Conductivity 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. Conductivity, also referred to as specific conductance, is the reciprocal of resistivity (specific resistance) and is expressed in units of reciprocal ohm-cm or mho per centimeter. It is usually reported as micromhos/cm (µmho/cm) at a given temperature. In the International System of Units (SI), the reciprocal of the ohm is the Siemens (S), and therefore micromhos = microsiemens (µS). Conductivity standards can be made from various salts (or other ionizable chemicals), with Sodium Chloride or Potassium Chloride being the ones most frequently used. RICCA CHEMICAL COMPANY offers a choice of Conductivity standards made from Sodium Chloride (Group Nos. 2236 - 2249) or from Potassium Chloride (Group Nos. 5885.13 - 5896).
Conductivity is very dependent on temperature. In order to provide traceability to National Institute of Standards and Technology (NIST) Standard Reference Materials, which are certified at 25°C, all RICCA CHEMICAL COMPANY conductivity standards are tested and certified at 25°C. The accuracy of RICCA CHEMICAL COMPANY Conductivity Standards is typically ±0.5%. The manufacturing specifications are actually tighter than this accuracy limit in order to ensure minimal lot-to-lot variation. Due to the uncertainty inherent in even the best conductivity meters, the conductivity read on a different meter may not be within our published manufacturing specifications. For more information, see Technical Reference Document TRD18, available from our website www.riccachemical.com.
The greater the amount of a salt that is dissolved in water, the greater will be the conductivity. Conductivity can therefore be used to measure the Total Dissolved Solids (TDS) concentration. However, because different ions have different ionic conductances, when equal amounts of different salts are dissolved in the same amount of water, the TDS values will be the same, but the conductivities will be different. Also, ion pairing effects lead to a non-linear relationship between conductivity and TDS. Most RICCA CHEMICAL COMPANY Conductivity Standards made from Sodium Chloride (Group Nos. 2236 - 2249) have both the conductivity value and the TDS concentration as ppm NaCl on the product labels. The TDS concentration as ppm NaCl is useful for salt water samples. Other TDS scales are also used to simulate the relative abundances of ions in other types of samples, such as fresh water. Where appropriate, RICCA CHEMICAL COMPANY conductivity standards are also labeled with other TDS values.
In addition to the items listed here, custom prepared conductivity standards are available for the entire range from 5 µmho/cm (µS/cm) to 500,000 µmho/cm (500 mS/cm).