An-Najah Staff

A method for the spectrophotometric determination of cobalt(II) is presented, with a comparison of the binary complexes formed by cobalt(II) with di-2-pyridyl ketone nicotinoylhydrazone (DPKNH), di-2-pyridyl ketone 2-thiophenoylhydrazone (DPKTH) and di-2-pyridyl ketone benzoylhydrazone (DPKBH) in 50% (v/v) ethanolic solution. Cobalt(II) forms 1:2 complexes with the three reagents. Maximum absorbance is at 372 nm for Co(II)-DPKNH, at 389 nm for Co(II)-DPKTH and at 370 nm for Co(II)-DPKBH. Ranges of linearity, effects of pH and excess reagent, sensitivity, stability of the complexes and tolerance limits of ions are reported. It was concluded that the system with DPKTH is the best, followed by DPKNH and then DPKBH. The method was applied to the determination of cobalt in different alloys.

The spectral and differential pulse polarographic (DPP) behaviour of di-2-pyridyl ketone 2-thienoylhydrazone (DPKTH) has been investigated in 30% (v/v) ethanol-buffer mixtures over a wide range of pH (2.0-12.0). The spectral bands located at 325, 276, 230 and 208 nm of the DPKTH in ethanol are assigned to the possible electronic transitions of the molecule. The spectral data at various pH values indicates that the molecule characterized by the keto = enol tautomerization and the pK_a value for the enolic form of the compound is determined in aqueous ethanol and was found to be 10.6. The effect of various operational parameters on the reduction current and the mechanism of the electrode reaction of DPKTH at the DME are discussed. The main reduction peaks are attributed to the reduction of C=N centre of both the keto and enol forms. This behaviour is compared with the DPP behaviour of the other related acid hydrazone compounds. The applicability of DPP technique for the trace determination of DPKTH was tested under the optimum experimental conditions and the detection limit is found to be 0.09 μm.

Two spectrophotometric methods are described for spectrophotometric determination of tiopronin in pharmaceuticals. They are based on the oxidation-reduction reaction between tiopronin and iron(III)-ferrozine complex [system I] or iron(III)-di-2-pyridyl ketone-2-thiophenoyl-hydrazone complex [system II]. The produced colored iron(II)-Ferrozine or iron(II)-di-2-pyridyl ketone-2-thiophenoyl-hydrazone complex absorbs at 562 nm and 656 nm, respectively. The effect of different factors such as; pH, reagent concentration, times, temperature and the tolerance amount of the common excipients have been reported. Applying the optimum working conditions, tiopronin can be determined over the range 0.2-8.6 and 0.5-17.0 ppm when using system I and II, respectively. The two methods offer high selectivity, sensitivity and accuracy with RSD less than 1.5% for five measurements. The proposed methods were applied successfully for the determination of tiopronin in Captimer tablets.

Two methods are described for quantitative determination of nizatidine. The first is a cathodic stripping voltammetric method which is based on the accumulation of the compound at the hanging mercury drop electrode. The adsorptive stripping response was evaluated with respect of accumulation time, potential, concentration, pH and other variables. A linear calibration graph was obtained over the range 3.0×10⁻⁸–1.0×10⁻⁶ M with a detection limit 3.0×10⁻⁸  M after a 20s accumulation time at −0.2 V accumulation potential. On the other hand, it was found that the detection limit could be lowered to 1.0×10⁻⁸ M after 180s accumulation time at −0.2 V accumulation potential. The relative standard deviation was in the range 1.2−2.0% for six measurements. The tolerance amounts of the common excipients have also been reported.
The second is a spectrophotometric method which is based on the formation and extraction of the ion-pair complex formed between nizatidine and either bromocresol green or bromothymol blue. The extracted colored ion-pair complexes absorb at 416 nm. The effect of different factors such as: type of organic solvent, pH, reagent concentration, number of extraction times, shaking time, temperature and the tolerance amount of the common excipients have been reported. The calibration graph was linear in the range 6.0×10⁻⁷–1.8×10⁻⁵ M with a detection limit of 6.0×10⁻⁷ M and molar absorptivity of 2.1×10⁴ lċmol⁻¹ċcm⁻¹ when using bromocresol green, while the calibration graph was linear in the range 3.0×10⁻⁷–1.1×10⁻⁵ M with a detection limit of 3.0×10⁻⁷ M and molar absorptivity of 3.2×10⁴ lċmol⁻¹ċcm⁻¹ when using bromothymol blue. The spectrophotometric methods offer alternative methods with reasonable sensitivity, selectivity and accuracy with relative standard deviation in the range 2.1−6.0% and 1.2−4.7% (for six measurements) when using bromothymol blue and bromocresol green, respectively. The proposed two methods were applied for the determination of nizatidine in commercially available dosage forms. A comparison between the voltammetric and the extraction-spectrophotometric methods was also reported.