🧂 Salt (NaCl) in food

Food Chemistry PN-ISO 1841-1

Overview

Determination of table salt (NaCl) content is one of the basic tests in food chemistry. Salt affects taste, stability, and safety of food products, and its excessive consumption is associated with cardiovascular disease risk. EU regulations (Reg. 1169/2011) require salt content declaration on labels.

Two main argentometric methods are Mohr method (direct titration) and Volhard method (back titration). In Mohr method, the sample is titrated with AgNO₃ solution in presence of K₂CrO₄ indicator — after binding all Cl⁻ ions, excess Ag⁺ reacts with CrO₄²⁻ forming red-brown Ag₂CrO₄ precipitate (endpoint). In Volhard method, excess AgNO₃ is back-titrated with KSCN with Fe³⁺ indicator (red FeSCN²⁺).

Standard PN-ISO 1841-1 describes Volhard method for meat and meat products. Volhard method is preferred for colored and turbid samples (meat, products) because the endpoint is easier to observe than in Mohr method. Before titration, the sample is mineralized or extracted with hot water, filtered, and acidified with nitric acid.

Alternative methods include: potentiometric chloride titration (more precise), chloride ion-selective electrodes (ISE), conductometric measurement, and sodium determination by AAS/ICP (calculation Na × 2.54 = NaCl). WHO recommends limiting salt intake to 5 g/day, making this determination important for public health.

Method principle

Volhard method (PN-ISO 1841-1): The sample is mineralized or extracted with hot water, filtered, and acidified with HNO₃. A measured excess of standardized AgNO₃ is added, which reacts with Cl⁻ ions forming white AgCl precipitate. Excess unreacted AgNO₃ is back-titrated with standardized KSCN (potassium thiocyanate) solution in presence of indicator — Fe³⁺ ions (ferric-ammonium alum solution). Endpoint is appearance of persistent red-brown coloration from FeSCN²⁺. Cl⁻ content is calculated from difference: n(Cl⁻) = n(AgNO₃ added) − n(KSCN consumed). NaCl [%] = Cl⁻ × 1.6485.

Applications

• NaCl determination in meat and meat products (PN-ISO 1841)
• Salt content control in cheeses and dairy products
• Testing of fish products (salted, marinated, smoked)
• Control of bread and bakery products
• Salt content declaration on labels (EU Reg. 1169/2011)
• NaCl monitoring in ready meals and convenience products
• Control of salting and curing processes

Key parameters

Standard

Standard number

PN-ISO 1841-1:2002

Title (PL)

Mięso i przetwory mięsne — Oznaczanie zawartości chlorków — Część 1: Metoda Volharda

Title (EN)

Meat and meat products — Determination of chloride content — Part 1: Volhard method

Step-by-step procedure

1. Sample preparation

   Homogenize sample. Weigh 2–5 g into conical flask with 0.001 g accuracy.

   ⏱ Time: 5 min

2. Chloride extraction

   Add 100 mL hot distilled water. Boil for 15 min with stirring. Cool to room temperature.

   ⏱ Time: 30 min

3. Filtration

   Filter through quantitative filter into 250 mL volumetric flask. Wash residue with hot water. Fill to mark.

   ⏱ Time: 15 min

4. Aliquot collection

   Pipette 25–50 mL filtrate into 250 mL conical flask.

   ⏱ Time: 3 min

5. Acidification

   Add 5 mL HNO₃ (1:1) — acidic environment required for Volhard method.

   ⏱ Time: 1 min

6. Excess AgNO₃ addition

   Add measured volume (e.g., 25 mL) of standardized 0.1 mol/L AgNO₃ — excess relative to expected Cl⁻ content.

   ⏱ Time: 2 min

7. AgCl filtration (option)

   Filter AgCl precipitate or add nitrobenzene to cover precipitate (prevent Ag⁺ readsorption on AgCl).

   ⏱ Time: 5 min

8. Indicator addition

   Add 2 mL saturated ferric-ammonium alum solution (Fe³⁺ indicator).

   ⏱ Time: 1 min

9. KSCN titration

   Titrate excess Ag⁺ with 0.1 mol/L KSCN solution to persistent red-brown coloration (FeSCN²⁺).

   ⏱ Time: 5–10 min

10. Calculations

    Cl⁻ [mg] = (V_AgNO₃ × c_AgNO₃ − V_KSCN × c_KSCN) × 35.453. NaCl [%] = Cl⁻ × 1.6485 × 100 / sample mass.

11. Quality control

    Perform duplicates. Analyze NaCl standard solution every series. Duplicate difference < 0.1% NaCl.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• AgNO₃ — corrosive, stains skin black (irreversibly). Use nitrile gloves.
• HNO₃ — corrosive and oxidizing (H272, H314). Work under fume hood.
• K₂CrO₄ (Mohr method) — carcinogenic (H350), CMR. Minimize contact. Waste as hazardous.
• Nitrobenzene — toxic (H301, H311, H331). Use only under fume hood.
• Silver-containing waste — collect separately (silver recovery).

Frequently asked questions