13 Aug

Laboratory Water

Water types used in the laboratory can be type I water, type II, type III determined by the levels of impurities.  The impurities are described as the following:

Particulates – suspended particles which could be debris from piping, sit, colloids. . . this is what will make water turbid, cloudy.  Microfiltration, distillation, and reverse osmosis will remove particulates.

Resistivity (conductivity) – The tendency of the water to conduct electricity in the absence of added ions.  The unit for resistivity is megohm-centimeter or MΩ-cm.  Units for conductivity are microSiemens/centimeter or μS/cm another unit is the reciprocal of ohms, the mho; 1 mho/m = 1 S/m.  A measurement of dissolved inorganics, largely Calcium and Magnesium which make hard water, although other inorganics such as Sodium and Chloride can be present as well.  These can be removed by deionization or to a lesser extent by reverse osmosis

Total Organic Carbon – Dissolved organic molecules measured in parts per million, ppm, for feed water and parts per billion, ppb, for treated water.  The organic molecules come from decayed plant and animal material, pesticides, plasticizers from piping, and other sources.  Carbon filtration, distillation, as well as UV oxidation can remove dissolved organics.

Microorganisms and endotoxin – Bacteria enter the water treatment system through feedwater and by exposure to air.  The system must be designed to inhibit growth of the microorganisms.  Endotoxins are molecules released by the bacteria or produced when the bacteria dies.  Both Bacteria and endotoxins are removed by distillation and ultrafiltration at point of use.

ASTM Standards for Reagent Water

Measurement (Unit)                       Type I                   Type II                  Type III

Colloids – Silicia/μg/ml                   <3                          <3                          <500

Resistivity (MΩ-cm) at 25° C         >18                      >1                         >4

Total Organic Carbon (ppb)           <50                       <50                       <200

The ASTM standards are further subdivided into A, B, and C.

Measurement (Unit)                                                     A                            B                            C

Heterotrophic Bacteria Count (CFU/ml)                10/1000               10/100                1000/10

Endotoxin (units per ml)                                             0.03                      0.25                      n/a

Robin Prymula