Determination of iron (III) content in water and soil by the rhodanite method.

Determination of iron (III) content in water and soil by the rhodanite method.

The method is based on the colored complex formation during the interaction of thiocyanate ions of ammonium or potassium with Fe ³⁺ ions in an acidic medium.

1. Preparation of working solution.

The solution of iron ammonium alum (NH₄)Fe(SO₄)₂*12H₂O with an iron concentration of 0. 1 mg / ml is used as the main standard solution. The working solution is prepared by diluting this solution. For this, put 25 ml of the standard solution with a concentration of 0. 1 mg / ml into a measuring flask for 250 by pipet and add distiller water so it goes up to the mark. The preparing solution contains 10 μ g / ml or 10 mg / l iron (III).

2. Water analysis.

1. Building of calibration graph

Take 6 flasks with the volume of 50 ml, put with the pipette 0; 0. 5; 1. 0; 1. 5; 2. 5; 5. 0 ml of working standard solution with an iron concentration of 10 mkg / ml. Add 0. 5 ml HNO₃ solution (1: 1) and 2. 5 ml 10% solution of NH₄SCN or KSCN in each flask. Bring water to a tag and mix. The obtained solutions respectively contain 0; 5; 10; 15; 25; 50 μ g Fe^{3 +} in the sample. In 10 minutes measure optical density of solutions at the wavelength of 480 – 500 nm relative to the zero solution. Select the size of the working cuvette yourself. Experimental data enter in the table and build a calibration graph A = f (C).

2. Determination of iron in water.

Put 25 ml of the water in a 50 ml flask, add all the necessary reagents, as in the of the calibration soilutions, bring distilled water to the mark, mix and measure A solution with relative to the zero solution. By the amount of A find according to the calibration graph concentration of Fe³⁺ in μ g/test.

Table 1

№	Volume of iron working solution	С Fe³⁺ μ g/test	А_{1. 2. 3}_.	А_av	r = a= b=

3. Calculations:

X = C / V, where X is the concentration of iron, mg / l; C - iron content in the analyzed sample, found according to the graph; V - volume of water taken for analysis, ml. Compare with the MAC of iron 0. 3 mg / l.

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