EN ISO 18724:2025

SLOVENSKI STANDARD
01-november-2025
Kakovost vode - Določanje kroma (VI), raztopljenega v vodi - Fotometrijska
metoda (ISO 18724:2025)
Water quality - Determination of dissolved chromium(VI) in water - Photometric method
(ISO 18724:2025)
Wasserbeschaffenheit - Bestimmung des gelösten Chrom(VI) in Wasser -
Photometrisches Verfahren (ISO 18724:2025)
Qualité de l'eau - Détermination du chrome dissous(VI) dans l'eau - Méthode
photométrique (ISO 18724:2025)
Ta slovenski standard je istoveten z: EN ISO 18724:2025
ICS:
13.060.50 Preiskava vode na kemične Examination of water for
snovi chemical substances
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18724
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2025
EUROPÄISCHE NORM
ICS 13.060.50
English Version
Water quality - Determination of dissolved chromium(VI)
in water - Photometric method (ISO 18724:2025)
Qualité de l'eau - Détermination du chrome dissous(VI) Wasserbeschaffenheit - Bestimmung des gelösten
dans l'eau - Méthode photométrique (ISO 18724:2025) Chrom(VI) in Wasser - Photometrisches Verfahren (ISO
18724:2025)
This European Standard was approved by CEN on 14 September 2025.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18724:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 18724:2025) has been prepared by Technical Committee ISO/TC 147 "Water
quality " in collaboration with Technical Committee CEN/TC 230 “Water analysis” the secretariat of
which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2026, and conflicting national standards shall be
withdrawn at the latest by April 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document has been prepared under a standardization request addressed to CEN by the European
Commission. The Standing Committee of the EFTA States subsequently approves these requests for its
Member States.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 18724:2025 has been approved by CEN as EN ISO 18724:2025 without any modification.

International
Standard
ISO 18724
First edition
Water quality — Determination of
2025-10
dissolved chromium(VI) in water —
Photometric method
Qualité de l'eau — Détermination du chrome dissous(VI) dans
l'eau — Méthode photométrique
Reference number
ISO 18724:2025(en) © ISO 2025
ISO 18724:2025(en)
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 18724:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Interferences . 1
4.1 General .1
4.2 Sampling, preservation and storage .2
4.3 Interferences in photometry .2
5 Principle . 2
6 Reagents . 3
7 Apparatus . 6
8 Sampling, sample pretreatment and sample storage . 6
8.1 Sampling .6
8.2 Samples with reducing or oxidizing substances .6
8.3 Consideration of the self-absorption of coloured samples .7
8.4 Sample storage .7
9 Procedure . 7
9.1 General .7
9.2 Required concentrations of reagents in the reaction vessel .7
9.3 Required concentrations of reagents in the reaction vessel for measuring the intrinsic
absorption .8
9.4 Measurement .8
9.5 Calibration .8
9.6 Measurement of chromium(VI).8
9.7 Validity check of the calibration function .8
9.8 Calculation considering the ordinate intercept .9
9.9 Calculation with zero adjustment of the photometer .9
10 Expression of results . 9
11 Test report . 10
Annex A (normative) Manual static technique .11
Annex B (normative) Automated static techniques .13
Annex C (normative) Automated dynamic techniques .15
Annex D (normative) Calculations for adjusting reagent concentrations, volumes and flows .22
Annex E (informative) Performance data .25
Bibliography .28

iii
ISO 18724:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see 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, in collaboration with the European Committee for Standardization
(CEN) Technical Committee CEN/TC 230, Water analysis, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
This first edition of ISO 18724:2025 cancels and replaces ISO 11083:1994, ISO 18412:2005 and
ISO 23913:2006.
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
ISO 18724:2025(en)
Introduction
Chromium is an important raw material in the industrial manufacturing process that can contaminate
sources of drinking water (e.g. groundwater, surface water). The most common oxidation states of chromium
are +3 and +6. The hexavalent form (Cr(VI)) is far more toxic and water-polluting than the trivalent form
(Cr(III)).
The chromium content depends on several factors: the availability of chromium in the rock, the weathering
stage and the vicinity. Chromium(VI) occurs mainly in the aerobic environment; under reducing or anaerobic
conditions, chromium(VI) is reduced to chromium(III).
