ISO 23697-2:2023

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
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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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This document was prepared by Technical Committee ISO/TC 147, Water quality, Subcommittee SC 2,
Physical, chemical and biochemical methods.
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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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