Spectrophotometric Determination of Sodium Nitroprusside by Coupling with Diazotized p-Nitroaniline and Determination of Thermodynamic Parameters

The coupling reaction of sodium nitroprusside (SNP) with diazotized p-nitroaniline (DPNA) in basic media has been studied using spectrophotometric method. The intense yellow water-soluble associated species which was formed, showed maximum absorbance at 278 and 396 nm, depending on the concentration of SNP. Two calibration curves have been obtained for the determination of SNP with DPNA. The first curve (max=278 nm) obeys Beer's law over the range 25-300 g/25 ml, with a regression equation of A=0.0027C+ 0.0747 ; R2 = 0.9853. The second curve (max=396 nm) obeys Beer's law over the range 300-5000 g/25 ml, with a regression equation of A= 0.0002C+ 0.0439 ; R2=0.9943. The stability constants (K) were determined at different temperatures (283, 288, 293, 298 and 303K). Van't Hoof plot was obtained from which the thermodynamic parameters (free energy change G, enthalpy change H, and entropy change S) were obtained.