|| The anthor found that azocal-A reacts only with calcium, and devised a direct colorimetric, and a direct titration method for calcium determination using azocal-A as an indicator, and obtained a satisfactory result. Accuracy : detectable amount in NaOH……0.28 γ, of Ca ; detectable amount in NH(4)OH……2 γ, of Ca. Sensibility……1 : 125,000 & 1 : 17,500 respectively. Interfering substances : Fe, U, Ni, Co, Hg, Ag, citrate, tartarate, oxalate, large amount of NH(4) salts, Sr, Mg. Not interfering subst. : Ba, F, acetate and silicate. Reagents : Saturated soln. of azocal-A in weak NaOH soln.. Spot test procedure : Place a drop of the acid or neutral test soln. upon white spot plate, add 1 drop of azocal-A soln., and mix them. If a yellow color appears, calcium is present. Colorimetric method: Transfer water sample containg not more than 0.05 mg of calcium, and not more than 5 mg of Fe, Mn, Mg, etc. to a 20 cc colorimetric tube, add 0.5-1 cc of azocal-A soln. and 0.5 cc of 6N-NaOH soln.. Allow it to stand for 10 minutes. Then compare the resulted yellow color with standard Ca soln. prepared similarly and simultaneously. Volumetric method : Amount determinable : 0.1 g of calcium. Transfer 40 cc of sample containg 5-100 mg to a 100 cc Erlenmeyer's flask, add 0.5 cc of 6N-HCl, add azocal-A to the amount of pink color as caused by methyl orange in acid soln.. Make alkaline with 1 cc of 6N-NaOH. Add 10 cc alcohol to every 40 cc of the sample solution. Then its color turns to yellow. Titrate with 0.1 N oxalate soln.. 1.0 cc of 0.1 N oxalate soln. is equivalent to 2.0 mg of Ca. Absorption band of the acid soln. at 4900 A, alkaline soln. at 5000 A, Ca-compound at 4300 A. The azocal-A is o-carboxy-benzol-azo-2-naphtol 3, 6-disulfonic acid prepared from anthranilic acid and R-salt.