03 Dec

Conductivity

Conductivity of water is the measurement of the amount of dissolved charge carrying particles in water.  A conductivity meter and conductivity cell are used to carry out this measurement.  Conductivity is important in rivers, lakes, and ocean environments as well as ground water.  The  particles measured can be:  Fl-, Na+, Cl-, Ca++, K+, NO3, PO43- or any one of a number of charge carrying molecules (ions).  These ions come from a number of sources including natural run off, agricultural and residential run off, discharge from industrial or sewage treatment plants, and leaching from soil contamination.

Laboratory conductivity  cells generally have 4 electrodes, 2 of which are charged, one positively, one negatively.  The potential is measured across the other 2 electrodes and compared to the charge applied.  The difference between the potential applied and the potential measured is converted to the desired reading by the conductivity meter.  Conductivity meters can read in conductivity, milliSiemens/centimeter – mS/cm; or resistance, MegaOhm centimeters – MOhm*cm.  Some meters also measure salinity, parts per thousand Sodium Chloride – ppt NaCl and total dissolved solids – ppt tds.  For all of these readings Standards are used to develop a standard curve and the unknown sample is compared to the standard curve.  Since tds may be also dissolved non-ionic particles which do not conduct, it is best to assume the readings are accurate to +/- 10% .  If a large amount of non-ionic particles are suspected tds should be determined gravimetrically.

 

Robin Prymula