Novelette P. Sadler

Application of the Folin-Ciocalteau reagent to the determination of salbutamol in pharmaceutical preparations

A method for the determination of salbutamol in both tablets and syrups is described. It utilizes the reduction of the Folin-Ciocalteau reagent by the phenolic group, monitoring the absorbance of the resulting complex at 760 nm. Results obtained are linear over the range 0-6 mug ml(-1) salbutamol. Coloring material was removed by anionexchange chromatography prior to analysis and there was no interference from sucrose, neutral flavorings or the common preservative sodium benzoate. This method appears suitable as a general assay for salbutamol.

Cobalt(III)-promoted hydrolysis of 4-nitrophenyl phosphate: the role of dinuclear species

Abstract The reactions which follow on mixing aqueous solutions of Co III (tn) 2 (aq) and 4-nitrophenyl phosphate (NPP) have been reexamined using spectrophotometry and 31 P NMR (here ‘(aq)’ refers to (H 2 O) 2 , (H 2 O)(OH), or (OH) 2 depending on pH; charges omitted). When solutions of Co III (tn) 2 (aq) and NPP are mixed with pH maintained in the neutral region, equilibrium formation of the mononuclear monodentate complex Co III (tn) 2 (OH 2 )(NPP) ( cis and trans ) is established in a few minutes. This 1:1 complex (or its deprotonated hydroxo form) undergoes NPP hydrolysis by two parallel paths: (a) direct rate determining hydrolysis to produce 4-nitrophenol(ate) (NP) and Co(tn) 2 PO 4 ; and (b) reaction with additional Co III (tn) 2 (aq) to form a dinuclear complex of NPP, which never builds up to directly detectable levels and which undergoes hydrolysis to produce NP and the dinuclear complex [(tn) 2 Co] 2 (μ 4 -PO 4 ) 3+ . Path (b) is favored by high reactant concentrations and by high cobalt to NPP ratios. Additional dinuclear complex of orthophosphate is produced by reaction of Co(tn) 2 PO 4 with Co III (tn) 2 (aq). At pH 7.3 and at 25°C: for path (a) k =1.75×10 −4 s −1 (a rate enhancement of ∼3×10 4 over the unpromoted NPP hydrolysis rate); for path (b), the first order rate constant for hydrolysis of the dinuclear complex of NPP is estimated to be >5.8×10 −3 s −1 (a rate enhancement of at least 1.0×10 6 over the unpromoted NPP hydrolysis rate).

Oxidation of biological substrates by chromium(VI). Part 1. Mechanism of the oxidation of L-ascorbic acid in aqueous solution

The kinetics of oxidation of L-ascorbic acid (H2A) by potassium chromate has been studied under aerobic and anaerobic conditions over the range 0.002 ⩽[A]T⩽ 0.040 mol dm–3, 3.50 ⩽ pH ⩽ 8.70, 17.8 ⩽T⩽ 35.1 °C, 0.06 ⩽l⩽ 0.50 mol dm–3(NaClO4) and 0.05 ⩽[O2]⩽ 0.12 mmol dm–3. The experimental rate constants obtained in the presence of oxygen are about 10 times smaller than those obtained in its absence. A mechanism involving the formation of a chromium(VI)–ascorbate–oxygen intermediate is discussed in an attempt to explain this. The disappearance of chromium(VI) under aerobic conditions follows the rate law (I) where kf=(k1Ka1[H+]+k2Ka1Kc+k3[H+]2)/(Ka1+[H+])(Kc+[H+]). –d[CrVI]/dt=kf[A]T[CrVI]T(I) At 25 °C and l= 0.50 mol dm–3(NaClO4), k1= 59.4 ± 0.5, k2= 0.10 ± 0.02 and k3= 115 ± 3 dm3 mol–1 s–1.

