Based on preliminary experiments, the three wavelengths were selected to be 710, 625 and 530 nm. The
net absorbance of the azo dye at 625 nm is found to
correlate linearly with the concentration of nitrite. For
optimization of the analytical procedure the various
reaction condition involved were thoroughly investigated. Fig. 1 shows that within the concentration range
studied, 1.38–6.9 mg l −1 nitrite, the optimum concentration of 4-nitroaniline was found to be 2.76 mg l −1 ,
using 1 ml of the reagent. Adjustment of the concentration of hydrochloric acid during the diazotization
Fig. 2. Effect of the concentration of 1-naphthol using 2× 10−3 M 4-nitroaniline, 0.05 mg ml−1 NO2− , 0.06 M NaOH, and a shaking period
of 20 min.
step is also important where the maximum absorbance
of the azo dye is found to be at pH 0.3–0.5. In the
coupling step, the concentration of 1-naphthol must be
adjusted to 8 mg l−1 , above and below which a great
decrease in the absorbance occurs, as is clear from
Fig. 2. While optimum and constant absorbance values were obtainable at pH=11–12 using sodium hydroxide, as can be seen in Fig. 3, dye formation is
completely hindered below pH 9