Determination of Free Iron by Formic Acid Extraction Method

This method is suitable for analyzing free iron present in Frisis and Crisis. Free iron is selectively extracted in 0.41 N formic acid. Iron oxide is insoluble under these conditions.


Formic acid – 0.41 N (Dilute 16 ml of 99% formic acid  to 1000 ml)

Ammonia – concentrated (Specific gravity 0.90)

Citric acid – 50% (w/v) aqueous solution

Thioglycolic acid – 10% (w/v) aqueous solution Iron stock solution - Dissolve exactly 8.635 ± 0.001 gm Ferric ammonium sulfate   (FeNH4(SO4)2  12H2O), in water, add 3.5ml of sulphuric acid (50% v/v), and dilute to 1000 mL with water. (1 ml contains 1 mg of Fe)

Iron Working standard solution – Dilute stock solution 100 times by successive dilution. This must be freshly prepared whenever required for calibration. 1.0 ml contains 10 µg Fe. (Note: All reagents used should be of AR, LR grade, and only deionized or distilled water should be used for dilutions)

Formic Acid Extraction Method


  • Standard laboratory glassware
  • Filter paper – Whatman No. 40.0
  • Spectrophotometer
  • Water bath
  • Muffle furnace
  • Platinum crucible
  • Bunsen flame
  • Pipeclay triangle

In-process Quality Assurance  (IPQA) | In-process quality control responsibilities


10.1 Preparation of the calibration curve

10.1.1 Pipette into a series of 25 mL volumetric flasks 0, 1, 2, 3, 5, and 10 mL of iron working standard solution from step 7.6.

10.1.1 To each flask, add 4 mL of 50% citric acid solution and 5 mL ammonia. Thoroughly mix after the addition of each reagent.

 Free Iron by Formic Acid Extraction Method

10.1.2 Cool and add 5 mL of 10% thioglycolic acid solution, dilute to volume with water, and mix well. (Ensure that the final solution is alkaline (smell of ammonia)).

10.1.3 Measure the absorbance at 540 nm using the standard with 0.0 mL iron solution as the reference.

10.1.4 Plot calibration curve relating absorbance absorbance  (y-axis) to µg iron (x-axis).

10.2 Sample Preparation

10.2.1 Weigh accurately about 0.25 to 0.30 g sample into a 250 ml conical flask 

10.2.2 Add 40 ml of 0.41N formic acid solution, and heat in a water bath for 15 minutes at 80 to 90 deg. C(count time only when the temperature reaches 80oC). Swirl the flask every five minutes.

10.1.1 Quantitatively transfer to a 100 ml volumetric flask, make up to volume with distilled water and mix well and filter.

10.1.2  Transfer 10 mL of the filtered solution to a 25 mL volumetric flask.

10.1.3 Add 4 mL of 50% citric acid solution and 5 mL of ammonia. Thoroughly mix after the addition of each reagent.

10.1.4 Cool add 5 mL of 10.% thioglycolic acid solution. dilute to volume with water and mix well. (Ensure that the final solution is alkaline (smells like ammonia)).

10.1.5 Measure the absorbance at 540 nm using the standard with 0 mL iron solution as the reference.

10.1.6 From the standard calibration graph estimate the amount of iron in the solution in mg/ g 


Free iron in ppm = mc g from graph x 100

                                                      W x 10


 W = Weight of sample taken in g


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