ISO 23697-2:2023

Water quality - Determination of total bound nitrogen (ST-TNb) in water using small-scale sealed tubes - Part 2: Chromotropic acid colour reaction

This document specifies a method for the determination of total bound nitrogen (ST-TNb) in water of various origins: groundwater, surface water and wastewater, in a measuring range of concentration generally between 0,5 mg/l and 150 mg/l of ST-TNb using the small-scale sealed tube method. Different measuring ranges of small-scale sealed tube methods can be required. The measuring ranges can vary depending on the type of small-scale sealed tube method of different manufacturers. It is up to the user to choose the small-scale sealed tube test with the appropriate application range or to adapt samples with concentrations exceeding the measuring range of a test by preliminary dilution. NOTE The results of a small-scale sealed tube test are most precise in the middle of the application range of the test. All small-scale sealed tube methods are based on a heated alkaline potassium persulfate oxidation in a heating block at 100 °C and different digestion times are applicable. Chromotropic colour reaction is applied, depending on the typical operating procedure of the small-scale sealed tube used, see Clause 9.

Qualité de l’eau — Détermination de l’azote lié total (ST-TNb) dans l’eau à l’aide de tubes fermés à petite échelle — Partie 2: Réaction colorimétrique à l’acide chromotropique

General Information

• Status: Published
• Publication Date: 01-Feb-2023
• ICS: 13.060.50 - Examination of water for chemical substances
• Technical Committee: ISO/TC 147/SC 2 - Physical, chemical and biochemical methods
• Drafting Committee: ISO/TC 147/SC 2/WG 81 - TN, nitrate, ammonium using cell tests
• Current Stage: 6060 - International Standard published
• Start Date: 02-Feb-2023
• Due Date: 18-Jul-2022
• Completion Date: 02-Feb-2023

Overview

ISO 23697-2:2023 specifies a laboratory method for determining total bound nitrogen (ST-TNb) in water (groundwater, surface water, wastewater) using small-scale sealed tubes and the chromotropic acid colour reaction. The method covers a typical measuring range of 0.5 mg/L to 150 mg/L ST-TNb and is based on heated alkaline potassium persulfate oxidation in a heating block at 100 ± 5 °C. Nitrate produced by oxidation reacts with chromotropic acid to produce a measurable colour read by a photometer or spectrophotometer.

Key topics and technical requirements

• Principle: Alkaline potassium persulfate oxidation converts organic and inorganic nitrogen to nitrate; nitrate is detected by the chromotropic acid colour reaction.
• Digestion conditions: Heating block at (100 ± 5) °C; recommended digestion time noted in Clause 9 (typical 30 min for chromotropic reaction).
• Detection wavelengths: Filter photometer ~430 ± 5 nm; spectrophotometer ~410 ± 5 nm.
• Measuring range: Generally 0.5–150 mg/L ST-TNb; manufacturers may provide different small-scale tube ranges. Users must select tubes appropriate for sample concentration or dilute samples as needed.
• Reagents and reagent ranges (mass fraction ranges in digestion/sample mix):
  • Sulfuric acid: 50–70%
  • Sodium hydroxide: 0.3–2%
  • Potassium persulfate: 0.8–3%
  • Sodium metabisulfite: 0.01–5%
  • Chromotropic acid: 0.01–1%
• Apparatus: Photometer/spectrophotometer, convective heating block (100 ± 5 °C) with carousel for small tubes, digestion tubes, calibrated pipettes, homogenizer.
• Interferences: Metal ions, chloride and high organic load (COD) can affect results. Example maximum tolerable concentrations (guidance): Na+/K+ ~500 mg/L, Cl- ~500 mg/L, COD ~200 mg/L, Ca2+ ~50 mg/L, Ag+ ~100 mg/L. Users should identify and mitigate interferences (e.g., sample dilution, spiking checks).
• Quality control: Verify calibration with reference materials (potassium nitrate, ammonium chloride, EDTA standards), use blanks, and follow ISO/IEC 17025 practices.

Applications and users

• Routine monitoring of water quality and eutrophication risk (total nitrogen assessment).
• Regulatory compliance testing, environmental monitoring programs, and wastewater treatment plant process control (denitrification efficiency).
• Laboratories performing chemical analysis, manufacturers of small-scale sealed tube kits, and water quality consultants.
• Useful where rapid, small-volume sealed-tube testing is preferred for throughput or field-proximate analysis.

