It is a fact that all ions conduct differently. The ion composition of your water sample should be the primary factor when selecting a calibration standard. To ensure your instrument is calibrated accurately, a standard of similar ion composition to the solution being tested should be used. If the predominate ion in a solution to be tested is NaCl (seawater, brackish water, etc.) the correct choice for a calibration standard is NaCl.
The three values on the bottle represent a cross-reference of the predominate ion composition of the three most common solutions tested. These cross-reference values are correct at 25°C only.
442™ Natural water - this standard solution was developed by the Myron L Company to simulate the properties of natural water (rivers, lakes, wells, etc.). The acronym 442 represents (4) 40% Sodium Bicarbonate, (4) 40% Sodium Sulfate, (2) 20% Sodium Chloride. The 442 standard is the best choice to use when measuring water samples such as city water, rivers, lakes, and wells.
NaCl - this standard solution is made of Sodium Chloride and is to be used to calibrate an instrument that measures solutions such as seawater and brackish water where predominate ions are NaCl (sodium chloride).
KCI Standard - this standard solution is made from Potassium Chloride and is an international standard to calibrate instruments that measure conductivity. The unit of measure for KCl is micromhos/microsiemens or millimhos/millisiemens.
To choose the proper standard contact your Myron L distributor.
Source: Myron L FAQ