Chromium(VI) occurs naturally in groundwater aquifers, mostly in low concentrations depending on the
occurrence in geological formations, e.g. volcanic rocks.
Other sources of chromium(VI) contamination for the environment are industrial activities, e.g. the
production of textiles, leather tanning or electroplating.
A sensitive and specific photometric method for the determination of hexavalent chromium is the reaction
with 1,5-diphenylcarbazide (DPC). Most of the standardized procedures are based on DPC chemistry,
specifying different reaction conditions.
This procedure describes a uniform procedure that can be used for different photometric measuring
devices, such as static or dynamic techniques. The available techniques vary in sensitivity. The choice of the
technique used for the measurement depends on the chromium(VI) concentration expected in the sample.
The choice of the analytical technique to be used and the needs-based sample preparation (e.g. matrix
elimination) enables the determination of chromium(VI) in concentrations ≥ 0,02 µg/l in raw water, drinking
water, surface water, aqueous eluates, cooling water and treated wastewater, provided that the matrix does
not contain any reducing substances. Typical areas of application for the static techniques as well as FIA
and CFA are samples with chromium(VI) concentrations ≥ 2 µg/l. When using cuvettes with large optical
path lengths, for example > 100 mm, the range of application can be extended to concentrations < 2 µg/l
chromium(VI) (see Annex A, Annex B, Clause C.2 and Clause C.3). When using coupled techniques [e.g. ion
chromatography with post-column reaction (IC-PCR)], chromium(VI) concentrations ≥ 0,02 µg/l can be
determined (see Clause C.4).
v
International Standard ISO 18724:2025(en)
Water quality — Determination of dissolved chromium(VI) in
water — Photometric method
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 photometric determination of dissolved chromium(VI) using
manual, (e.g. hand photometry), automated static (e.g. discrete analyser system) or automated dynamic
[e.g. flow injection analysis (FIA), continuous flow analysis (CFA)] or ion chromatography with post-column
reaction (IC-PCR)] techniques.
The method described in this document is applicable for other matrices, such as leachates from landfills and
raw wastewater, after appropriate method validation.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
the 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 8466-1, Water quality — Calibration and evaluation of analytical methods — Part 1: Linear calibration
function
ISO 8466-2, Water quality — Calibration and evaluation of analytical methods and estimation of performance
characteristics — Part 2: Calibration strategy for non-linear second-order calibration functions
ISO/TS 13530, Water quality — Guidance on analytical quality control for chemical and physicochemical water
analysis
3 Terms and definitions
No terms and definitions are listed in this document.
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/
4 Interferences
4.1 General
The influence of the matrix interferences can vary considerably and depends on the type of sample. Matrix
interferences can be detected by recovery or spiking attempts.

ISO 18724:2025(en)
4.2 Sampling, preservation and storage
Oxidizing agents (e.g. peroxyacetic acid, permanganate), or chlorine, chlorine dioxide, hydrogen peroxide
and ozone used for disinfection in drinking water treatment can interfere. If necessary, their presence can
be checked with potassium iodine starch test paper (6.21) and eliminated by adding sodium sulfite (6.18 or
6.19). Excess sulfite is removed with sodium hypochlorite solution II (6.15).
NOTE 1 Determination of free chlorine and total chlorine is described in ISO 7393-2.
The oxidizing agents listed in Table 1 generally do not interfere if the concentrations are not exceeded.
[7],[8]
Table 1 — Upper limits for mass concentrations of individual disinfectants up to which no
significant interference occurs under the given reaction conditions
Disinfectant Concentration
mg/l
Chlorine 0,6
Chlorine dioxide 0,4
Hydrogen peroxide 0,2
Ozone 0,1
Sulfide in concentrations > 0,2 mg/l interferes with the method.
To avoid changes in chemical valency, ammonium hydroxide or ammonium sulfate solution (6.7) can be
added and pH can be adjusted between 8 and 9,5 (see Clause 8).
NOTE 2 Slow reduction of chromium(VI) can occur in some waters.
Acidic preservation of the samples should be avoided to prevent increased oxidation potential of
[11],[12]
chromium(VI) in acids and the associated reduction of chromium(VI) .
