ISO 25319:2026

• Abstract:

  This method is applicable to a concentration range of 52,80 % mass fraction to 86,40 % mass fraction of the metallic Fe.

• Status: Published
• Publication Date: 04-Nov-2026
• ICS: 73.060.10 - Iron ores
• Technical Committee: ISO/TC 102/SC 2 - Chemical analysis
• Drafting Committee: ISO/TC 102/SC 2/WG 55 - Fe Metal in sponge iron and briquettes
• Current Stage: 6060 - International Standard published
• Start Date: 05-Nov-2026
• Completion Date: 05-Nov-2026

ISO 25319 is an ISO International Standard for determining metallic Fe in reduced iron, including sponge Iron and briquette, by an iron(III) chloride titrimetric method. It is written for laboratories that measure metallic iron content in these materials and need a controlled procedure, calculation rule, and result-checking approach. The document covers sample preparation, the titration sequence, calculation of the metallic iron content, acceptance of duplicate results, and the test report.

What does ISO 25319 specify?

ISO 25319 specifies a titrimetric method for metallic Fe in reduced iron and briquette samples within a metallic Fe concentration range of 52.80 % to 86.40 % by mass. The method uses iron(III) chloride to release metallic iron into solution and potassium dichromate to titrate the ferrous iron formed.

The document is organized into clauses for scope, normative references, terms and definitions, principle, reagents, apparatus, sampling and sample preparation, procedure, expression of results, and test report. It also includes three annexes.

Annex	What it covers
Annex A	Flowsheet for acceptance of test results
Annex B	Calculation of the correction factor F for potassium dichromate
Annex C	Interlaboratory study results, calculations, and statistical assessment

What are the key requirements of ISO 25319?

Sample preparation and test portions

ISO 25319 requires the sample to be dried at or below 105 °C until constant mass, then cooled in a desiccator. For analysis, the laboratory sample must be finer than 160 µm and taken and prepared in accordance with ISO 10835.

This matters because moisture and coarse particles can change the measured metallic Fe content. A dry, fine, representative laboratory sample reduces variation between test portions.

The test portion mass is 1.000 g ± 0.0050 g. The method requires duplicate determinations, and further determinations if needed according to the flowsheet in Annex A.

Titration principle and reagents

ISO 25319 uses iron(III) chloride solution and ammonium chloride to convert the metallic iron in the sample, then titrates the resulting iron(II) with standard potassium dichromate solution. The endpoint is detected with sodium diphenylamine sulfonate or barium diphenylamine sulfonate indicator, which the document treats as equivalent.

This means the analyst must prepare the stated reagents correctly and watch for the permanent purple endpoint. The standard also requires a blank test in parallel with the sample, and a new blank for each new batch of iron(III) chloride solution.

The reagent list includes:

• iron(III) chloride solution 7 %
• standard potassium dichromate solution 0.01667 mol/L
• sulfuric acid-phosphoric acid mixed acid
• sodium diphenylamine sulfonate solution
• barium diphenylamine sulfonate solution
• ammonium chloride solution
• ammonium iron(II) sulfate hexahydrate for Annex B

Procedure

ISO 25319 instructs the analyst to add 100 mL of iron(III) chloride solution and 50 mL of ammonium chloride solution to the test portion, then stir with a magnetic stirrer for at least 30 minutes at high speed, around 600 to 700 rpm. After decomposition, the method calls for adding the mixed acid, indicator, and distilled water before titration.

In practice, the stirring time, stirring speed, and endpoint recognition affect the result. The blank run is part of the same procedure and is used to correct the titration volume.

Calculation and result acceptance

ISO 25319 calculates metallic iron from the difference between the sample titration volume and the blank titration volume, together with the dichromate concentration, the correction factor F, the sample mass, the molar mass of iron, and the stoichiometric factor given in the principle. The standard does not give the calculation as a narrative estimate - it requires use of the stated formula.

The correction factor F is not assumed; Annex B gives the procedure for determining it using ammonium iron(II) sulfate hexahydrate. That matters because the actual concentration of the dichromate titrant must be verified for accurate results.

Clause 9.2 links the reported result to repeatability and reproducibility data. It tells users to compare independent replicate results with the independent duplicate limit Rd using the acceptance procedure in Annex A. In practice, this is the control step that tells the laboratory whether the two determinations are close enough to report.

Test report

ISO 25319 requires the test report to include the laboratory identity, test report date, sample name, test date, reference to the standard, sample identification details, the test result to one decimal place, any observations that may have influenced the result, any optional or non-specified operations, and reference to verification activities.

For users, this means the report must show enough detail to trace the measurement and understand anything that may have affected the value, such as drying time or the titration endpoint.

What terms does ISO 25319 define?

ISO 25319 uses the terms and definitions from ISO 11323 for this subject area. The key terms used in the method are:

• metallic Fe - the iron in the sample that the method measures as the analyte of interest.
• reduced iron - the material type covered by the standard, including sponge Iron and briquette.
• briquette - a compacted reduced-iron product that can be tested by this method.
• laboratory sample - the prepared sample used for analysis after sampling and size reduction.
• test portion - the weighed portion taken from the laboratory sample for one determination.
• blank test - a parallel run without sample, used to correct the titration volume.
• correction factor F - the factor used to adjust the potassium dichromate concentration in the calculation.
• independent duplicate limit Rd - the limit used in Annex A to judge whether duplicate results are acceptable.

Who uses ISO 25319?

