What is purified water & its parameters ?

Last modified: 10/05/2020
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Purified water is a type of water that has been mechanically filtered or processed to remove impurities and make it suitable for use.

Purification Methods

There are many types of purification process are used in order to purify the water such as :

  1. Capacitive deionization
  2. Reverse osmosis
  3. Carbon filtering
  4. Microfiltration
  5. Ultrafiltration
  6. Ultraviolet oxidation
  7. Electrodeionization.

Combinations of a number of these processes have come into use to produce ultrapure water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt) & sometimes parts per million ( PPM ).

Uses of purified water

Purified water is suitable for many applications, including autoclaves, hand-pieces, laboratory testing, laser cutting, and automotive use. Purification removes contaminants that may interfere with processes, or leave residues on evaporation. Although water is generally considered to be a good electrical conductor—for example domestic electrical systems are considered particularly hazardous to people if they may be in contact with wet surfaces—pure water is a poor conductor.

Laboratory use

Technical standards on water quality have been established by a number of professional organizations, including the American Chemical Society (ACS), ASTM International, the U.S. National Committee for Clinical Laboratory Standards (NCCLS) which is now CLSI, and the U.S. Pharmacopeia (USP). The ASTM, NCCLS, and ISO 3696 or the International Organization for Standardization classify purified water into Grade 1–3 or Types I–IV depending on the level of purity.

Maximum contaminant levels in purified water
ContaminantParameterISO 3696 (1987)ASTM (D1193-91)NCCLS (1988)Pharmacopoeia
Grade 1Grade 2Grade 3Type I*Type II**Type III***Type IVType IType IIType IIIEP (20 °C)USP
IonsResistivity at 25 °C [MΩ·cm]1010.>10>1>0.1>0.23>0.77
Conductivity at 25 °C [μS·cm−1]<0.1<1<10<4.3<1.3
Acidity/AlkalinitypH at 25 °C5.0–7.55.0–8.05.0–8.0
OrganicsTotal Organic Carbon/p.p.b.(μg/l)1050200<50<200<1000<500<500
Total Solidsmg/kg120.115
ColloidsSilica [μg/ml]<2<3<500<0.05<0.1<1
BacteriaCFU/ml\ –<10<1000<100<100

Electrical conductivity

The electrical conductivity of purified water is very low , so it is used in the areas where risk of getting shocked is very high. The electrical conductivity of ultra-pure water is 5.5 × 10−6 S/m (18 MΩ·cm in the reciprocal terms of electrical resistivity) and is due only to H+ and OH ions produced in the water dissociation equilibrium.[10][11] This low conductivity is only achieved, however, in the presence of dissolved monatomic gases. Completely de-gassed ultrapure water has a conductivity of 1.2 × 10−4 S/m, whereas on equilibration to the atmosphere it is 7.5 × 10−5 S/m due to dissolved CO2 in it.

Industrial uses

Some industrial processes, notably in the semiconductor and pharmaceutical industries, need large amounts of very pure water. In these situations, feedwater is first processed into purified water and then further processed to produce ultrapure water.

Another class of ultrapure water used for pharmaceutical industries is called Water-For-Inject (WFI), typically generated by multiple distillation or compressed-vaporation process of DI water or RO-DI water. It has tighter bacteria requirement as 10 CFU per 100 mL, instead of the 100 CFU per mL per USP.

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