Chromium(III) and other metal ions present in the sample can be removed at the sampling point using a
[13]
cation exchange cartridge (7.7) or precipitated in applying the mixed solution of ammonium hydroxide
and ammonium sulfate (6.7) with aluminium sulfate solution (6.17) as a precipitation aid, and removed by
filtration.
Chromium(VI) losses can occur a few hours after sampling in waters which contain reducing substances.
4.3 Interferences in photometry
Interferences caused by the sample's own colour shall be taken into account (see 8.3).
The pH value of the samples with a value > 9,5 shall be adjusted to a pH value between 8 and 9,5 by adding
sulfuric acid (6.3) before analysis to conform with the requirements of 9.2.
Vanadium ions interfere when their concentration is 10 times higher than that of chromium(VI) and
generally in concentrations > 4 mg/l.
Molybdenum and mercury ions react with the reagent to form a red dye. The intensity of the dye is
significantly lower than that formed with chromium(VI), so that concentrations of up to 200 mg/l
molybdenum and mercury can be tolerated at the adjusted pH value. However, such concentrations are
extremely unlikely in the aqueous matrices covered by this document.
Iron in concentrations > 1 mg/l can produce a yellow colour, but this is not a significant interference at the
specified wavelength. Iron in particulate form should be removed by filtration.
5 Principle
Chromium(VI) reacts with 1,5-diphenylcarbazide to form a red-violet chromium-1,5-diphenylcarbazone
complex. The absorbance of this complex is measured at (540 ± 10) nm (absorbance maximum at 544 nm).

ISO 18724:2025(en)
The reaction can optionally be carried out using manual (e.g. classical photometry) or automated static
(e.g. discrete analysis system) or automated dynamic techniques (e.g. FIA technique, CFA technique or IC).
Annex A provides information on manual static techniques, Annex B on automated static techniques, and
Annex C describes dynamic techniques.
The chromium(VI) concentration is determined by calibration according to ISO 8466-1 or ISO 8466-2.
Control experiments are necessary to ensure the validity of the calibration function. Replicate
determinations can be necessary. The method of standard addition can be applied if matrix interferences
are expected (according to ISO 8466-1:2021).
Other measurement techniques than those described in Annexes A, B, and C may be applied as long as no
changes are made to the sampling and pre-treatment (see Clause 8) or to the required reagent concentrations
in the reaction vessel (see 9.2 and 9.3).
The methods described have been validated. Performance data are given in Annex E.
6 Reagents
6.1 General
If available, reagents of quality “for analysis” (or better quality) that are free of chromium(VI) compounds
are to be used. Carry out weighing with an accuracy of 1 % of the nominal mass, unless stated otherwise.
If necessary, alternative concentrations and volumes to the solutions described in this clause may be
prepared and used. Alternatively, use commercially available solutions with the appropriate and required
specification. The reagents described in this clause are used to set the reagent concentrations specified in
9.2 in the reaction vessel.
6.2 Water, with a specific electrical resistance ≥ 18,2 MΩ cm (25 °C). The water shall not have a measurable
content of chromium(VI) or interfering compounds above one third of the reporting limit of the method, in
accordance with ISO/TS 13530.
6.3 Sulfuric acid, c(H SO ) = 5 mol/l.
2 4
6.4 Ethanol, c(C H OH) = 17,2 mmol/l.
2 5
6.5 Ammonium sulfate, (NH ) SO .
4 2 4
6.6 Ammonium hydroxide solution, c(NH OH) = 13,3 mol/l.
6.7 Mixed solution of ammonium hydroxide and ammonium sulfate.
Dissolve 3,3 g ammonium sulfate (6.5) in approximately 75 ml of water (6.2) in a 100 ml volumetric flask,
add 6,5 ml ammonium hydroxide solution (6.6) and dilute to volume with water (6.2).
The solution contains 250 mmol/l ammonium sulfate and 865 mmol/l ammonium hydroxide.
The solution is stable for one month if stored at room temperature in polyethylene or glass bottles.
6.8 1,5-diphenylcarbazide, C H N O.
13 14 4
6.9 Acidified colour reagent solution.
Dissolve 0,125 g of 1,5-diphenylcarbazide (6.8) in 25 ml of ethanol (6.4), in a 250 ml volumetric flask add
about 190 ml of water (6.2) and 25 ml of sulfuric acid (6.3) and dilute to volume with water (6.2). The solution
contains 2 mmol/l 1,5-diphenylcarbazide, 1 720 mmol/l ethanol and 500 mmol/l sulfuric acid.

