🔬 Water for analytical laboratory use — three grades of purity per PN-EN ISO 3696

Physicochemistry PN-EN ISO 3696

Overview

WHY A STANDARD FOR WATER. In an analytical procedure water is a reagent, not a background — and the reagent used in the largest quantity. Its impurities enter the blank, shift the limit of quantification and can distort a trace-level result more than any other factor. The words "distilled water" say nothing about conductivity, silica content or the presence of organic substances — and at trace level the difference between grade 1 and grade 3 water is the difference between a result and noise.

THE SCOPE FROM THE PKN CATALOGUE CARD, IN FULL: "Requirements and the corresponding test methods are given for water of three grades of purity, used in laboratories for the analysis of inorganic products. The standard does not cover requirements and test methods for water used in the analysis of surface-active agents, nor for water used in biological and medical analyses."

TWO EXCLUSIONS THAT MATTER MORE THAN THE REQUIREMENTS THEMSELVES. The scope excludes two areas outright: the analysis of surface-active agents and biological and medical analyses. This means that citing this standard in a microbiological procedure, or in the testing of detergents, is citing a document outside its scope. Water of very low conductivity may at the same time be microbiologically contaminated — conductivity says nothing about the number of micro-organisms, and the standard sets no microbiological requirements. A microbiology laboratory needs water meeting requirements from the standards of its own field, not from this one.

WHY THREE GRADES AND NOT ONE. Water of the highest purity is expensive to produce and unstable — its parameters deteriorate within hours of collection, because it dissolves carbon dioxide from the air and leaches constituents from vessels. Grading allows the water to be matched to the task: one grade is needed for rinsing glassware, another for preparing standard solutions, another again for trace determinations by mass spectrometry. Demanding the highest grade where a lower one suffices is a cost without benefit; the opposite mistake is a methodological error.

THE STANDARD ALSO CONTAINS TEST METHODS — AND THAT IS ITS SECOND PART. The title says "Specification and test methods": the document not only states the parameters the water must meet, but also how to verify them. A laboratory declaring the use of water of a given grade should be able to demonstrate this by a measurement carried out in accordance with this standard, and not solely by the declaration of the equipment manufacturer.

WHAT WE DO NOT GIVE. We do not have the text of the standard — PKN supplies it for a fee. We therefore DO NOT QUOTE limit values for any grade: neither conductivity, nor silica content, nor absorbance, nor evaporation residue. Such values circulate in secondary publications and in the materials of water-purification equipment manufacturers, but their source is then the publication, not the standard — and an entry presenting them as the content of the standard would claim more than we have verified. A laboratory setting requirements for its own water has to buy the standard.

THE STANDARD HAS A CORRIGENDUM — AND A REFERENCE WITHOUT IT IS INCOMPLETE. The PKN catalogue card has a "Corrigenda and Errata" section and an "Additional elements" row reading PN-EN ISO 3696:1999/Ap1:2004P. The corrigendum is a separate document, just as amendment Az1 is for PN-Z-04008-7, which has its own entry in this database; the standard as applied today is the standard TOGETHER WITH Ap1. WE READ THE TEXT OF THE CORRIGENDUM ON 22.09.2026 — it is free of charge and it is substantive. Next to the "Additional elements" row on the card there is a link to a PDF file; a direct download returns an error, and the file opens through the r.jina.ai reader. The corrigendum was prepared by KT 249 for General Chemical Analysis and approved on 16 December 2004, and it changes three things: • on page 4, in clause 4 (Requirements), in the TABLE, in column 1, in row 4 the temperature value changes from 250 to 25 — that is, the original text of the standard gave, for this requirement, a temperature ten times too high (an obvious misprint, but in the TABLE OF REQUIREMENTS); • in note 2, "przewodności elektrolitycznej" [electrolytic conductivity] is replaced by "przewodności elektrycznej" [electrical conductivity]; • on page 5, the title of subclause 7.2 changes to "Pomiar przewodności elektrycznej właściwej" [Measurement of specific electrical conductivity]. The last two changes are terminological; the first concerns a number in the table of requirements. A laboratory reading the standard without the corrigendum sees "250" in that row instead of "25". HOW WE FOUND THIS — because the route is more general than this entry: with PN-EN 872 it turned out (22.09.2026) that the PKN card has, next to the "Additional elements" field, a free-of-charge file with the text of the corrigendum. We then went through our own entries citing standards with an "Ap" element and, by the same route, read the text of further corrigenda, including this one.

AGE OF THE DOCUMENT. The Polish edition dates from 1999 and adopts EN ISO 3696:1995 and ISO 3696:1987. The catalogue card retrieved on 15 September 2026 carries neither the heading "Withdrawn" nor a "Date of withdrawal" row — the standard is in force. It replaced PN-C-06510:1991.

Method principle

Water is classified into one of three grades of purity on the basis of a set of physicochemical parameters, verified by the methods given in the same standard. A higher grade means stricter requirements for all parameters simultaneously — meeting one of them is not enough. The classification applies to the water at the moment of use, not at the moment of production: water of the highest grades deteriorates during storage through dissolution of atmospheric constituents and leaching from the vessel material.

Applications

• Setting requirements for water used in a laboratory's analytical procedures — giving substance to the words "grade 1/2/3 water"
• Qualifying a water purification system and periodically verifying its parameters by the methods in the standard
• Preparing standard solutions and blanks for trace determinations, where the water background governs the limit of quantification
• Reviewing another laboratory's procedure: whether the grade of water cited matches the sensitivity required by the method

Key parameters

Standard

Standard number

PN-EN ISO 3696:1999 wraz z poprawką PN-EN ISO 3696:1999/Ap1:2004 (wersja polska; wprowadza EN ISO 3696:1995 [IDT] oraz ISO 3696:1987 [IDT])

Title (PL)

Woda stosowana w laboratoriach analitycznych -- Wymagania i metody badań

Title (EN)

Water for analytical laboratory use — Specification and test methods

Step-by-step procedure

1. Establish the required grade

   Check in the analytical method which grade of water it requires. Match the grade to the sensitivity of the determination, not to "the highest available".

2. Verify the scope of the standard

   Make sure the determination falls within the scope: analysis of inorganic products. For biological, medical and surfactant analyses this standard does NOT apply.

3. Take the water sample

   Sample directly at the point of use. Storage alters the parameters of high-grade water — testing a sample that has stood for hours does not describe the water used in routine work.

4. Perform the tests per the standard

   Verify the parameters by the methods given in the standard. THE SPECIFIC PARAMETERS AND VALUES ARE NOT GIVEN HERE — they are in its text.

5. Record in the documentation

   Give the purity grade together with a reference to the standard and the date of verification. The equipment manufacturer's declaration does not replace your own measurement.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Laboratory water poses no chemical hazard; the hazard lies in the installation — reverse osmosis systems operate under pressure
• TECHNICAL NOTE: water of very low conductivity may be microbiologically contaminated — the standard sets no microbiological requirements and must not be cited as evidence of microbiological purity
• TECHNICAL NOTE: the parameters of high-grade water deteriorate during storage — a declared grade refers to the moment of use, not the moment of production

Frequently asked questions