SOP: Determination of Sodium Chloride in Tomato Products by the Indirect Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

Where a more precise figure is required, as in the determination of salt-free solids, the modified Volhard method — the official method of the AOAC — is followed. The sample is ashed with sodium carbonate, and the chloride in the ash is precipitated with a known excess of standard silver nitrate; the excess silver is back-titrated with thiocyanate using ferric alum as indicator. The end point is very clear, which makes the determination very accurate. A rapid variant based on the same principle extracts the sample with alcohol and centrifuges instead of ashing.

Apparatus required

• Platinum or silica dish
• Muffle furnace (not more than 550°C)
• 100-ml volumetric flask
• 250-ml conical flask
• Gooch crucible with asbestos pad
• Burette
• Centrifuge bottle and centrifuge (1,800 rpm) — for the rapid method only

Reagents

a. Sodium carbonate — about 0.5 g, in excess, added before ashing.

b. Dilute nitric acid (1 + 4) — for dissolving the ash.

c. 0.1 N silver nitrate — standard solution; 25 ml is added in known excess.

d. Dilute nitric acid (1 + 4) — for washing the precipitate (in the full method).

e. Ferric alum indicator — 2 ml of a saturated solution of ferric ammonium sulphate, FeNH₄(SO₄)₂·12H₂O, in water.

f. Potassium thiocyanate standard solution — 9.719 g of KCNS per litre — or ammonium thiocyanate standard solution — 7.613 g of NH₄CNS per litre — for back-titrating the excess silver.

g. 80% alcohol — for the rapid method.

h. Dilute nitric acid (1 + 1) — 5 ml, for the blank titration.

Sample preparation

For the full method: note the tare weight of a platinum or silica dish and accurately weigh 20 g of puree, or 5 g of paste.

For the rapid method: weigh approximately 5 g of material.

Procedure — Full method (ashing)

Add excess of sodium carbonate (about 0.5 g) to the weighed sample in the dish; ash in a muffle furnace at not more than 550°C

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Dissolve the ash in dilute (1 + 4) HNO₃; make up to 100 ml in a volumetric flask

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Pipette 20 ml of the ash solution into a 250-ml conical flask

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Add a known excess of 0.1 N standard silver nitrate solution (25 ml)

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Filter through a previously prepared Gooch crucible (containing an asbestos pad and washed with dilute HNO₃); wash the precipitate repeatedly with dilute HNO₃ until the filtrate is about 150 ml

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Add 2 ml of ferric alum solution to the filtrate; titrate the excess of silver nitrate with the potassium thiocyanate (or ammonium thiocyanate) standard solution until a pale rose colour is formed

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Carry out a blank titration using a similar volume (25 ml) of silver nitrate solution, 2 ml of indicator (ferric alum solution), and 5 ml of dilute HNO₃ (1 + 1)

Procedure — Rapid method (alcohol extraction, no ashing)

Transfer the weighed sample (approximately 5 g) to a 100-ml volumetric flask with 80% alcohol

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Add enough 80% alcohol until the volume is approximately 50 ml

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Add 1 ml of HNO₃; with the help of a pipette, add a measured excess of 0.1 N AgNO₃

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Make up to volume with alcohol; transfer the mixture to a centrifuge bottle and centrifuge for 5 min at 1,800 rpm

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Pipette 50 ml of the clear supernatant; add 2 ml of saturated ferric indicator solution and 2 ml of HNO₃

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Titrate with 0.1 N ammonium thiocyanate standard solution; multiply the ml of 0.1 N ammonium thiocyanate required by 2 (A)

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Carry out a blank titration using the same volume of silver nitrate solution as added to the 100-ml flask (B)

Calculation — Full method

Sodium chloride % = [(Blank titre − Sample titre) × Normality of AgNO₃ × Total volume of ash solution × 100 × 58.45] / (ml of ash solution taken × Wt of sample × 1,000)

Where,

• Blank titre = ml of thiocyanate consumed in the blank titration
• Sample titre = ml of thiocyanate consumed in the sample titration
• Normality of AgNO₃ = normality of the silver nitrate solution
• Total volume of ash solution = 100 ml
• ml of ash solution taken = 20 ml
• Wt of sample = weight of the sample taken, in g

Calculation — Rapid method

Sodium chloride % = [(B − A) × 58.45 × 100] / (Wt of sample × 1,000)

Where,

• B = ml of 0.1 N NH₄CNS in the blank titration
• A = ml of 0.1 N NH₄CNS for the sample, multiplied by 2
• Wt of sample = weight of the sample taken, in g

Flag: as printed, this rapid-method formula carries no normality factor for the 0.1 N thiocyanate titrant, which would make the result ten times too high in the usual Volhard arithmetic. The formula is carried exactly as read; verify against the printed book before use.

Precautions

• Do not exceed 550°C when ashing; higher temperatures volatilise the chloride.
• Prepare the Gooch crucible pad carefully and wash the precipitate well, or silver will be retained on the filter.

References

Official Methods of Analysis, Association of Official Analytical Chemists, 11th edn. (1970).