ISO 18724:2025(en)
Depending on the analytical technique used, it can be necessary to prepare other concentrations in the
acidified colour reagent solution. This is permissible provided the concentrations in the cuvette specified in
9.2 are observed.
The solution is stable for five days if stored in glass bottles at (5 ± 3) °C.
Depending on the analytical technique used, it can be necessary to prepare the sulfuric acid solution and the
colour reagent solution (from the components DPC, ethanol and water) separately.
NOTE The use of an ultrasonic device can accelerate the dissolution of the 1,5-diphenylcarbazide.
6.10 Reagents for preparing chromium(VI) standard and calibration solutions.
Solutions prepared with potassium dichromate (K Cr O ) or potassium chromate (K CrO ) as well as
2 2 7 2 4
commercially available chromium(VI) solutions of the appropriate and required specification are suitable
for the preparation of chromium(VI) standard and chromium(VI) calibration solutions. In 6.10.2 to 6.11,
examples for the preparation of solutions with potassium dichromate (K Cr O ) are given.
2 2 7
Depending on the chromium(VI) concentration to be expected in the sample, additional standard solutions
with a suitable chromium(VI) concentration may be prepared from the chromium(VI) standard solution I
(6.10.2). It should be noted that the risk of concentration changes due to interaction with the vessel material
increases with decreasing chromium(VI) concentration.
6.10.1 Potassium dichromate, K Cr O .
2 2 7
6.10.2 Chromium(VI) standard solution I, ρ(Cr(VI)) = 1 000 mg/l.
Dry about 1 g of potassium dichromate (6.10.1) at (105 ± 5) °C for 2 h. Cool in the desiccator. In a 100 ml
volumetric flask, dissolve (0,283 ± 0,001) g of dried potassium dichromate in about 80 ml of water (6.2) and
dilute to volume with water (6.2).
The solution is stable for at least 12 months when stored in polyethylene or glass bottle at (5 ± 3) °C.
6.10.3 Chromium(VI) standard solution II, ρ(Cr(VI)) = 10 mg/l.
Pipette 1,0 ml of the chromium(VI) standard solution I (6.10.2) into a 100 ml volumetric flask and dilute to
volume with water (6.2).
The solution is stable for at least three months if stored in polyethylene or glass bottle at (5 ± 3) °C.
6.10.4 Chromium(VI) standard solution III, ρ(Cr(VI)) = 1 mg/l.
Pipette 10 ml of the chromium(VI) standard solution II (6.10.3) into a 100 ml volumetric flask and dilute to
volume with water (6.2).
The solution is stable for at least one month if stored in polyethylene or glass bottle at (5 ± 3) °C.
6.11 Chromium(VI) calibration solutions.
Depending on the expected chromium(VI) concentration in the sample, prepare 5 to 10 calibration solutions
with concentrations distributed as evenly as possible over the calibration range.
For example, for the working range of 10 μg/l to 55 μg/l chromium(VI) or 0,5 μg/l to 5 μg/l chromium(VI),
proceed as follows: pipette the volumes to a series of 100 ml volumetric flasks in accordance with Table 2 or
Table 3 and fill up to the mark with water (6.2).

ISO 18724:2025(en)
Table 2 — Example for the preparation of the calibration solutions for the working range
10 µg/l to 55 µg/l of chromium
Volume of chromium(-
VI) standard solution II 100 µl 150 µl 200 µl 250 µl 300 µl 350 µl 400 µl 450 µl 500 µl 550 µl
(6.10.3) in 100 ml
ρ(Cr(VI)) 10 µg/l 15 µg/l 20 µg/l 25 µg/l 30 µg/l 35 µg/l 40 µg/l 45 µg/l 50 µg/l 55 µg/l
Table 3 — Example for the preparation of the calibration solutions for the working range 0,5 µg/l to
5 µg/l of chromium
Volume of chromium(VI)
standard solution III 50 µl 100 µl 150 µl 200 µl 250 µl 300 µl 350 µl 400 µl 450 µl 500 µl
(6.10.4) in 100 ml
ρ(Cr(VI)) 0,5 µg/l 1 µg/l 1,5 µg/l 2 µg/l 2,5 µg/l 3 µg/l 3,5 µg/l 4 µg/l 4,5 µg/l 5 µg/l
Prepare the calibration solutions on the day of use.
