90. When determining manganese in steel, a sample of it x g was dissolved, manganese was oxidized to MnO4-, the solution was diluted to V cm3. Photometry was carried out by the differential method with a standard solution of manganese C(Mn)=20 mg/dm3 and a zero solution of KMnO4- with a concentration of C1/2=0.0005 mol/dm3. The optical densities of the test and standard solutions with respect to zero were, respectively, A'st and A'x.
X=0.115 g V=210 cm3 A’st=0.22 A’x=0.18

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