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Relationship Between Conductivity

Measurement, Total Dissolved Solids and

Salinity

Definition of ConductivityConductivity is a measure of the ability of a solution to conduct an electrical current and it is influenced bythe charge on ions in solution. Water is a rather poor conductor of electricity but the presence of ions in thewater increases its conductance considerably. The principle for conductivity measurement is to place twometal electrodes in water and apply a voltage across them. The current flow between the electrodes ismeasured and conductivity is then determined from the voltage (V) and current (I) values according to Ohm’slaw where R is resistance.

R = V/I

Conductivity is the inverse of resistance. As an example, highly purified water containing little or no ions hasan extremely high resistance or, conversely, an extremely low conductance. Another example would behighly contaminated water such as seawater having an extremely low resistance to current flow fromelectrode to electrode but an extremely high conductance.

The conductivity of fresh distilled water is 0.5 to 2 µS/cm but it can have a conductivity of up to 5µS/cm whenstored for several days. The absorption of atmospheric carbon dioxide causes the increase in conductivity ofthe stored water.

Conductivity Units of MeasurementThe basic unit of measurement of conductivity is the siemens (S) but it was originally designated as “mho” orohm spelled backwards. Conductivity in water solutions is normally expressed as micromhos /cm

(µmhos/cm) or microSiemens/cm (µS/cm) or milliSiemens per cm (mS/cm).

 As seen in the list below, there is no difference between micromhos (µmhos) and microSiemens (µS). Theterm micromhos is more common in the United States and the term microSiemens is more common inEurope.

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1 Siemen (S) = 1,000,000 µS

1 mhos = 1,000,000µmhos1 Siemen (S) = 1 mhos

1µS/cm = 1µmhos/cm

1000 µS/cm = 1 mS/cm

Temperature Influence on ConductivityCurrent (I) flow increases with increasing temperature thereby making conductivity measurementtemperature dependent. Therefore, the conductivity of a solution will increase with increasing temperature.Conductivity meters have an automatic temperature compensation at a reference temperature of 25°C or20°C or they have adjustable temperature compensation that is adjusted manually according to thetemperature of the test solution.

Conductivity Meter Calibration and Standard SolutionsThe conductivity meter should be calibrated using a fresh standard solution in the range of the sample that is

being measured. For example, if you are measuring conductivity in the range of 0-200 µS, calibration with a

84 µS standard is appropriate. If you are measuring conductivity in the range of 0-2000µS, calibration with

a 1413 µS standard is appropriate.

Calibration should be done when a new electrode is used and when readings are questionable.

The conductivity electrode should not be placed into the bottle of calibration standard. An aliquot of standardsolution should be poured into a separate, clean container such as a beaker and then it should be discardedafter the calibration is complete. Never pour the aliquot of standard solution back into the bottle of calibrationstandard.

Conductivity Electrode ConstantsEvery conductivity electrode has a constant, K. The constant reflects a particular electrode’s physicalconfiguration. For example, a constant of K=1.0 cm

-1 is formed by two 1 cm square surfaces spaced 1 cm

apart. The K of a specific electrode is determined by a comparison measurement to a standard solution ofknown conductivity. Electrode’s with a constant of K=1.0 cm

-1 are appropriate for measuring conductivity in

the range of 1 µS to 100 mS (or 100,000 µS).

Conductivity and Total Dissolved SolidsTotal Dissolved Solids (TDS) are determined by evaporating a sample to dryness at 180°C and weighing theresidue. Conductivity may be used to estimate total dissolved solids by multiplying the conductivity by a

suitable factor. For example, the total dissolved solids of raw water with a conductivity greater than 1000µScan be estimated by multiplying the conductivity by 0.85.

When measuring the conductivity of boiler water, the sample needs to be neutralized by the addition of gallicacid. This is done to avoid the excess conductivity of hydroxide ions or of alkaline salts. The total dissolvedsolids of boiler water can then be estimated by multiplying the neutralized conductivity of the sample by afactor of 0.7.

Conductivity and SalinityConductivity is used to determine the level of contamination in water from dissolved solids. Salinitymeasurement is used when the dissolved solids contamination comes from seawater although it can also beused to determine contamination from natural or industrial waters.

Salinity measurement is based on a Practical Salinity Scale (PSS) that was developed from themeasurement of conductivity of a range of solutions of known salinity. The practical scale forms the basis ofan internationally accepted method for calculating salinity from conductivity. The equations for calculatingsalinity include measurements of conductivity and temperature.

Salinity is expressed in parts per thousand (ppt) by weight. For example, a salinity of 35 ppt means thatthere is 35 pounds of salt per 1,000 pounds of seawater. Generally, open ocean salinity is in the range of 32to 37 ppt.

Salinity meters are available that calculate salinity based on the Practical Salinity Scale.

SummaryConductivity measurement is a valuable tool in assessing levels of contamination in water. It offers animmediate indication of salt water contamination in marine shipboard technical water treatment applications.