6.12 Sodium hydroxide, NaOH.
6.13 Sodium hydroxide solution.
Dissolve 20 g sodium hydroxide (6.12) in 100 ml water (6.2).
6.14 Sodium hypochlorite solution I, w(NaOCl) = 10 %.
6.15 Sodium hypochlorite solution II, w(NaOCl) = 0,7 %.
Dissolve 70 ml sodium hypochlorite solution I (6.14) in 1 000 ml water (6.2).
The solution is stable for five days if stored in amber glass bottles at (5 ± 3) °C.
6.16 Aluminium sulfate 18-hydrate, Al (SO ) ·18H O.
2 4 3 2
6.17 Aluminium sulfate solution.
Dissolve 247 g of aluminium sulfate 18-hydrate (6.16) in 1 000 ml of water (6.2).
6.18 Sodium sulfite, Na SO .
2 3
6.19 Sodium sulfite solution.
Dissolve 11,8 g of sodium sulfite (6.18) in about 80 ml of water (6.2) in a 100 ml volumetric flask and dilute
to volume with water (6.2).
The solution can be kept for a week.
6.20 Sulfite test paper.
6.21 Potassium iodine starch test paper.
6.22 Blank solution.
Fill a volumetric flask (e.g. 100 ml) with water (6.2).

ISO 18724:2025(en)
6.23 Solution for measuring the intrinsic colour of samples.
Place 25 ml of ethanol (6.4) and about 190 ml of water (6.2), add 25 ml of sulfuric acid (6.3) in a 250 ml
volumetric flask and dilute to volume with water (6.2).
The solution contains 1 720 mmol/l ethanol and 500 mmol/l sulfuric acid.
The solution is stable for two months if stored at (5 ± 3) °C.
Depending on the analytical technique used, it can be necessary to prepare other concentrations in the
solution to measure the inherent colour. This is permissible provided the concentrations in the cuvette
specified in 9.3 are observed.
7 Apparatus
All materials used shall not affect the chromium(VI) concentration.
Use the usual laboratory equipment as follows.
7.1 Membrane filtration device with 0,45 µm membrane filter.
7.2 Automatic pipettes, nominal volume, e.g. between 50 µl and 1 000 µl, 2 ml, 5 ml and 10 ml.
7.3 Volumetric pipettes, nominal volume, e.g. 25 ml, 40 ml or 50 ml.
7.4 Volumetric flask, nominal volume, e.g. 100 ml and 1 000 ml.
7.5 Analytical balance, suitable for weighing from 0,000 1 g.
7.6 pH meter.
7.7 Cation exchanger in the Na form (cartridge).
8 Sampling, sample pretreatment and sample storage
8.1 Sampling
Use clean containers for sampling [e.g. glass, polyethene, polypropene, polytetrafluoroethene (PTFE)].
Immediately after sampling, add 1 ml of the ammonium hydroxide/ammonium sulfate solution (6.7) to
100 ml of sample and mix. The ammonium hydroxide/ammonium sulfate solution (6.7) may also be placed
in the sample container before sampling. The pH shall be ≥ 8. If the pH is < 8, adjust the pH by adding sodium
hydroxide solution (6.13).
Other volumes may be used while maintaining the ratio of sample to ammonium hydroxide/ammonium
sulfate solution.
If necessary, filter the sample with a 0,45 µm membrane to reduce the risk of biodegradation and to remove
particulate matrix components.
NOTE In the presence of lead, barium and silver ions, insoluble chromates can be formed. The chromium(VI)
contained therein cannot be determined in the filtrate.
The sample containers shall not be completely filled.
8.2 Samples with reducing or oxidizing substances
Groundwater samples with reducing substances usually do not contain chromium(VI).

ISO 18724:2025(en)
Organic substances present, e.g. in surface water, can be removed by flocculation with aluminium sulfate
solution (6.17) and subsequent filtration. For flocculation, add 1 ml ammonium hydroxide/ammonium
sulfate solution (6.7) and 0,1 ml aluminium sulfate solution (6.17) to 100 ml sample. Other volumes may be
used while maintaining the sample to reagent ratio.