Related standards

• ISO 5667-1, ISO 5667-3, ISO 5667-10 (sampling and sample handling)
• ISO 8466-1 (calibration and analytical method evaluation)
• ISO/IEC 17025 (laboratory competence)
• ISO 11905-1 (related digestion methods; noted differences)

Keywords: ISO 23697-2:2023, water quality, total bound nitrogen, ST-TNb, small-scale sealed tubes, chromotropic acid, potassium persulfate oxidation, heating block, photometer, spectrophotometer.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23697-2
First edition
2023-02
Water quality — Determination of
total bound nitrogen (ST-TN ) in water
b
using small-scale sealed tubes —
Part 2:
Chromotropic acid colour reaction
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Interferences . 2
6 Sampling and sample preparation . 3
7 Reagents . 3
8 Apparatus . 3
9 Oxidation and colour reactions principle . 4
9.1 General . 4
9.2 Chromotropic acid colour reaction. 4
10 Quality control . 5
10.1 V erification of the calibration . 5
10.2 System and interferences checks . 5
11 Calculation . 5
12 Expression of results .6
13 Test report . 6
Annex A (informative) Performance data . 7
Bibliography . 8
iii
Foreword
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 147, Water quality, Subcommittee SC 2,
Physical, chemical and biochemical methods.
A list of all parts in the ISO 23697 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
Total bound nitrogen (ST-TN ) includes all inorganic forms, nitric nitrogen, nitrous nitrogen,
b
ammoniacal nitrogen and all organic forms of nitrogen in a water sample. Nitrate in particular is one
of the fundamental nutrients of algae, which, in the presence of sufficient quantities of phosphates and
other favourable conditions, determine the eutrophication of water. The main sources of nitrogen are
both natural and anthropogenic. Of the anthropogenic sources, particular importance can be attributed
to domestic wastewater and the use of fertilizers in agriculture. Reduction of nitrogen load is carried
out in domestic and industrial wastewater treatment plants through special denitrification processes.
The presence of significant concentrations of nitrogenous substances in water can indicate pollution
and pose a risk to health and environment.
v
INTERNATIONAL STANDARD ISO 23697-2:2023(E)
Water quality — Determination of total bound nitrogen
(ST-TN ) in water using small-scale sealed tubes —
b
Part 2:
Chromotropic acid colour reaction
WARNING — Persons using this document should be familiar with normal laboratory practice.
This document does not purport to address all of the safety problems, if any, associated with its
use. It is the responsibility of the user to establish appropriate safety and health practices.
IMPORTANT — It is absolutely essential that tests conducted in accordance with this document
be carried out by suitably qualified staff.
1 Scope
This document specifies a method for the determination of total bound nitrogen (ST-TN ) in water of
b
various origins: groundwater, surface water and wastewater, in a measuring range of concentration
generally between 0,5 mg/l and 150 mg/l of ST-TN using the small-scale sealed tube method. Different
b
measuring ranges of small-scale sealed tube methods can be required.
The measuring ranges can vary depending on the type of small-scale sealed tube method of different
manufacturers.
It is up to the user to choose the small-scale sealed tube test with the appropriate application range or
to adapt samples with concentrations exceeding the measuring range of a test by preliminary dilution.
NOTE The results of a small-scale sealed tube test are most precise in the middle of the application range of
the test.
All small-scale sealed tube methods are based on a heated alkaline potassium persulfate oxidation in
a heating block at 100 °C and different digestion times are applicable. Chromotropic colour reaction is
applied, depending on the typical operating procedure of the small-scale sealed tube used, see Clause 9.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 5667-1, Water quality — Sampling — Part 1: Guidance on the design of sampling programmes and
sampling techniques
ISO 5667-3, Water quality — Sampling — Part 3: Preservation and handling of water samples
ISO 5667-10, Water quality — Sampling — Part 10: Guidance on sampling of waste water
ISO 8466-1, Water quality — Calibration and evaluation of analytical methods — Part 1: Linear calibration
function
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
small-scale sealed tube
glass tube commercially available on the market prefilled by manufacturers with reagent(s) to develop
a colour to be read by a photometer or spectrophotometer
3.2
digestion tube
glass tube used to heat and digest samples in an alkaline solution
Note 1 to entry: This tube is capable to keep temperature and pressure generated by the reagents and by the
heating block.
3.3
sealed tube-total bound nitrogen
ST-TN
b
sum of organically bound and inorganically bound nitrogen present in water or suspended matter
measured under the conditions of this method
4 Principle
The sample to be analysed is heated under specific condition (see Clause 9) in a strongly alkaline
solution with potassium persulfate. Organic and inorganic nitrogen are oxidized to nitrate.
Nitrate ions react in acidic solution with chromotropic acid which is read at maximum absorption by a
photometer or a spectrophotometer.
The met
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