SOP: Calcium Analysis by EDTA Titration

Calcium fortification claims on milk, juice, and plant-based drinks all trace back to this titration. EDTA locks onto calcium ions one-to-one, and the indicator color flip marks the point precisely. It is a staple of dairy and beverage labs.

Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.

Purpose

To determine calcium content by complexometric titration for fortification verification and nutrition labeling.

Scope

Applies to milk, fortified beverages, dairy products, and calcium supplements after appropriate sample digestion or ashing.

Equipment and Materials

• Burette, 25 mL, Class A
• Standardized EDTA solution, 0.01 M
• Calcon or Patton-Reeder indicator
• Sodium hydroxide solution to raise pH above 12
• Conical flasks, 250 mL
• Muffle furnace and acids for sample preparation
• Calcium carbonate primary standard

Procedure

1. Prepare the sample: dry ash 2 to 5 g at 550 C, dissolve the ash in dilute hydrochloric acid, and make to a known volume. Alternatively, wet-digest with nitric acid.
2. Standardize the EDTA against dried calcium carbonate. Record the exact molarity.
3. Pipette an aliquot containing roughly 2 to 10 mg of calcium into a conical flask and dilute to about 100 mL.
4. Add sodium hydroxide to bring the pH above 12. At this pH, magnesium precipitates as hydroxide and does not interfere.
5. Add a small amount of Patton-Reeder indicator; the solution turns wine red.
6. Titrate with 0.01 M EDTA to the change from wine red to pure blue.
7. Record the volume. Run duplicates; agreement within 0.2 mL is required.

Recording Results

Milligrams of calcium equals EDTA volume in mL times molarity times 40.08 times dilution factor. Convert to percent or mg per 100 g as needed. Report to two significant figures of the specification.

Quality Control

Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.

Troubleshooting

• Endpoint sluggish or unclear: pH below 12 lets magnesium interfere. Add more sodium hydroxide and re-titrate.
• Indicator blocked: old indicator or heavy metal contamination. Use fresh indicator and ensure clean glassware.
• Low results: incomplete ashing or calcium trapped in silica residue. Extend ashing and consider hydrofluoric acid treatment for high-silica samples (specialist procedure).

Safety Notes

• Sodium hydroxide and acids are corrosive. Wear face protection during sample prep.
• Muffle furnace work requires heat-resistant gloves and tongs.