🍎 Nitrogen content of liquid whole or skimmed milk — semi-micro rapid routine method: smaller test portion digested in 250 ml tubes in a heating block with concentrated sulfuric acid, hydrogen peroxide (about 30 g/100 ml) and potassium sulfate with a catalyst (e.g. tablets with copper(II) sulfate and titanium dioxide, without selenium or mercury), liberation of ammonia with excess NaOH, steam distillation into boric acid and titration with 0.1 or 0.05 mol/l hydrochloric acid (manually, semi-automatically or automatically with photometric or pH end point); result in % by mass; PN-EN ISO 8968-3:2008

Food chemistry PN-EN ISO 8968-3

Overview

WHAT THE STANDARD COVERS. We read the text of the standard in two iTeh samples: ISO 8968-3:2004 | IDF 20-3:2004 (English text) — ISO and IDF forewords, clauses 1–5 and clause 6 up to 6.5, up to page 3 — and SIST EN ISO 8968-3:2007 (title page of EN ISO 8968-3:2007, European foreword and endorsement notice, then the same ISO text up to clause 3). PN-EN ISO 8968-3:2008 adopts EN ISO 8968-3:2007 and ISO 8968-3:2004 as identical (PKN search). We do not read the rest of the apparatus, the procedure (including digestion, distillation and titration), calculation, precision or the test report.

ACCORDING TO THE TEXT OF EN ISO 8968-3:2007 AND ISO 8968-3:2004. The standard was prepared by ISO/TC 34 "Food products", Subcommittee SC 5 "Milk and milk products", jointly with IDF and in collaboration with AOAC International (action team on nitrogen compounds of the Standing Committee on main components of milk); this edition replaces IDF 20-3:1993, technically revised. The ISO 8968 | IDF 20 series "Milk — Determination of nitrogen content": Part 1 — Kjeldahl method, Part 2 — block digestion (macro method), Part 3 — block digestion (semi-micro rapid routine method), Part 4 — non-protein nitrogen, Part 5 — protein nitrogen. The ISO text was taken over as EN ISO 8968-3:2007 by CEN/TC 302 "Milk and milk products — Methods of sampling and analysis" (secretariat NEN); CEN approved it on 19.05.2007 without modification, and national implementation was due by December 2007 at the latest. Warning: use of the standard may involve hazardous materials, operations and equipment; the user establishes safety practices. Scope (1): determination of the nitrogen content of liquid whole or skimmed milk by a semi-micro rapid routine method based on block digestion; the method is faster than ISO 8968-1 and 8968-2 because the digestion time is reduced by a smaller test portion and hydrogen peroxide used together with sulfuric acid and a catalyst. Reference (2): ISO 385 (burettes). Definition (3.1): nitrogen content — mass fraction of nitrogen determined by the procedure of this part, expressed as a percentage by mass. Principle (4): the test portion is digested in a heating block with a mixture of concentrated sulfuric acid, hydrogen peroxide and potassium sulfate with a catalyst, converting organic nitrogen to ammonium sulfate; excess sodium hydroxide is added to the cooled digest to liberate ammonia; ammonia is distilled in a manual, semi-automatic or fully automatic steam distillation unit — in the first two cases into excess boric acid and titrated with hydrochloric acid, in an automatic unit titration is automatic with end-point detection photometrically or by pH; the nitrogen content is calculated from the amount of ammonia produced. Reagents (5): Kjeldahl catalyst tablets — suitable e.g. with 3.5 g potassium sulfate, 0.105 g copper(II) sulfate pentahydrate and 0.105 g titanium dioxide; other tablets if 7 g of potassium sulfate can be dispensed by a whole number of tablets and they contain no salts of toxic metals such as selenium or mercury; sulfuric acid of mass fraction at least 95–98 %, nitrogen-free (about 1.84 g/ml); hydrogen peroxide about 30 g/100 ml; antifoaming agent (silicone preparation recommended, e.g. 30 % aqueous emulsion); nitrogen-free NaOH about 40 g/100 ml; boric acid 40.0 g/l with 3 ml indicator (light orange solution, in borosilicate glass, protected from light and ammonia fumes) and 10.0 g/l for titration with photometric end point (with methyl red and bromocresol green, neutralized with 0.1 mol/l NaOH to green — usually 3 ml per 1 l of 1 % acid); indicator for pH end-point titration — methyl red and bromocresol green in 95 % ethanol, 1 part to 5 parts; standard hydrochloric acid c(HCl) = (0.1 ± 0.0005) mol/l, and for manual titration (0.05 ± 0.0005) mol/l is recommended; ammonium sulfate of at least 99.9 % purity on dry matter, dried before use for at least 2 h at (102 ± 2) °C; tryptophan or lysine hydrochloride of at least 99 % purity (not dried in an oven); sucrose with nitrogen content not more than 0.002 % (not dried). Apparatus (6, to 6.5): water bath 38–40 °C, analytical balance (1 mg, readability 0.1 mg), digestion block (aluminium alloy or equivalent) with temperature control and measurement, 250 ml digestion tubes, exhaust manifold fitting the tubes.

