Techniques for Establishing Purity of Laboratory use Water

Elga water purification system

Water grades for laboratory use have been discussed earlier.ASTM 1193 Laboratory grade specifications have been laid down for TypesI, II and III and are outlined below:

Parameter Type-1 Type-II Type-III
Resistivity,min
(Megohm-cm at 250C) 18.0 >1.0 >0.05
TOC,max(μg/l) <10 50 200
Na, max(μg/l) 1 5 10
Chloride, max(μg/l 1 5 10
Total silica, (μg/l) 3 3 500
Bacteria(CFU/ml) <10 100 <1000
Endotoxin(Eu/ml 0.03 NA NA

Similar specifications have been laid down by other global and national standardization bodies such as ISO,CLSI, BS, BIS, Japanese, EU, etc.

It is important to know what test procedures are carried out to measure the specified values.In this article methods adopted to test such parameters are briefly discussed.

Elga water purification system
Elga water purification system

Electrical properties

Resistivity and conductivity are important control parameters . Resistivity is expressed as megohmcentimeter (mΩ-cm) at 25 deg C.It is the resistance offered by the water sample to flow of current. Theoretically it lies around 18.2 mΩ-cm at 25 degC, which is the maximum value for water sample free from conducting species. The greater the presence of ions the lower is the resistivity.

Conductivity is the tendency of water to conduct electricity. The unit for conductivity is microsiemens/cm or micromho/cm. Conductivity value is higher the higher the ionic strength of the water sample.The instrument for measuring the conductivity or resistivity is a conductivity meter. Commercial ultrapure purity water generators have online conductivity measurement and dispalay facility.

Inorganic Salts

The inorganic composition of water depends on its source. It includes salts of Ca or Mg which result in water hardness, Ferrous and Ferric ions from underground deposits and rusty pipelines, Al from dosing chemicals, phosphates from detergents, nitrates, silicates, etc. Commonly used techniques for detection and estimation are Ion chromatography,Inductively coupled Mass spectrometry and Graphite furnace atomic absorption spectroscopy.The inorganic species can be detected in the ppm, ppb or even sub-ppb levels.

Organic Contaminants

TOC-analyser-for-total-organics-estimation-
TOC analyser for total organics estimation

Individual compound identification is not necessary for water classification. Presence of total organics is sufficient and for this purpose Total Organic Analyser (TOC) is recommended.As an example water for HPLC analysis should have TOC values in the range of 1-5ppb.

Bacteria

Common bacterial contamination sources are gram negative bacteria, fungi, algae and endotoxins.
Methods like microscopic examination or plate count method are commonly employed. Some bacteria are not free floating but exist in biofilms. Plate count methods in such cases under-report microbial activity so require complimentary endotoxin testing.

Endotoxin testing proves useful for gram negative bacteria as well as fungi and algae. The Limus Amebocyte lysate(LAL) test is commonly adopted.

A subsequent article will discuss the suitability of different grades of water for different analytical applications.

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