If necessary, remove chromium(III) during storage to prevent oxidation of chromium(III) to chromium(VI)
as follows.
— Add 0,1 ml sodium sulfite solution (6.19) to 100 ml sample, pretreated in accordance with 8.1.
— Check for sulfite with the sulfite test paper (6.20). If no sulfite is present, add more sodium sulfite solution
until an excess is obtained.
Alternatively, proceed as follows.
— Pass the sample over a cation exchange cartridge (7.7) at a constant flow rate of 1 ml/min to 1,5 ml/min.
— Discard the first 5 ml of the eluate.
8.3 Consideration of the self-absorption of coloured samples
Systematic interferences due to self-absorption shall be compensated. To do this, measure the sample
again using solution (6.23) instead of solution (6.9). The result of self-absorption is to be taken into account
according to 9.8, Formula (2) or according to 9.9, Formula (4).
When applying ion chromatography or a dialyzer for CFA [e.g. with chromium(VI) concentrations > 50 µg/l],
it is not necessary to pay attention to the self-absorption, since the chromium(VI) can be separated on the
separation column or by the dialyzer from the sample matrix. Discrete analysis systems can be programmed
to perform colour correction automatically.
8.4 Sample storage
Drinking water and groundwater samples may be stored for two weeks, surface water samples and
wastewater samples from the sewage treatment plant effluent for one week.
Raw wastewater (e.g. sewage treatment plant inflow) should be analysed immediately after sampling.
9 Procedure
9.1 General
The reaction rate increases with increasing reaction temperature. The reaction shall be carried out at a
constant temperature between 20 °C and 50 °C.
Annex D shall be used to establish calculations for setting the required volumes, reagent concentrations that
apply static and dynamic techniques. Unknown variables can be calculated by rearranging Formula (D.1)
and Formula (D.2) appropriately.
9.2 Required concentrations of reagents in the reaction vessel
The reagent concentrations specified below are indispensable for the application of this method and shall be
set with an accuracy of ±10 % in the reaction vessel or in the reaction coil or the reaction cuvette.
1,5-diphenylcarbazide: 0,50 mmol/l
Ethanol: 428 mmol/l
Sulfuric acid: 125 mmol/l
ISO 18724:2025(en)
9.3 Required concentrations of reagents in the reaction vessel for measuring the intrinsic
absorption
The reagent concentrations specified below are indispensable for the application of this method and shall be
set with an accuracy of ±10 % in the reaction vessel or in the reaction coil or the reaction cuvette.
Ethanol: 428 mmol/l
Sulfuric acid: 125 mmol/l
9.4 Measurement
Set up the analytical equipment to be used in accordance with Annex A, Annex B, Annex C and the
manufacturer's instructions.
9.5 Calibration
Measure the chromium(VI) calibration solutions (6.11). The measured absorption is proportional to the
concentration of chromium(VI).
Establish a calibration function in accordance with ISO 8466-1 or ISO 8466-2 after the initial setup of the
analysis system and at intervals afterwards.
Determine the regression function in accordance with ISO 8466-1 or ISO 8466-2 from the data obtained.
Subsequently, verify the continuing validity of the established calibration function (9.7).
NOTE Generally, the calibration method is not restricted to a calibration strategy covering a single concentration
decade. When calibrating over a larger range than one concentration decade, a loss of accuracy, compared to that
specified in ISO 8466-1:2021, Annex A or ISO 8466-2, can occur.
9.6 Measurement of chromium(VI)
After establishing the calibration function, measure the pretreated sample (see Clause 9).
If necessary, membrane filter the sample to remove undissolved materials prior to injection into the analyser.
Possible contamination of the sample from the membrane shall be avoided (e.g. rinse the membrane with a
small sample volume and discard the first fraction of the filtrate).
Samples with a pH value > 9,5 should be adjusted to a pH value between 8 and 9,5 by adding sulfuric acid
(6.3). If the chromium(VI) concentration exceeds the calibration range, dilute the sample and re-analyse.
