ISO 3696 Water for Analytical Laboratory Use - Specification and Test Methods

The International Organization for Standardization (ISO) standard "ISO 3696 Water for analytical laboratory use - Specification and test methods" describes the requirements and associated test methods for three classes of water for the analysis of inorganic chemicals in laboratories. It does not apply to water for organic trace analysis, water for the analysis of surfactants, or water for biological or medical analysis. For some purposes (e.g., for certain analytical methods or for tests requiring water to be sterile, pyrogen-free, or to have a specified surface tension), additional specialized testing and further purification or other processing may be necessary.

The material should be a clear, colorless liquid when evaluated by visual inspection.

This international standard covers three classes of water:

• Grade 1: Essentially free of dissolved or colloidal ionic and organic contaminants and meets the most stringent analytical requirements, including high-performance liquid chromatography. Grade 2 must be produced by further purification of the water (e.g., reverse osmosis or deionization followed by a pore size of 0,2 to remove particulate matter). µm membrane filter or redistillation through a fused silica apparatus).
• Class 2: Very low in inorganic, organic, or colloidal contaminants and suitable for sensitive analytical purposes, including atomic absorption spectrometry and the determination of trace components. For example, it must be produced by multiple distillation or by deionization or reverse osmosis followed by distillation.
• Class 3: Suitable for most laboratory wet chemistry work and the preparation of reagent solutions. For example, they should be produced by single distillation, deionization, or reverse osmosis. Unless otherwise specified, they should be used for ordinary analytical work.

The initial feedwater is assumed to be potable and pure. If it is heavily contaminated in any way, some pretreatment may be necessary.

The material must comply with the relevant requirements in the table. Testing for conformity is carried out using the methods specified in this standard.

Due to the difficulties encountered in measuring the pH value of high purity water and the questionable significance of the obtained value, limits for the pH values ​​of 1st and 2nd degree waters have not been determined.

It is appropriate that pollutants such as atmospheric carbon dioxide and alkalis in glass containers dissolve during storage and changes in conductivity occur.

Because compliance testing at this purity level is difficult, no limits are specified for oxidized matter and residue after evaporation for Class 1 water. The quality of Class 1 water is assured by its compliance with other requirements and the preparation method.

In accordance with this standard, a representative sample of at least 2 liters is taken from the stockpile for inspection purposes. This sample is not used for conductivity measurements in Grade 1 and Grade 2 waters.

The sample should be placed in a suitable, clean, airtight container designated solely for water samples and should be large enough to completely fill the container with the sample. Care should be taken to avoid any risk of contamination of the contents.

Aged containers made of borosilicate glass (i.e. containers boiled in hydrochloric acid solution for at least 2 hours, followed by boiling in distilled water twice for 1 hour) and suitable inert plastic containers (e.g. polyethylene or polypropylene) may be used, but it is essential to determine that the sample is not affected by storage in them, particularly with respect to oxidizable matter and absorbance.

Contamination of water during storage can primarily occur from the dissolution of soluble components in glass or plastic containers or from the absorption of atmospheric carbon dioxide and other contaminants present in the laboratory atmosphere. Therefore, storing Class 1 and Class 2 water, which is readily available for immediate use when needed, is not recommended. However, a reasonable amount of Class 2 water can be prepared and stored in suitable, inert, clean, airtight, filled containers that have been thoroughly rinsed with water of similar quality.

Storing Class 3 water presents little problem, but the containers and storage conditions should be the same as those used for Class 2 water. It is recommended that any storage container be dedicated solely to storing water of a specific quality.

It is essential that the determinations specified in this standard be carried out in a dust-free, clean environment and that appropriate precautions be taken to prevent contamination of the sample and test pieces.

The ISO 3696 standard includes the following test methods for determining the quality of water used in laboratories:

• Determining pH: This test is performed to measure the hydrogen ion concentration of water. A calibrated pH meter is used for measurements. This test is generally performed for Type 2 and Type 3 water (Type 1 water may not have a measurable pH value due to a lack of ions).
• Determination of conductivity (electrical conductivity): This test is performed to assess the ionic content (impurities) in water. During the test, it is measured in microsiemens per centimeter (µS/cm) at 25 degrees Celsius using a conductivity meter. The limitations are as follows:
  • Type I: up to 0,1 µS/cm
  • Type II: up to 1,0 µS/cm
  • Type III: up to 5,0 µS/cm
• Determination of oxidizable substances (oxidizable substances): This test is performed to detect organic and some inorganic contaminants. This method is based on oxidation by heating with potassium permanganate in an acidic solution. The amount of potassium permanganate consumed indicates the level of oxidizable substances. The limitations are as follows:
  • Type I: up to 0,08 mL
  • Type II: up to 0,08 mL
  • Type III: up to 0,4 mL
• Determination of absorbance: This test is performed to detect the presence of organic substances that absorb UV light. This test uses UV spectrophotometry at 254 nm and a maximum optical path length of 1 cm. Limitations include:
  • Type I: up to 0,01
  • Type II: up to 0,1
  • Not usually necessary for Type III
• Determination of evaporation residue: This test is performed to evaluate total dissolved solids. During the test, the residue remaining after evaporation of a known volume of water (usually 100 mL) and drying at 105°C is weighed. The limitations are as follows:
  • Type I: up to 1 mg/L
  • Type II: up to 1 mg/L
  • Type III: up to 2 mg/L
• Determination of silica: This test is performed to measure the silica content in water. The colorimetric method is measured spectrophotometrically using reagents that produce a blue color with silica. Limitations include:
  • Type I: up to 0,01 mg/L
  • Type II: up to 0,1 mg/L
  • Type III: up to 1 mg/L

Our organization, which has been supporting businesses across all sectors for years with a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the ISO 3696 Water for Analytical Laboratory Use - Specification and Test Methods standard.