7. When analyzing steel for chromium according to the method of three standards on a microphotometer, blackening (S) of the lines of a homologous pair was measured in the spectra of the standards and the test sample. Find the percentage of chromium using the following data:

Reference 1 2 3
СCr,% 0.50 1.23 4.17
SCR 0.07 0.37 0.86
SFe 0.27 0.23 0.27

The analyzed sample has SCr=0.61 and SFe=0.25.

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