Five new unsymmetric derivatives of carboxylazo, p- carboxycarboxylazo, p-chlorocarboxylazo, p-methylcarboxylazo, p- sulfocarboxylazo and m-nitrocarboxy-lazo were synthesized. A new type, β-type reactions of calcium ion with these reagents were studied. The relationship between the molecular structure of reagents and reaction conditions was detected. The β-type reaction behavior between calcium ion and p-carboxycarboxylazo was investigated in detail. calcium ion reacts with it to form a 1:2 sensitive and stable β-type complex in citric acid medium, having an absorption maximum at 718nm (Δλ=157nm). Beer's law is obeyed for 1-13μg of calcium in 25mL of solution with an apparent molar absorptivity oof 1.51× 10^5L.mol^-^1.cm^-^1 at 718nm. The complex formation is complete within 15 min in a small volume and the resulting complex is stable for 4h after dilution. Kinetic study on this system has shown that calcium ion reacts with p-carboxycarboxylazo to form only a β-type complex and the order of reaction is 2.

Key words: CALCIUM ION, DERIVATIVES, CARBOXYAZO III, COLOR REACTION, REACTION PROPERTIES, CITRIC ACID, ORDER OF REACTION