A polarographic investigation of the alkaloids of Ungernia

Summary In the investigation of the polarographic behavior of nine ungerine alkaloids, it was found that the reduction of narwedine forms the corresponding carbinol, while the reduction of hippeastrine, ungerine, and unsevine forms the corresponding tetrahydro derivatives. The catalytic hydrogen waves that all the ungerine alkaloids form can be used for analytical purposes.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 365–367, May–June, 1976.     

AC polarographic reduction of biguanides

Biguanides present ac polarographic activity in buffered media. At acid pH the reduction process is controlled by diffusion. The equation of waves and measurement criteria are discussed.     

Electrochemical study and polarographic determination of ranitidine

In the polarographic reduction of ranitidine, an H₂-antagonist of histamine, three waves are observed; their half-wave potentials and limiting currents depend strongly on the pH of the solution. The first and second waves are due to reduction of teh protonated, CHNO₂H⁺, and unprotonated, CHNO₂, nitroethene group of ranitidine, respectively; the origin of the third wave is unknown. The characteristics of the second and third waves are studied in acetic acid/acetate buffer at pH 5.5; the first wave does not appear at this pH. The second wave (E_{$\text{1}\text{2}$} = ?0.90 V, vs. Ag/AgCl) is useful for determining ranitidine in the range 2.4–4.9 × 10^?4 M by direct current polarography and in the range 2.5 × 10^?7?2.05 × 10^?5 M by differential pulse polarography.     

Effects of surface-active substances on polarographic waves of some complexes

The effect of the charge type of surface-active substances on the polarographic waves of various complexes has been investigated. The results indicate that the polarograms are practically unaffected by the presence of a aurfactant, provided that the adsorbed layer has a charge opposite to that of the depolariser. Polarographic waves of depolarisers with the same charge as the adsorbed layer are, however, shifted to more negative values or greatly distorted. The distorting effect is qualitatively interpreted in terms of an electrostatic repulsion between the adsorbed layer and the depolariser. The charge type of a surfactant seems to be important for the appropriate choice of a nondistorting maximum suppressor.     

A study of the polarographic reduction of methaqualone

Methaqualone [2-methyl-3-o-tolyl-4(3H)-quinazolinone] is reduced at pH 1.5–5 in a single two-electron step. At pH 1.5–3, wave i₁ appears and is gradually replaced by wave i₂ which predomiantes in the solution between pH 4.5 and 6.5. At pH > 5, the height of wave i₂ decreases in the shape of a dissociaton curve with increasing pH. For both waves, the diprotonated form of methaqualone (I_a) is reduced to 1,2,-dihydromethaqualone as the final product. In wave i₁, the monoprotonated form I_b is further protonated at the electrode surface before it accepts the first electron; in wave i₂, the free base form I accepts two protons at the electrode surface before the first electron uptake. Polarographic curves are complicated by the presence of three waves of catalytic hydrogen evolution. Wave i_{1, cata} appears at pH < 5, waves i_2,cata at ?1.5 V and i_3,cata at ?1.7 V at pH > 5. Citrate buffers pH 1.5–3 or Britton-Robinson buffers at pH 2.6–3.6 are most suitable for quantitative work with eitehr d.c. polarography or differential pulse polarography.     

Mathematical treatment of polarographic reversible adsorption waves and considerations on their origin

Polarographic current-potential characteristics and current-time curves for a Nernstian charge transfer O+neR with Langmuir or Frumkin adsorption of reactant and/or product are derived via a mathematical procedure based on the diffusion-layer approximation. It is shown that all features of reversible adsorption waves can be explained by the effect of adsorption of reactant and product upon the only rate-determining stage, namely the diffusion stage. Conversely, no conclusion can be drawn as to whether charge transfer takes place in the adsorbed or else in the non-adsorbed state from a comparison between theoretical predictions and experimental behaviour. The origin of adsorption prewaves and post-waves is justified by the concept of entropy production, which can also be used to explain the origin of the so-called adsorption pseudo-prewaves.     

Effect of hydration on polarographic reduction of some carbonyl compounds

Hydration and acid-base equilibria of the hydrated form govern the reduction and oxidation of numerous carbonyl compounds. Aliphatic and aromatic aldehydes are reduced in the free carbonyl form and oxidized as geminal diol anions. In haloacetaldehydes and haloacetones the C?X bond can be reductively cleaved both in the free carbonyl and in the hydrated form, oxidized form is again the geminal diol anion. Schemes (6), (7), (10), (11), (12) and (13) summarize the mechanism of the hydration and its relation to electrooxidation and reduction. Differential pulse of polarographic waves enabled measurement of heights of waves close to the initial or final rise of current and following the variations of their height with changes in activity of the solution.     

