Water is an indispensable resource in laboratory settings, playing a crucial role including chemical and biological experiments, analytical techniques and equipment maintenance.
Its quality greatly impacts the accuracy and reliability of experimental results. However, not all water is created equal. Laboratories require different types of water depending on the applications and the level of purity required.
However, various levels of high purity water exist, and multiple organisations have established standards to define them. These criteria instil confidence in the water’s purity and its suitability for diverse applications.
In laboratory settings, two widely adopted water purity standards come from ASTM International and ISO.
Both ASTM International and ISO have established standards to categorise and describe water purity in laboratory-grade water. While ASTM International employs a scale ranging from Types I to IV, the ISO uses Grades 1 to 3, with Type I/Grade 1 representing the highest quality.
Additionally, ASTM offers a substandard with different grades to account for varying levels of microbiological contaminants.
|ASTM International – D1193-06||TYPE 1||TYPE 2||TYPE 3||TYPE 4|
|Conductivity - min µS/cm (25°C)||0.056||1||0.25||5|
|Resistivity - min MΩ-cm (25°C)||18||1||4||0.2|
|Total Organic Carbon (TOC) - max µg/l||50||50||200||No limit|
|Sodium - max µg/l||1||5||10||50|
|Silica - max µg/l||3||3||500||No limit|
|Chloride - max µg/l||1||5||10||50|
|pH value (25°C)||-||-||-||5.0 - 8.0|
|Bacteria - max (CFU/ml)||1||10||1000|
|Endotoxin - max (EU/ml)||0.03||0.25||-|
International Organisation for Standardisation (ISO) ISO 3696
|ISO 3696: 1987||GRADE 1||GRADE 2||GRADE 3|
|pH value (25°C)||-||-||5.0 - 7.0|
|Conductivity - min µS/cm (25oC)||0.1||1||5|
|Oxidizable matter Oxygen content - max mg/l||-||0.08||0.4|
|Absorbance - max absorbance units 254 nm & 1 cm optical path length||0.001||0.01||-|
|Residue post-evaporation - max (110°C) mg/kg||-||1||2|
|Silica - max mg/l||0.01||0.02||-|
What do the parameters mean?
Conductivity [ASTM and ISO]
This parameter quantifies the water’s ability to conduct electrical current. Much like resistivity, conductivity reveals the amount of dissolved conductive substances in the water. Conductivity is the inverse of resistivity and essentially represents the same metric but with reversed calculation. Thus, if water demonstrates high conductivity, it indicates low resistivity. The unit of measurement for conductivity is µS/cm (microSiemens per centimetre) at 25°C.
Resistivity [ASTM only]
Total Organic Carbon (TOC) [ASTM only]
Sodium, Chloride and Silica [ASTM and ISO]
Both the ASTM and ISO standards set specific thresholds for the concentration of silica in laboratory water, measured in ppb or µg/L (ppm in the ISO). Additionally, the ASTM standard defines limits for chlorine and sodium content in ppb. These contaminants, being cations or anions, are also accounted for in resistivity/conductivity measurements. In practical terms, a high purity water system capable of producing 18+ megohm resistivity water should exhibit nearly negligible levels of each of these three dissolved contaminants.
pH [ASTM and ISO]
Further ASTM standards
Additional ISO parameters
Oxidizable matter oxygen content
Residue after evaporation
Understanding these grades and standards is important to ensure the proper selection and utilisation of water suitable for specific applications. Our team of knowledgeable professionals is here to guide you through the intricacies of high purity water, helping you identify the most appropriate grades and standards for your unique requirements.
Stay tuned for Part 2 of our guide to laboratory water, where we delve into the different types of high purity water and their applications, as well as explore the various purification methods to achieve optimal water quality.