A Guide to the Types of Water Used in Laboratories - part one

A Guide to the Types of Water Used in Laboratories part 1 – Quality Standards

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.

Types of Water Used in Laboratories

ASTM International

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
ASTM Sub-Standards A B C
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?

Deciphering the parameters outlined by the ASTM or ISO is crucial for establishing and upholding water quality standards. Here are concise explanations of each parameter in relation to high purity water applications.

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]

Resistivity refers to the water’s ability to resist the flow of electrical current. A high resistivity indicates low conductivity and vice versa. The measurement is expressed in MΩ-cm (Megohms per centimetre) at 25°C. The maximum resistivity of high purity water is approximately 18.3 megohms, which means it is almost non-conductive and entirely free from dissolved cations and anions.

Total Organic Carbon (TOC) [ASTM only]

Total organic carbon, often referred to as TOC, gauges the organic content within a water source. The measurement is expressed in ppb (parts per billion) or µg/L (micrograms per litre). Managing and monitoring TOC is vital in many applications as it indicates the level of organic substances present.

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]

pH is a measure of the concentration of hydrogen ions in water, which helps determine its acidic or alkaline nature. A pH value above 7.0 indicates alkalinity, while a pH below 7.0 indicates acidity. The pH parameter is relevant mainly to less pure water classifications, as ultrapure water lacks dissolved content that could influence its pH level.

Further ASTM standards

Bacteria count

A water’s heterotrophic bacteria count, measured in CFU/ml, is a relative measure of how much bacteria is in a given feed water source. This is a critical metric for most laboratory, medical, and pharmaceutical applications as most uses of high purity water in these applications are micro-biologically sensitive.


Endotoxins are substances produced as part of the bacterial lifecycle. As bacteria go through their life stages, they release these endotoxins, making it essential to measure them in numerous high purity water applications. Endotoxin levels are typically measured in endotoxin units per ml (EU/ml).

Additional ISO parameters

Oxidizable matter oxygen content

Quantified in mg/L, this parameter evaluates the quantity of oxidizable substances in water. Its significance pertains primarily to ISO Grade 2 and ISO Grade 3 applications, as Grade 1 applications should exhibit minimal oxidizable matter.


This parameter gauges the water’s capacity to absorb light at a particular wavelength. High purity applications (Grade 1) exhibit minimal light absorption as they contain very few dissolved substances.

Residue after evaporation

This simple parameter quantifies the residue left behind when water is heated to 110 degrees Celsius and subsequently evaporated.
A Guide to the Types of Water Used in Laboratories - part one

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.

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