ISO 25319 is used by chemical analysis laboratories that test direct reduced iron products. It is also relevant to iron and steel producers, quality control teams, and buyers or inspectors who need a reported metallic Fe content for reduced iron or briquette.

In practice, these users apply the standard for routine quality control, incoming inspection, product release, and dispute checking. The method is especially useful when a consistent wet-chemical result is needed for materials in the specified metallic Fe range.

Which standards are used with ISO 25319?

ISO 25319 refers to several companion standards that support the method:

• ISO 385 - specifies burettes, which supports the titration equipment.
• ISO 1042 - specifies one-mark volumetric flasks, which supports solution preparation.
• ISO 10835 - gives sampling and sample preparation for direct reduced iron and hot briquetted iron.
• ISO 11323 - provides the vocabulary used for the terms and definitions in this document.

These references matter because they set the glassware and sample-preparation framework that the method depends on.

What does the ISO 25319 document contain?

ISO 25319 contains the full wet-chemical procedure for metallic Fe determination, including reagent preparation, apparatus requirements, sample drying, size reduction, duplicate testing, decomposition with iron(III) chloride, titration with potassium dichromate, and the calculation of metallic iron content. It also gives the blank correction step and the acceptance approach for duplicate results.

The document includes the formula for calculating metallic iron, a precision table, and the rules for expressing and reporting results. Annex B provides the correction-factor calculation for the dichromate solution using ammonium iron(II) sulfate hexahydrate.

Annex C presents interlaboratory study data, calculation work, and statistical assessment for the method. That material supports users who need to understand how the method behaved in collaborative testing and how the precision values were derived.

International
Standard
ISO 25319
First edition
Determination of metallic Fe in
2026-11
reduced iron (sponge Iron) and
briquette — Iron (III) chloride
titrimetric method
Dosage du Fe métallique dans le fer préréduit (fer spongieux) et
dans la briquette — Méthode titrimétrique au chlorure de fer
(III)
Reference number
© ISO 2026
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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 . 2
7 Sampling and sample preparation . 3
7.1 Sample preparation .3
7.2 Laboratory sample .3
8 Procedure . 3
8.1 Number of determinations .3
8.2 Preparation of test portions .3
8.3 Determination .3
9 Expression of results . 4
9.1 Calculation of the metallic iron content .4
9.2 General treatment of result .4
9.2.1 Repeatability and Reproducibility .4
9.2.2 Determination of analytical result .5
10 Test report . 5
Annex A (Normative) Flowsheet of the procedure for the acceptance of test results . 6
Annex B (Normative) Calculation F correction factor of potassium dichromate . 7
Annex C (Informative) Results obtained, calculations performed and statistical assessment . 8
Bibliography . 14

iii
Foreword
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iv
International Standard ISO 25319:2026(en)
Determination of metallic Fe in reduced iron (sponge Iron)
and briquette — Iron (III) chloride titrimetric method
1 Scope
This method is applicable to a concentration range of 52,80 % mass fraction to 86,40 % mass fraction of the
metallic Fe.
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 385, Laboratory glassware — Burettes
ISO 10835, Direct reduced iron and hot briquetted iron — Sampling and sample preparation
ISO 11323, Iron ore and direct reduced iron — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11323 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
0 +2
Iron present in the oxidation state Fe in the sample is oxidized to Fe by the action of FeCl . This reaction is
catalysed by ammonium chloride according to the following reaction:
2FeCl + Fe  3FeCl
3 2
2+
Fe is oxidized and titrated with potassium dichromate solution using the sodium diphenylamine sulfonate
indicator (or barium diphenylamine sulfonate) according to the following reaction:
2+ 2- + 3+ 3+
6Fe + Cr O + 14H  6Fe + 2Cr + 7H O
2 7 2
5 Reagents
During the analysis, use only reagents of recognized analytical reagent grade and only distilled water or
water of equivalent purity.
5.1 Iron (III) chloride solution 7 %
Add 117 g of iron (III) chloride hexahydrate to a 2 000 ml container holding 600 ml of water and agitate until
it is completely dissolved. Dilute with water to a volume of 1 000 ml.
NOTE For better dissolution of the iron (III) chloride, use hot water (60 °C to 80 °C). The ideal condition is a
complete dissolution of the salt resulting in a translucent solution.
5.2 Standard potassium dichromate solution, 0,016 67 mol/L
Add 4,903 1 g of dried potassium dichromate to a 2 000 ml container holding 600 ml of water and agitate
until it is completely dissolved. Dilute with water to a volume of 1 000 ml.
Prepare a standard potassium dichromate solution following the directions in Annex B
5.3 Mixed acid
Add 700 ml of water to a 2 000 ml beaker. Place the beaker in a cool place and add slowly with stirring
150 ml of phosphoric acid, 1,7 b/ml. Allow cooling. Add slowly with stirring 150 ml of sulfuric acid, 1,84 g/
ml and allow cooling (it is optional: placing the beaker in an ice-water bath). Dilute this mixture to 1 000 ml
with stirring.
5.4 Sodium diphenylamine sulfonate 2 g/l
Dissolve 0,2 g of powdered sodium diphenylamine sulfonate (C H NHC H SO Na) in a small volume of water
6 5 6 4 3
and dilute to 100 ml. Store this solution in a brown glass bottle.
5.5 Barium diphenylamine sulfonate 2 g/l
Dissolve 0,2 g of powdered barium diphenylamine sulfonate ((C H NHC H SO ) Ba)
...