3. Analysis of soil extract.

1. Building of calibration graph

Take 6 flasks with the volume of 50 ml, put with the pipette 0; 1. 0; 2. 0; 3. 0; 4. 0; 5. 0 ml of standard solution with an iron concentration of 100 μ g / 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; 2; 4; 6; 8; 10 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. Preparation of soil extract.

Take 5 g of the analyzed soil and transfer the sample to a conical flask. Add 50 ml of a 0. 5 M nitric or sulfuric acid, cover with a stopper and shake for 30 minutes. Filter the extract through a dry folded filter (you should roil the flask content before filtering). If the extract is turbid, filter it once again through the same filter.

3. Determination of mobile iron in the soil.

Take 5 ml of soil extract and place in a 50 ml volumetric flask. Add 10 ml of distilled water and all the necessary reagents, as in the calibration graph. Bring the volume in the flask to the mark with distilled water and mix. Measure the optical density with relative to the zero solution. By the amount of A find according to the calibration graph concentration of Fe³⁺ in μ g/test.

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

3. Calculations:

X = C*V₁*50 / g*V₂, where X is the concentration of iron, mg /kg; C - iron content in the analyzed sample, found according to the graph, μ g/ml; V₁ - total volume of soil extract, ml; V₂ - volume of soil extract, taken for analysis, ml; g – soil mass, g.

Write down the calculation of the sample to be taken for the preparation of the standard solution with the concentration 100 μ g / ml and 10 μ g / l of iron (III).

What compound is the photometric measurement form in this laboratory work?

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