Citric Acid: Quality Specification & Test Methods

Procedure:

1. Dissolve 0.80 g in 4 ml of water
2. Add 3 ml of hydrochloric acid and 1 g of zinc granules
3. Boil for 1 minute
4. Allow standing for 2 minutes
5. Transfer supernatant to test tube with 0.25 ml of phenyl hydrazine hydrochloride solution (10 g/l)
6. Heat to boiling, cool rapidly
7. Add equal volume of hydrochloric acid and 0.25 ml of potassium ferricyanide solution (50 g/l)
8. Shake and allow to stand for 30 minutes

Acceptance: Test solution color ≤ Standard solution color

Procedure:

1. To 1.5 ml of sulfate standard solution (10 ppm SO₄), add 1 ml of barium chloride solution (25% w/v)
2. Shake and allow standing for 1 minute
3. Dissolve 1 g sample in distilled water, dilute to 15 ml
4. Add 1.5 ml of sample solution to the above mixture
5. Add 0.5 ml of 5M acetic acid
6. Allow standing for 5 minutes

Procedure (Gutzeit Method):

1. Dissolve 10 g sample in 50 ml water
2. Add 10 ml of stannated hydrochloric acid
3. Add 5 ml of 1M potassium iodide and 10 g zinc AsT
4. Immediately assemble the apparatus
5. Immerse flask in water bath (maintain uniform gas evolution)
6. After 40 minutes, observe mercuric chloride paper

Acceptance: Test stain intensity ≤ Standard stain

Procedure:

1. Dissolve 5.0 g in 39 ml of 2M sodium hydroxide (in portions)
2. Dilute to 50 ml with distilled water → Solution A
3. To 5 ml of Solution A add 5 ml of 1M sulphuric acid
4. Allow standing for 1 hour

Acceptance: Test opalescence ≤ Control opalescence

Procedure:

1. Prepare Solution A as in Barium test
2. Dilute 5 ml of Solution A to 10 ml with distilled water
3. To 0.2 ml of ethanolic calcium standard (100 ppm Ca), add 1 ml of ammonium oxalate solution (4% w/v)
4. After 1 minute, add 1 ml of 2M acetic acid and 5 ml of diluted sample solution
5. Shake and allow standing for 15 minutes

Acceptance: Test opalescence ≤ Standard opalescence

Procedure:

1. Prepare Solution A as in Barium test
2. Dilute 8 ml of Solution A to 10 ml with water
3. Transfer to Nessler cylinder
4. Add 2 ml of citric acid solution (20% w/v, iron-free)
5. Add 0.1 ml of thioglycollic acid
6. Make alkaline with iron-free ammonia solution
7. Dilute to 50 ml with water
8. Allow standing for 5 minutes

Acceptance: Test color intensity ≤ Standard color

Procedure:

1. Dissolve 5.0 g in 10 ml water
2. Add 1 ml of 2M nitric acid, dilute to 15 ml
3. Transfer to Nessler cylinder
4. Add 10 ml dilute nitric acid (unless already used)
5. Dilute to 50 ml with water
6. Add 1 ml of 0.1M silver nitrate
7. Stir immediately with glass rod
8. Allow standing for 5 minutes protected from light

Viewing: Examine transversely against black background

Acceptance: Test opalescence ≤ Standard opalescence