PURPOSE: To measure the concentration of nickel simply, quickly and accurately in a nickel plating liquid or the like by employing an aqueous solution of nickel sulfate, chloric acid, ammonium chloride and boric acid as nickel reference liquid for atomic absorption analysis.

CONSTITUTION: For example, 4.7845g of NiSO4.7H2O is weighed accurately and dissolved into water to make 1l of a solution. Amounts of 1, 2 and 5ml are sampled exactly from the solution and 1ml of (1+1) chloric acid, 1ml of a 1W/V% NH4Cl solution and 1ml of a 1W/V% H3BO3 solution are added separately to those sampled. Then, water is added thereto to reach 50ml and the products thus obtained are used as reference liquids for atomic absorption analysis. In addition, an Ni plating liquid, diluted exactly to 1/5,000 is used to performs an atomic absorption analysis at a wavelength of 232nm, an acethylene flow rate of 2.5l/min and an air flow rate of 18l/min. Thus, the measurement of Ni concentration can be performed at a high accuracy and simply, quickly and safely in the same manner as the capacity analysis using NaCN.