Thank you for spending your time on my problem. I need to develop the purple color for samples containing CN- before UV-VIS measurement. And here is my procedure:
Firstly, 5ml of sample is added to volumetric flask (25ml), which contains a certain volume of NaOH (dilution solution). Then 0.5 ml of Acetate buffer (pH = 5.5) and 1ml of Chloramine-T, respectively, are added. Mix and keep the flask stand still in 2 min. Finally, 2.5 ml Pyridine/Barbituric acid is added for color development. Add distilled water to the line and mix evenly. Wait for full color development in 8 mins then measure the sample within 20+-5 mins.