Method F-021F The Determination of Calcium in Fruit Juice

Equipment Required:

1. Sherwood Flame photometer with calcium filter (622 nm)
2. Graduated stoppered centrifuge tube

Reagents:

1. Standard calcium solution 100 mg/L (containing perchloric acid)
2. 1% ammonium oxalate solution
3. 0.5% ammonium oxalate solution
4. 25% ammonia solution
5. 4M perchloric acid

Method:

1. Pipette 5 ml fruit juice into a 10 ml graduated stoppered centrifuge tube.
2. Add 5 mls 1% ammonium oxalate and 3 drops ammonia solution.
3. Shake, and allow to stand for 30 minutes.
4. Centrifuge at 2000 rpm for 2 minutes.
5. Decant the supernatant and allow the tube to drain inverted for 30 seconds.
6. Add 0.5 ml 4M perchloric acid and shake.
7. Heat for 1 minute in a boiling water bath.
8. Cool and dilute to the mark with distilled water.
9. Calibrate the flame photometer using a 100 ppm Ca²⁺ standard containing 50 mls 4M HCIO₄ per litre from which 75, 50 and 25 ppm Ca²⁺ solutions have been prepared.
10. Aspirate the sample directly into the flame photometer.
11. The calcium concentration is then calculated by multiplying the flame photometer reading on the calibrated scale by the dilution factor.