4. When determining nickel by the method of differential spectrophotometry, 100.0 ml of a colored solution was obtained from a sample of steel (αst) 0.2542 g after appropriate processing. The relative optical density of this solution was found to be 0.55. To construct a calibration graph, five standard solutions were taken with a nickel content of 6.0; 8.0; 10.0; 12.0;14.0 mg in 100 ml. The optical density of the obtained solutions is equal to 0.16, respectively; 0.32; 0.48; 0.62; 0.78. The reference solution contained 4.0 mg nickel per 100 ml. Determine the percentage of nickel in steel.

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