosion or encrustation of metallic surfaces by water high in dissolved solids causes problems with industrial equipment and boilers as well as domestic plumbing, hot water heaters, toilet flushing mechanisms, faucets, and washing machines and dishwashers.

Indirect effects of excessive dissolved solids are primarily the elimination of desirable food plants and habitat-forming plant species. Agricultural uses of water for livestock watering are limited by excessive dissolved solids. High dissolved solids can be a problem in water used for irrigation.

For testing support for this or other chemical or biological parameters, please Contact Us.

Table 2 | Data We Generated in the Laboratory for Different Solution Strengths with Measured Total Dissolved Solids and Measured Conductivity.

Solution Concentration (Percent) Total Dissolved Solids (mg/L) Conductivity (µohms/cm)
NaCl 1% 10090 16920
NaCl 0.10% 1020 1980
NaCl 0.01% 102 207
NaCl 0.00% 10.2 30.7
KCl 1% 10010 16270
KCl 0.10% 1040 1890
KCl 0.01% 104 191.3
KCl 0.00% 10.4 31.9
CaCO3 1% 10060 No Data
CaCO3 0.10% 1030 35.4
CaCO3 0.01% 103 10.3
CaCO3 0.00% 10.3 4.7
CaSO4 0.10% 1040 1200
CaSO4 0.01% 104 167.6
CaSO4 0.00% 10.4 34.6
CaCl2 1.00% 10090 13320
CaCl2 0.10% 1050 1713
CaCl2 0.01% 105 188.6
CaCl2 0.00% 10.5 27.9

A graphical analysis of the data yields the following: NaCl Solution - Ratio TDS/Conductivity = 0.59 KCL Solution - Ratio TDS/Conductivity = 0.61 (Note: Certified Solutions have a ratio of 0.7) CaCO3 Solution - Ratio TDS/Conductivity = 0.34  (Solubility Issues) CaSO4 Solution - Ratio TDS/Conductivity = 0.89

CaCl2 Solution - Ratio TDS/Conductivity = 0.76

Natural Water TDS/Conductivity Ratio - 0.55 to approximately 1*

Freshwater TDS/Conductivity Ratio - 0.55* - others use 0.65**  (Standard Methods for the Examination of Water and Wastewater indicates the value ranges from 0.55 to 0.7, but can be as high as 0.8 if the solution is dominated by Calcium and Sulfate, see our data).

Seawater Water TDS/Conductivity Ratio - 0.75*

(* Source )

(** Source )


1. When measuring conductivity using a conductivity sensor, it is best to make sure the sensor and the solution are at a constant temperature and we recommend standardizing the unit with a known standard with a known ratio of TDS to Conductivity, such as a KCL standard solution.   The standard solution should be at the same temperature as the samples or you need to be using a sensor system with automatic temperature compensation and you need to document the field temperature.

2. We recommend conducting laboratory testing of both the total dissolved solids and the conductivity so you can develop a better understanding of how this ratio may apply to your natural samples and the specific anion/cation chemistry.

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