color of sweet paprika

iron in condiments

iron in flour

phosphates in apple juice

phosphates in meat products

nitrite in meat products (1)

nitrite in meat products (2)

alcohol in spirit

color of beer

glucose


3. Spectrometric determination of iron in flour

PRINCIPLE OF THE METHOD

The determination is based on the preparation a coloured product resulting from the reaction between the sulphosalicylic acid and Fe2+ or Fe3+ and on the transmittance measurement of the solution.

EQUIPMENT

  • a SpektraTM spectrometer,
  • a crucible,
  • a 5-mL measuring cylinder,
  • a 50-mL measuring flask,
  • a water bath,
  • a glass funnel,
  • a filter paper.


REAGENTS

  • 20 % HCl,
  • 5 % HCl,
  • 10 % 5-sulphosalicylic acid (C7H6O6S·2 H2O),
  • 25 % NH3.

     

STANDARD SOLUTION
 

0.967 g FeCl3·6 H2O / 100 mL

1 mL corresponds to 2 mg Fe

Standard is diluted 10 times (10x).

1 mL corresponds to 0.2 mg Fe.


 

HAZARDS
 

Hydrochloric acid causes burns, irritates eyes and the respiratory organ. Hold in a tightly closed container. Wear a protective uniform, gloves and glasses.

R: 34-37S: (1/2)-26-45

Ammonia causes burns, irritates eyes and the respiratory organ. Wear a protective uniform, gloves and glasses.

R: 34-37S: (1/2)-7-26-45

5-sulphosalicylic acid irritates eyes and the respiratory organ. Wear a protective uniform, gloves and glasses.

R: 36/37S:22-24


PROCEDURE

Weigh 5-10 g (± 0.1 mg) of flour into a crucible, initially ignite the sample on an electric plate, and then place in an oven at 550°C (6-8 hours) until a white or grey ash is obtained. Let the crucible cool down in a desiccator and dissolve the ash in 5-mL 20 % HCl. Dry the content of the crucible on a water bath (~60 minutes) and then dissolve the sample in 5-mL 5 % HCl. Filter the solution through a filter paper into a 50-mL measuring flask and fill it with distilled water to the mark line. Mix well. Transfer an aliquant part (e.g., 10 mL) of the sample solution into a clean, 50-mL measuring flask. Add 5-mL 10 % sulphosalicylic acid to achieve a red coloration of the solution. After that, add the surplus NH3 0.5-1 mL to the flask. The addition of NH3 changes the colour of the solution to yellow. Afterwards, fill the flask to the mark line with distilled water and mix the solution well. Pipette 400 mL of the prepared solution into the hollow of a blister and measure the transmittance at 430 nm (using the blue LED). A similar procedure is performed with standard solutions. Calculate the concentration of iron in the sample using Lambert-Beer’s law or read the results from the calibration curve.
 

PREPARATION OF THE STANDARD SOLUTION

Standard solutions are treated in the same way as a sample solution; the only difference is that instead of the sample solution the diluted standard solution is added, as demonstrated in the table below.

mL st. solu.

0

0.1

0.2

0.3

0.4

0.5

0.7

0.8

1.0

mg Fe/50 mL

0

0.02

0.04

0.06

0.08

0.1

0.14

0.16

0.2

 

CALCULATION

mg Fe/100g sample = Ax x cs x 100 x R / As x weighed

Ax = sample absorbance
As = standard absorbance
cs = standard concentration
R = diluttion

VALUES

Wheat contains approximately 3.5 mg of Fe per 100 g. Iron is mainly in the external membrane of the grain. The orientation values for flour are the following:

  • wheat flour type "400": 0.6 mg Fe/100 g
  • wheat flour type "1100": 2.1 mg Fe/100 g
  • millet flour: 6.8 mg Fe/100 g
  • soya flour: 8.4 mg Fe/100 g
  • rye flour: 2.0 mg Fe/100 g
  • buckwheat flour: 2.8 mg Fe/100 g
  • maize flour: 1.2 mg Fe/100 g


Developed and prepared by: Avrelija GOJKOVIČ KUMPERGER, Živilska šola Maribor