ISO 13825:2025

International
Standard
ISO 13825
First edition
Petroleum and related products —
2025-10
Determination of arsenic in
crude petroleum using atomic
fluorescence spectrometry
Pétrole et produits connexes — Détermination de l'arsenic dans
le pétrole brut par spectrométrie de fluorescence atomique
Reference number
© ISO 2025
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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 . 1
5 Reagents . 1
6 Apparatus . 3
7 Sampling and sample preparation . 4
7.1 Sampling .4
7.2 Mixing samples .4
7.3 Preparation of test solution .4
8 Instrumental set-up . 5
9 Procedure . 5
10 Calculation using the calibration curve . 6
11 Expression of results . 6
12 Precision . 6
12.1 Repeatability, r.6
12.2 Reproducibility, R .6
13 Test report . 7
Annex A (normative) Homogenization of samples . 8
Bibliography . 9

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 28, Petroleum and related products, fuels and
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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
Arsenic compounds have a wide range of contents in crude oil. During the refining process, it is easy for
the arsenic compounds to be adsorbed by the catalyst, leading to catalyst poisoning. It has been confirmed
−3
that arsenic compounds in crude oil at a concentration level of 10 mg/kg can cause permanent poisoning
and deactivation of platinum-based catalysts for reforming. The presence of arsenic compounds can directly
lead to contamination of straight run fractions of oil products such as naphtha or heavy fraction oil.
The determination of arsenic content plays an important role in guiding the removal of arsenic, determining
the arsenic tolerance of catalysts, and extending the service life of catalysts.
This document provides a method to determine the arsenic content in crude oil. Since arsenic occurs
naturally in organic and inorganic compounds, a closed state is maintained during the digestion process in
order to fully decompose all arsenic containing compounds and avoid losses.

v
International Standard ISO 13825:2025(en)
Petroleum and related products — Determination of arsenic
in crude petroleum using atomic fluorescence spectrometry
WARNING — It is absolutely essential that tests conducted according to this document be carried out
by suitably trained staff. 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 and to ensure compliance with any national regulatory conditions.
1 Scope
This document specifies a method for the determination of arsenic content in crude oil pretreated by a
closed microwave digestion method by atomic fluorescence spectrometry.
The precision statement of this test method was determined in an interlaboratory study and is valid for
samples with an arsenic content between 0,35 mg/kg and 3,57 mg/kg. The test method can also be applied
to samples with either a higher or lower arsenic content, however, no precision data has been determined at
levels outside of this range.
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 3696, Water for analytical laboratory use — Specification and test methods
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/
4 Principle
After adding acid to the sample and pre-treating it by a closed microwave digestion method, adding
thiourea-ascorbic acid mixed solution to reduce the arsenic (V) to arsenic (III) in the solution, add sodium
tetrahydroborate solution under acidic conditions to convert the arsenic into gaseous arsenic hydrogen. At
the same time, when the hydrogen generated by the reaction of sodium borohydride and acid is brought into
the atomizer by argon, the hydrogen is ignited and forms an argon hydrogen flame, and arsine leads into
the atomizer by means of argon gas flow. The arsenic content in the sample is quantitatively measured by
atomic fluorescence spectrometry.
5 Reagents
It is important to use high purity reagents in all cases. Use only reagents of recognized analytical grade,
unless otherwise specified.
5.1 Water, complying with grade 1 as defined in ISO 3696, for all sample preparation and dilutions.
5.2 Nitric acid, ρ(HNO ) = 1,4 g/ml.
NOTE Nitric acid is available both as ρ(HNO ) = 1,40 g/ml, approximately mass fraction of 65 % and
ρ(HNO ) = 1,42 g/ml, approximately mass fraction of 69 %. Both are suitable.
5.3 Hydrogen peroxide, H O , approximately mass fraction of 30 %.
2 2
5.4 Hydrochloric acid, ρ(HCl) = 1,16 g/ml, approximately mass fraction of 36 %.
5.5 Hydrochloric acid solution (1 + 1), obtained by mixing one volume of hydrochloric acid (5.4) with
one volume of water (5.1).
5.6 Hydrochloric acid solution, ρ(HCl) = 50 ml/l. This solution is obtained by adding 50 ml of hydrochloric
acid (5.4) to 250 ml of water (5.1) and making up a final volume of 1 000 ml with water (5.1).
5.7 Sodium hydroxide, NaOH.
5.8 Sodium tetrahydroborate, NaBH , kept dry and stored in a cool, dark place.
5.9 Sodium tetrahydroborate solution, ρ(NaBH ) = 10 g/l. This solution is obtained by weighing 2,5 g of
sodium hydroxide (5.7) and dissolving it in water (5.1), adding 5,0 g of sodium tetrahydroborate (5.8) after
it is completely dissolved, and diluting to 500 ml with water.
Prepare appropriate quantities on day of use, smaller volumes can be prepared on a pro rata basis.
Do not keep in a closed container because of potential pressure build-up due to hydrogen evolution.
The concentration of NaBH is dependent on the hydride generator manifold and flow-rate conditions. See
recommendations from the manufacturer.
5.10 Thiourea, CH N S.
4 2
5.11 Ascorbic acid, C H O .
6 8 6
5.12 Thiourea-ascorbic acid solution, ρ(thiourea-ascorbic acid) = 50 g/l. This solution is obtained by
dissolving 25,0 g of thiourea (5.10) and 25,0 g of ascorbic acid (5.11) in water (5.1) and diluting to 500 ml.
This solution should be prepared on the day of use.
5.13 Reagent blank solution, for each 100 ml, prepare a solution containing 5 ml of hydrochloric acid (5.4)
and 20 ml of thiourea-ascorbic acid solution (5.12) per 100 ml.
The reagent blank solution is run as background.
5.14 Arsenic standard solutions, as follows:
a) Arsenic stock solution A, ρ[As(III)] = 1 000 mg/l. Use a quantitative stock solution with a traceable
arsenic (III) content of (1 000 ± 10) mg/l.
This solution is considered to be stable for at least one year.
Alternatively, use a stock solution prepared from high purity grade chemicals.
b) Arsenic standard solution B, ρ[As(III)] = 10 mg/l. Pipette 1 ml of arsenic stock solution A into a 100 ml
volumetric flask, add 5 ml of hydrochloric acid (5.4) and fill up to the mark w
...