SOP: Iron Content by Spectrophotometry

Iron fortification in flour and cereals is a public health measure in many countries, and the orange-red phenanthroline complex gives a reliable way to verify it. A spectrophotometer turns that color into a number with excellent precision.

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 iron content by visible spectrophotometry after complexation for fortification checks and nutrition analysis.

Scope

Applies to flours, cereals, and fortified foods after dry ashing or wet digestion.

Equipment and Materials

• Visible spectrophotometer, set at 510 nm, with matched cuvettes
• 1,10-phenanthroline solution, 0.25 percent
• Hydroxylamine hydrochloride, 10 percent (reducing agent)
• Sodium acetate buffer
• Iron standard solution (ferrous ammonium sulfate), 1000 mg per L stock
• Hydrochloric acid for sample dissolution
• Volumetric flasks, 100 mL

Procedure

1. Dry ash 2 to 5 g of sample at 550 C. Dissolve the ash in 6 N hydrochloric acid, filter if needed, and make to 100 mL.
2. Prepare calibration standards from 0 to 5 mg per L iron in 100 mL flasks, adding the same reagents as samples.
3. Pipette an aliquot of sample digest into a 100 mL flask. Add 1 mL of hydroxylamine hydrochloride to reduce all iron to Fe(II).
4. Add 5 mL of sodium acetate buffer and 2 mL of phenanthroline solution. Mix and make to volume.
5. Wait 15 minutes for full color development. The solution turns orange-red.
6. Measure absorbance at 510 nm against a reagent blank.
7. Read the iron concentration from the calibration curve. The curve must have a correlation coefficient of at least 0.995.
8. Run duplicates and a spiked recovery with each batch; recovery must be 90 to 110 percent.

Recording Results

Milligrams of iron per 100 g equals concentration from the curve in mg per L times final volume in liters times dilution factor times 100, divided by sample weight in grams. Report to one decimal place.

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

• Low absorbance on standards: phenanthroline degraded or pH wrong. Prepare fresh reagent and check buffer pH (3 to 5).
• High blanks: iron contamination from glassware or water. Acid-wash glassware and use deionized water.
• Non-linear curve: standards too concentrated. Dilute into the 0 to 5 mg per L working range.
• Color fades: read within 2 hours of development; strong light degrades the complex.

Safety Notes

• Hydrochloric acid is corrosive. Handle in the fume hood.
• Hydroxylamine is a skin sensitizer; wear gloves.