STATUS (as of 03.10.2026). The PKN search engine (query "PN-EN ISO 8968-3", 03.10.2026) returns two results: PN-EN ISO 8968-3:2008 — Polish version, "Introduces: EN ISO 8968-3:2007 [IDT], ISO 8968-3:2004 [IDT]", without a withdrawal note; and PN-EN ISO 8968-3:2007 — English version, "Withdrawn and replaced by PN-EN ISO 8968-3:2008 - Polish version".

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, load up to 17.09.2026, read 03.10.2026). The number 8968-3 appears in 2 records at 2 laboratories: AB 429 and AB 1334. In the current PCA documents (read 03.10.2026; AB 429 issue no. 26 of 02.04.2026, AB 1334 issue no. 31 of 04.05.2026) the number appears at the same 2 laboratories; in each document the number of page markers equals the number of PDF pages, without duplicates; searching all PCA documents downloaded from the BIP on 03.10.2026 found no laboratories outside the database copy. Form at both: "PN-EN ISO 8968-3:2008". None of these items is suspended. The "Laboratories" tab (query "PN-EN ISO 8968-3") shows both laboratories — checked by a tab query on the labcoda.pl production server on 03.10.2026.

WHAT THE LABORATORIES TEST (PCA items and database copy). AB 429: milk and milk products — total Kjeldahl nitrogen (2.0–75.0) g/kg and total protein (calculated), potentiometric titration method. AB 1334: milk and milk products — nitrogen content (0.06–16.00) % and protein content (calculated), potentiometric titration method.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. Matrix: the scope of the standard is liquid whole or skimmed milk (1), while both PCA items cover "milk and milk products" — for products the report should show on what basis the method was applied. Test portion and digestion: the speed of the method comes from the smaller test portion and hydrogen peroxide (1) — test portions from Parts 1 and 2 are not transferred to it. Catalyst: tablets without selenium or mercury, and a whole number of tablets should give 7 g of potassium sulfate (5.1). Reagents: sulfuric acid, NaOH and sucrose nitrogen-free (sucrose ≤ 0.002 % N) — nitrogen from reagents raises the result (5.2, 5.5, 5.11). Boric acid: the 40 g/l solution is protected from light and ammonia fumes (5.6.1); a separate 10 g/l solution neutralized to green is used for the photometric end point (5.6.2). Titrant: for manual titration 0.05 mol/l HCl is recommended (5.8). Control: ammonium sulfate is dried at 102 °C before use, while tryptophan, lysine and sucrose are not dried (5.9–5.11). Protein: both items give protein "calculated" — the conversion factor is not in the part of the standard read.

WHAT WE DO NOT GIVE. We do not reproduce digestion and distillation conditions, calculation formulas, precision data or the nitrogen-to-protein conversion factor — they are outside the samples. We do not summarize ISO 8968-1, -2, -4 or -5 in this entry.

Method principle

Milk is digested in tubes in a heating block with a mixture of concentrated sulfuric acid, hydrogen peroxide and potassium sulfate with a catalyst; organic nitrogen is converted to ammonium sulfate. After cooling, excess sodium hydroxide is added, and the liberated ammonia is steam-distilled into boric acid and titrated with hydrochloric acid — manually or in an automatic unit with a photometric or pH end point. The nitrogen content is calculated from the amount of ammonia.

Applications

• Liquid whole or skimmed milk — routine nitrogen determination (1)
• A faster alternative to ISO 8968-1 (Kjeldahl) and ISO 8968-2 (block, macro method) (1)
• Milk and milk products — total nitrogen, protein calculated — 2 accreditation scope records of 2 laboratories in PCA

Key parameters

Standard

Standard number

PN-EN ISO 8968-3:2008

Title (PL)

Mleko — Oznaczanie zawartości azotu — Część 3: Metoda z zastosowaniem bloku do mineralizacji (szybka metoda rutynowa półmikro)

Title (EN)

Milk — Determination of nitrogen content — Part 3: Block-digestion method (Semi-micro rapid routine method)

Step-by-step procedure

1. Preparation of reagents

   Boric acid with indicator, standard HCl, ammonium sulfate dried at 102 °C (5.6–5.9). PN-EN ISO 8968-3:2008 — no withdrawal note (PKN, 03.10.2026).

2. Digestion

   Test portion in a 250 ml tube, H2SO4 + H2O2 + catalyst, heating block under the manifold (4, 6.3–6.5).

3. Distillation

   Excess NaOH, ammonia with steam into boric acid (4).

4. Titration

   HCl manually or automatically (photometric or pH end point) (4).

5. Result

   Nitrogen in % by mass from the amount of ammonia (3.1, 4); calculation details — outside the sample.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Use of the standard may involve hazardous materials, operations and equipment — the user establishes safety and health practices (warning)
• Catalyst without salts of toxic metals such as selenium or mercury (5.1)

Frequently asked questions