ISO 5724:2023

INTERNATIONAL ISO
STANDARD 5724
First edition
2023-08
Jewellery and precious metals —
Determination of very high purity gold
— Difference method using ICP-MS
Joaillerie, bijouterie et métaux précieux — Dosage de l’or de très
haute pureté — Méthode par différence utilisant la spectrométrie de
masse à plasma à couplage inductif (ICP-MS)
Reference number
© ISO 2023
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 1
6 Apparatus . 3
7 Sampling . 3
8 Procedure .3
8.1 General . 3
8.2 Sample preparation . 3
8.3 Sample solution . 3
8.4 Blank solution . 3
8.5 Calibration solutions . 4
8.5.1 Blank calibration solutions . 4
8.5.2 Sample calibration solutions . 4
8.6 Measurement . 4
9 Calculation and expression of the results . 4
9.1 Calibration curves . 4
9.2 Calculation . 5
9.3 Repeatability . 5
10 Test report . 5
Annex A (informative) Procedures for preparation of monoelemental standard stock
solutions . 6
Annex B (informative) Measurement parameters. 9
iii
Foreword
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INTERNATIONAL STANDARD ISO 5724:2023(E)
Jewellery and precious metals — Determination of very
high purity gold — Difference method using ICP-MS
1 Scope
This document specifies an analytical procedure for the determination of very high purity gold with a
nominal content of and above 999,99 ‰ (parts per thousand, by mass), using an inductively coupled
plasma mass spectrometer (ICP-MS).
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 11596, Jewellery and precious metals — Sampling of precious metals and precious metal alloys
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
very high purity gold
gold with a content of and above 999,99 ‰ (parts per thousand, by mass)
4 Principle
The sample is weighed and dissolved in diluted aqua regia to prepare a 2 g/l solution. A blank calibration
curve is used to determine the impurities coming from the preparation procedures. The impurities in
the sample are determined by ICP-MS using internal standards (see Table B.1 for mass number and
internal standards), and the gold content is obtained by subtraction of the total content of impurities in
the sample from 1 000 ‰.
5 Reagents
During the analysis and for the preparation of all solutions, unless otherwise stated, use only reagents
of recognized analytical grade suitable for ICP-MS and ultrapure water with a resistivity around
.
18,2 MΩ cm.
5.1 Hydrochloric acid (HCl), with a mass fraction of approximately 30 % to 37 % of HCl.
5.2 Nitric acid (HNO ), with a mass fraction of approximately 65 % to 70 % of HNO .
3 3
5.3 Sulfuric acid (H SO ), with a mass fraction of approximately 95 % to 98 % of H SO .
2 4 2 4
5.4 Hydrofluoric acid (HF), with a mass fraction of approximately 40 % of HF.
5.5 Diluted hydrochloric acid, consisting of a mix of one volume of hydrochloric acid (5.1) and one
volume of water.
5.6 Diluted nitric acid, consisting of a mix of one volume of nitric acid (5.2) and one volume of water.
5.7 Diluted hydrochloric acid 1:9, consisting of a mix of one volume of hydrochloric acid (5.1) and
nine volumes of water.
5.8 Aqua regia, consisting of a mix of three volumes of hydrochloric acid (5.1) and one volume of
nitric acid (5.2), prepared freshly just before use.
5.9 Diluted aqua regia, consisting of a mix of one volume of aqua regia (5.8) and one volume of water.
5.10 Monoelemental standard stock solutions (1 mg/ml).
Monoelemental standard stock solutions can be either sourced externally or prepared in the laboratory.
NOTE Details on the preparation of monoelemental standard stock solutions can be found in Annex A.
5.11 Gold standard stock solution (20 g/l).
Weigh (10,0 ± 0,1) g of gold (of minimum purity 999,99 ‰) to the nearest 0,01 g. Transfer into a 250 ml
polytetrafluoroethylene beaker and add 50 ml of diluted aqua regia (5.9). Heat at an appropriate
temperature until complete dissolution of the gold. Add 100 ml of aqua regia (5.8), transfer the solution
into a 500 ml volumetric flask, make-up with water to 500 ml and mix thoroughly. Transfer the solution
into a clean plastic bottle.
5.12 Stock solutions and standard solutions.
5.12.1 Concentrated stock solution (10 µg/ml).
Ag, Al, As, Bi, Cd, Cr, Cu, Fe, Ir, Mg, Mn, Na, Ni, Pb, Pd, Pt, Rh, Sb, Se, Sn, Te, Ti, Zn (10 µg/ml each): Pipette
1 ml of each monoelemental standard stock solution (5.10) into a 100 ml volumetric flask, add 20 ml of
diluted aqua regia (5.9), make up with water to 100 ml and mix thoroughly.
5.12.2 Stock solution (0,1 µg/ml).
Ag, Al, As, Bi, Cd, Cr, Cu, Fe, Ir, Mg, Mn, Na, Ni, Pb, Pd, Pt, Rh, Sb, Se, Sn, Te, Ti, Zn (0,1 µg/ml each): Pipette
1 ml of Concentrated stock solution (5.12.1) into a 100 ml volumetric flask, add 20 ml of diluted aqua
regia (5.9), make up with water to 100 ml and mix thoroughly.
If other elements are present in the sample, they shall be added. Elements which shall not be analysed
can be omitted.
5.12.3 Concentrated internal standard solution (10 µg/ml).
Sc, Cs, Re (10 µg/ml each): Pipette 1 ml of each monoelemental standard stock solution of the standard
stock solutions (5.10) into a 100 ml volumetric flask, add 20 ml of diluted aqua regia (5.9), make up with
water to 100 ml and mix thoroughly.
5.12.4 Internal standard solution (0,1 µg/ml).
Sc, Cs, Re (0,1 µg/ml each): Pipette 1 ml of concentrated internal standard solution (5.12.3) into a
100 ml volumetric flask, add 20 ml of diluted aqua regia (5.9), make up with water to 100 ml and mix
thoroughly.
6 Apparatus
6.1 Customary laboratory apparatus
6.2 ICP-MS (inductively coupled plasma mass spectrometer), suitable for the quantification of trace
elements in a solution.
6.3 Analytical balance, with a reading accuracy of 0,1 mg.
7 Sampling
The sampling procedure shall be performed in accordance with ISO 11596.
8 Procedure
WARNING — Suitable health and safety procedures should be followed.
8.1 General
Unless mentioned otherwise, all vessels and reagent bottles shall be made of fluoropolymer (e.g. PTFE,
PFA, FEP). Special care should be taken to keep all vessels and reagent bottles as clean as possible, as
any contamination will impact the results.
8.2 Sample preparation
The sample should be etched, washed and dried before use to remove any surface contamination,
follo
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