If the chromium(VI) concentration is below the calibration range, establish a separate calibration function
for the lower working range, if necessary. If required, the technology shall be changed.
Matrix interferences can be solved according to Clause 8.
If matrix interference from the sample is expected, use the standard addition procedure to confirm the result.
If the filtrate is coloured, see 8.3.
Measure the blank solution (6.22) in the same way.
9.7 Validity check of the calibration function
To check the validity of the calibration function, measure independent standard solutions of different
concentrations in the lower and upper third of the working range. The recovery rate shall be between 90 %
and 110 % of the nominal value. If necessary, recalibrate. If necessary, carry out repeat determinations.

ISO 18724:2025(en)
9.8 Calculation considering the ordinate intercept
Calculate the mass concentration ρ of chromium(VI) in the sample in micrograms per litre or milligrams per
litre using the respective measured variable (e.g. absorption, peak area), in accordance with ISO 8466-1 or
ISO 8466-2.
Formula (1) is used for linear calibration according to ISO 8466-1.
ya−
ρ= (1)
b
where
ρ is the mass concentration of chromium(VI), in micrograms per litre (µg/l) or in milligrams per
litre, (mg/l);
y is the readout (or signal) of the sample;
a is the ordinate intercept of the calibration function;
b is the slope of the calibration line.
In the case of coloured samples, use Formula (2):
yy−−a
ρ= (2)
b
where y is the readout (or signal) of the sample when using solution (6.23) instead of solution (6.9).
All dilution steps shall be taken into account.
9.9 Calculation with zero adjustment of the photometer
If the photometer is adjusted with a reagent blank solution, Formula (3) is used.
y
1n
ρ= (3)
b
where
y is the readout (or signal) of the sample after zero adjustment of the photometer adjustment;
1n
b is the slope of the calibration line.
In the case of coloured samples, use Formula (4):
yy−
12nn
ρ= (4)
b
where y is the readout (or signal) of the sample when using solution (6.23) instead of solution (6.9) after
2n
zero adjustment of the photometer.
10 Expression of results
The analytical results obtained when applying this method are subject to measurement uncertainty to be
considered in the interpretation of the results.
Results are reported in micrograms per litre or milligrams per litre to no more than two significant figures.
EXAMPLE
Chromium(VI) 15 µg/l
Chromium(VI) 1,8 µg/l
ISO 18724:2025(en)
The results can also be given as chromate. To convert, the results are multiplied as follows:
2−
— ρ(Cr(VI)) is multiplied by 2,231 to get ρ(CrO );
2−
— ρ(CrO ) is multiplied by 0,448 3 to get ρ(Cr(VI)).
11 Test report
The analysis report shall contain at least the following information:
a) the test method used, together with a reference to this document, i.e. ISO 18724:2025;
b) the identity of the water sample;
c) the results in accordance with Clause 10;
d) a description of sample pretreatment, if relevant;
e) any deviation from this method;
f) a report of all circumstances that can have affect the results;
g) the date of the test.
ISO 18724:2025(en)
Annex A
(normative)
Manual static technique
A.1 General
The general information of Clauses 1 to 3 and the requirements of Clauses 4 to 11 remain valid to the manual
static technique.
A.2 Scope of manual static technique
See Clause 1 and the following.
The manual static technique is usually suitable for determining chromium(VI) in concentrations of ≥ 2 µg/l.
When using cuvettes with large optical path lengths, e.g. > 100 mm, the range of application can be extended
for the determination of concentrations < 2 µg/l chromium(VI).
A.3 Principle of the manual static technique
The chromium(VI)-containing sample is manually mixed with the acidified colour reagent solution (6.9), e.g.
mixed in a volumetric flask in which the chromium(VI) reacts with 1,5-diphenylcarbazide. The absorbance
of the dye formed is measured photometrically.
At room temperature, a reaction time of 3 min to 10 min shall be observed before the measurement. The
intensity of the staining persists for at least 30 min.
A.4 Interferences
See Clause 4.
A.5 Reagents
See Clause 6.
A.6 Equipment
See Clause 7 and the following.
A.6.1 Photometric measurement system, generally consisting of:
A.6.1.1 Photometer or filter photometer capable to monitor, (540 ± 10) nm.
A.6.1.2 Cuvettes with appropriate path length.
A.6.1.3 R
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