Application Notes: Reflectometry – Iron in Milk Powder and Milk

Reflectometric determination after reaction with Ferrospectral®

Reagents

Catalog No. 116982 Reflectoquant® Iron Test
Catalog No. 117246 Reflectometer RQflex® 20
Catalog No. 100456 Nitric acid 65 %
Catalog No. 100630 Nitric acid 10 mol/l, Titripur®
Catalog No. 1831 Ascorbic Acid, e.g. Calbiochem® Folded filters

This application note pertains to RQflex® 20 and all
discontinued instruments (RQflex® 10, RQflex® plus)

Milk powder

Preparation of a 3 N nitric acid

a) from nitric acid 65 % (Catalog No. 100456)

Place approx. 600 ml of distilled water in a 1-l volumetric flask and with stirring add 290.8 g of nitric acid 65 %. Make up this solution to 1 l with distilled water.

b) from nitric acid 10 mol/l (Catalog No. 100630)

Place approx. 600 ml of distilled water in a 1-l volumetric flask and with stirring add 300 ml of nitric acid 10 mol/l. Make up this solution to 1 l with distilled water.

Sample preparation

Weigh approximately 10 g (exact mass = m1) of the sample material, e.g. milk powder, into a 250-ml conical flask. 

With stirring heat up the sample to 98 °C and continue to stir at 98 °C for 30 minutes. Subsequently cool the samples to room temperature (approx. 25 °C), weigh out, and again make up to 110 g (= m3) with distilled water. Stir the sample again for 2 minutes and filter over a folded filter. To 20.0 ml of this filtrate add 2 g of ascorbic acid and stir for 5 minutes.

Blank value

The blank sample is prepared by subjecting water to the same pretreatment procedure as for the sample material.

 

Analysis

The iron content is determined by making two measurements (1 × sample material and 1 × blank sample). Press the START button of the reflectometer and - this is imperative - at the same time immerse both reaction zones of the test strip in the pretreated sample (5 - 40 °C) for 2 sec. Carefully allow excess liquid to run off via the long edge of the strip onto an absorbent paper towel. Immediately insert the strip all the way into the strip adapter with the reaction zones facing the display. After the end of the reaction time, read off the result from the display in mg/l Fe2+.

The result is automatically stored. (See RQflex® operating instructions and package insert of Reflectoquant® Iron Test.)

Notes

Direct determination without any pre-treatment using the Reflectoquant® Iron Test is not possible, since in many cases iron occurs in milk in strongly bound complexes (e.g. protein complexes) that cannot be quantitatively measured by the test. This method measures both Fe2+ as well as Fe3+, since the latter species is reduced to Fe2+ by the addition of ascorbic acid in step 3.2. Furthermore, care must be taken to ensure that the materials used are free from iron, since under certain circumstances this may be oxidized by the nitric acid to yield Fe2+/Fe3+ and hence also measured by the test.

Calculation

 

Results:

Sample Reflectoquant® [mg/kg] ICP-OES [mg/kg]
1 81 96
2 54 47
3 42 48
4 26 24
5 69 71

Materials