Oxidation of Thiol-containing Substrates by the Pentaamminechromatocobalt(III) Complex Ion

Abstract The reactions of the pentaamminechromatocobalt( III) complex ion with a series of thiols, RSH (where RSH = glutathione, L-cysteine, and DLpenicillamine) have been studied by spectrophotometry at pH > 7, over the range 20°C ≤ T ≤ 34.4°C, with the thiols in excess at ionic strength 0.50 Μ (NaClO4). The reactions occur in two stages; the first is a rapid reaction between reactants leading to formation of outer-sphere adducts, which are facilitated by electrostatic interaction between the Chromate complex and each thiol. For L-cysteine, the equation representing the first stage is rate = k2K1 [complex] [RSH]T/(1 + K1RSH]T). (i) At 25°C, the formation constant for the adduct is K1 = (800 ± 110) M-1 and the rate constant for its decomposition to pentaamminehydroxocobalt(III) and Chromate ions is k2 = (4.1 ± 0.2) x 10-3 s-1 (pH = 7.40). Rate parameters for the reactions of DL-penicillamine and glutathione with the complex are presented. The second slower stage of all three reactions is postulated to be oxidation of the thiols by the free chromium(VI) ion formed in the first stage. From schemes derived for the oxidation of the thiols, a general equation obtained may be expressed as follows: kobs = (k5/k6[RSH]2 + k5k7[RSH]) /(k-5 + k7 + k6[RSH]), (ii) where k5 represents the formation of a chromium(VI) ester intermediate, k -5 its reversible decomposition, k6 is the reduction to chromium(IV) by excess of the reductant and finally k7 represents intramolecular transfer of an electron resulting in the reduction of the intermediate to chromium(V). For L-cysteine and DL-penicillamine, k5= (0.47 ± 0.01) M-1 s-1 and (2.47 ± 0.01) x 10-1 M-1 s-1 , respectively. The pentaamminecobalt( III) moiety is not reduced, but reacts slowly with the thiols to form complexes. These latter complexation reactions are significantly slower than the electron transfer reactions.

Mechanism of the oxidation ofL-ascorbic acid by the pentaammineaquacobalt(III) ion in aqueous solution

SummaryThe kinetics of oxidation ofl-ascorbic acid (H2A) by the mononuclear complex [Co(NH3)5(OH2)] (ClO3)3 have been studied spectrophotometrically at 490 nm. The mechanism involves single one-electron transfers involving the pentaamminehydroxo complex and the ascorbate anions (HA− and A2−), subsequent formation of ascorbate radicals and > 90% CoII. The appropriate rate law in the 7.00 ⩽ pH ⩽ 8.40 and 0.005 ⩽ [AT] ⩽ 0.05 mol dm−3 ranges, has been established as:

Mechanism of the acid catalysed hydrolysis of the chromatopenta-amminecobalt(III) ion

\begin{gathered} - d [ complex] /dt = \{ (k_{\text{3}} {\text{[H}}^{\text{ + }} {\text{]}} \hfill \\ {\text{ + }}k_{\text{4}} K_{{\text{a1}}} {\text{)}}K_{\text{2}} {\text{[}}A{\text{]}}_{\text{T}} {\text{[complex]}}_T \} / \hfill \\ {\text{ \{ ([H}}^{\text{ + }} {\text{ + }}K_{{\text{a1}}} {\text{)([H}}^{\text{ + }} {\text{] + }}K_{\text{2}} {\text{)\} }} \hfill \\ \end{gathered}

Mechanism of the oxidation of DL-penicillamine and glutathione by chromium(VI) in aqueous solution

The rate is slower at lower pH as the less important reactionk1 (ROHinf2sup3 + HA− → products) becomes dominant, where R is (NH3)5CoIII. The more significant rate constantk4 (ROH2+ + A2− → products) was calculated at 25.7 °C as (2.7 + 0.5) × 102 dm−3mol−1 s−1, with ΔH# = 105 ± 11 kJ mol−1 and ΔS# = 93 +- 45 J mol−1K−1.

Iron(III)-Promoted Hydrolysis of 4-Nitrophenyl Phosphate

SummaryThe kinetics of formation of chromatopentaamminecobalt(III) from aquapentaamminecobalt(III) and chromate ions have been studied spectrophotometrically over the pH range 6.0 to 8.0 and temperature range of 16.7° to 25.8°C, with ionic strength 0.5 mol dm−3. Over this pH range, both the [Co(NH3)5(OH2)]3+ and [Co(NH3)5OH]2+ ions react with [CrO3(OH)]− to give the title complex, and the rate constants are 16.4±0.1 mol−1 dm3s−1 and 8.03±0.03 mol−1 dm3s−1 respectively at 25.8°C. These studies indicate that complexation is fairly rapid, suggesting a mechanism involving the cleavage of the CrVI−O and not the CoIII−O bond. Details of the mechanism are discussed.