SOP: Determination of Calcium in Canned Vegetables

GIFSQ — Global Institute for Food Safety & Quality

Principle

Calcium in canned vegetables (applicable to canned lima beans, potatoes and tomatoes) is determined by complexometric titration: the product is ashed, the ash solution is passed through an ion exchange column of high phosphate capacity, and the calcium is titrated with standard EDTA solution using hydroxynaphthol blue indicator.

Apparatus required

• Titration stand, fluorescent illuminated
• Ion exchange column, about 20 × 600 mm, with a coarse-porosity sintered glass disc and a Teflon stopcock. Pack it as follows: work 30–40 g of moist Amberlite IR-4B resin (an anion exchanger of high phosphate capacity) in a 600-ml beaker through three 250-ml portions of 5% Na₂CO₃ or NaOH; wash with water until the excess alkali is gone; then work through three 250-ml portions of 5% HCl, mixing well after each; rinse with water until the colour washes out, and transfer the resin to the column with water. The column is ready once the water level is drawn down to the top of the resin bed. (Phosphate exchange capacity is about 1,500 mg, so several aliquots can go through before regeneration is needed; before each use, rinse the column with about 250 ml of water until the eluate runs colourless.)
• pH meter
• Magnetic stirrer
• Platinum or porcelain dish
• Water bath
• Muffle furnace

Reagents

Prepare the reagents using glass-distilled or deionized water.

a. Potassium hydroxide–potassium cyanide solution: dissolve 280 g KOH and 66 g KCN in 1 litre of water.

b. Calcium carbonate: AR grade, dried for 2 hr at 285 °C.

c. Hydroxynaphthol blue–calcium indicator. Store in the dark. Use a fresh supply of this indicator after one year.

d. Ascorbic acid.

e. 5% HCl: mix 3 volumes of concentrated HCl with 22 volumes of water.

f. 10% HCl.

g. 5% w/v sodium carbonate.

h. 0.01 M disodium dihydrogen ethylenediamine tetraacetate (EDTA) standard solution: dissolve 3.72 g of EDTA (at least 99% purity) in water and make up to 1,000 ml in a volumetric flask. Standardise as follows: weigh accurately enough CaCO₃ to give a titre value of about 40 ml with 0.01 M EDTA and transfer to a 400 ml beaker; add 50 ml (the remainder of the printed standardisation passage — dissolution, adjustment and titration to the deep blue end point with magnetic stirring, the last few ml of EDTA added dropwise — is truncated in the available copy and is flagged for checking against a clean printed copy).

Molarity of EDTA solution = mg CaCO₃ / [EDTA (ml) × 100.09]

Note: the printed layout of this expression is scrambled in the available copy; the terms above are the book’s own.

i. 10% KOH solution — called for in the determination for the dropwise pH adjustment; the book gives no preparation for it.

Sample preparation

A. Liquid from canned whole tomatoes: drain off the tomato liquid, spin it down and filter through fast paper. Weigh 100 g of the filtrate into a platinum or porcelain dish and evaporate to dryness on a water bath. Ash at 500–525 °C until the residue looks free of carbon (grey to brown). After cooling, add 20 ml of water, stir with a glass rod, and cautiously add 10 ml of concentrated HCl under a watch glass. Rinse the watch glass down into the dish and evaporate to dryness on a steam bath. Add 50 ml of 10% HCl, warm on the steam bath for 15 min, and filter through quantitative filter paper into a 200-ml volumetric flask, washing the paper and dish well with hot water. Cool the filtrate, make up to the mark and mix.

B. Canned vegetables: reduce the entire can contents to a uniform mass (for cans of 8 cm diameter or more, blend a representative portion at high speed). Weigh 50 g of the sample — 100 g where no added calcium is declared — into a platinum or porcelain dish; evaporate to dryness, ash and treat exactly as the tomato liquid above.

Procedure — Determination

Pipette a 50 ml or 100 ml aliquot of the prepared sample into a 250-ml beaker and bring it to pH (the printed pH value is illegible in the available copy — it reads “pH *. 5” — and is flagged for checking against a clean printed copy) by adding 10% KOH solution dropwise, following a pH meter with magnetic stirring

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Run the sample through the resin column (in its chloride form) at 2–3 ml/min and collect the effluent in a 400-ml beaker

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Wash the column well with 100 ml of water in two 50-ml portions: the first at the same flow rate as the sample, the second faster at 6–7 ml/min

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Then run enough water freely through the column to bring the total collected volume to 250–300 ml

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Mix well and set the pH to 12.5–13.0 (pH meter and magnetic stirrer) with KOH–KCN solution (about 10 ml)

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Add 100 mg of ascorbic acid and 200–300 mg of hydroxynaphthol blue indicator, and titrate at once with 0.01 M EDTA solution from pink to the deep blue end point, stirring magnetically throughout

Calculation

For 50 ml aliquot, % w/w Ca = (titre value × 0.4008 × 4 × 100) / sample (mg)

For 100 ml aliquot, % w/w Ca = (titre value × 0.4008 × 2 × 100) / sample (mg)

Where,
– Titre value = ml of 0.01 M EDTA solution consumed in the titration
– Sample (mg) = weight of the sample represented by the aliquot, in mg

Results are expressed as per cent w/w Ca of the final product or of the packing medium, as appropriate.

Precautions

• Prepare the reagents using glass-distilled or deionized water.
• Titrate immediately after adding the indicator.
• Store the hydroxynaphthol blue indicator in the dark and use a fresh supply after one year.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

J. Assoc. Offic. Anal. Chem., 49, 211 (1966); 51, 494 (1968) — cited in the book.

FAO/WHO Codex Alimentarius Commission, CAC/RM 38-1970 — cited in the book.