Effect of substituents on the polarographic behaviour of α-arylhydrazononitriles: Further evidence for the mechanism of polarographic reduction of α-arylhydrazononitriles

The polarographic behaviour of the α-arylhydrazonomesoxalonitrile derivatives (IIb-i) and of 2-arylhydrazono-3-keto-3-phenylpropionitriles (IIIb-f) was investigated. With the exception of the nitro-substituted derivatives IIh, i and IIIf all the investigated compounds showed polarographic waves similar to that of their respective parent compounds IIa and IIIa. The m-nitro derivatives IIh and IIIf were firstly reduced in a 4 e wave to the corresponding hydroxylamino derivatives which were subsequently reduced in the manner common to other α-arylhydrazononitriles. On the other hand, the p-nitro derivative IIi was first reduced to the hydroxylamino derivative which then lost water to yield the corresponding quinoneimine. Reduction of the latter product in a 2 e wave gave the p-amino derivative IIe, which then underwent 4 e reduction to p-phenylenediamine and aminomalonitrile. The E_1/2 values corresponding to reduction of the arylhydrazonic moiety of compounds IIa-i and IIIa-f at different pH values were correlated to Hammett's different sigma sets. Analysis of the results provided evidence for the suggested mechanism for reduction of these compounds.     

The polarographic determination of copper and zinc in plants and soils

A method is described whereby the extraction and polarisation of copper and zinc with maximum, sensitivity is possible. Interference from cobalt is prevented by extracting with dithizonc in the presence of dimethylglyoxime.The supporting electrolyte used is dccimolar potassium hydrogen phthalate which permits satisfactory waves for both copper and zinc (with greater sensitivity for copper, than is the case with the ammonia-ammonium chloride electrolyte).     

Differential-pulse polarographic determination of the insecticide imidacloprid in commercial formulations

The reduction reaction of imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] at a mercury electrode shows two well-defined waves in the range of pH 2.0–11.0. The characteristics of the electrode processes were examined. The analyte captures four electrons in the first step and two in the second to give the hydroxylamine and amine derivatives, respectively. A differential-pulse polarographic method for the determination of imidacloprid based on the first reduction peak of this compound is presented. Britton-Robinson buffer was used as a supporting electrolyte and optimum pH value was found to be pH 8.O. The applicable concentration range was from 10 to 200 ng/ml, with a relative standard deviation of 1.5% (for a level of 80 ng/ml) and a detection limit of 3 ng/ml. The method has been satisfactorily applied to the determination of imidacloprid in commercial formulations.     

Simultaneous polarographic determination of cadmium and tellurium in electro-deposited cadmium telluride thin films

A polarographic method has been developed for the simultaneous determination of cadmium and tellurium in thin-film cadmium telluride. The procedure involves dissolution of the film with concentrated nitric acid, which is subsequently removed by evaporation. The Cd(II) and Te(IV) waves are well separated at pH 10, but sufficient ammonia must be present to prevent the precipitation of cadmium hydroxide.     

Extraction with long chain amines-VII polarographic determination of uranium

A polarographic determination of uranium is described, based on the highly selective extraction of uranium(VI) with a chloroform solution of trioctylammonium chloride, followed by re-extraction of uranium into an aqueous 0·5M KCl-0·5M HCl solution, which also serves as the polarographic supporting electrolyte. In this medium, uranium(VI) is reduced at the dropping mercury electrode to give two polarographic waves, the first of which is of analytical significance. In this way it is possible to determine 50–2400 μg of uranium in 1 ml of supporting electrolyte and in the presence of large amounts of accompanying elements.     

Studies on the polarographic adsorptive complex waves of germanium-alizarin red S

In a supporting electrolyte containing 6 × 10^?5 M Alizarin red S, 0.2 M NH₃-NH₄Cl, at pH 8.2-8.6, three adsorptive complex waves of germanium-Alizarin red S have been obtained by single-sweep polarograph. The most sensitive one among them is at E_p=?1.130 V (SCE). The compositions of the complexes and the mechanism of electrode processes were studied in detail.     

A polarographic study of Cd(II)-NTA complexes in alkaline solution

Up to four d.c. polarographic waves were observed on voltammetric curves of Cd(II)-nitrilotriacetate complexes on the DME in the pH range 7.0–11.5. These polarograms are analyzed using a non-linear least squares method and the mechanism of the electrode reactions and prior homogeneous reaction are discussed. The kinetic parameters of the separate reactions are calculated and compared with literature values. The accelerating influence of OH^? ions on the dissociation of the complexes is established.     

Logarithmic analysis of two overlapping d.c. polarographic waves: V. Application of a minicomputer to automatic analysis

A system for automatic analysis of d.c. polarographic waves, based on a DEC PDP-11/20 minicomputer and a PAR 170 Electrochemical system is described. More general expressions for analysis and separation of overlapping d.c. polarographic waves are derived than those previously reported in the literature. Corresponding programs are prepared in order to enable the automatic interpretation of polarographic experiments in an on-line system.     

D.C. and pulse polarographic studies of rhodium(III)-protein catalytic systems

The Rh(III) complexes of substituted ethylenediamines are observed to be catalytically active at the DME and studies of the effects of human serum proteins on this activity are reported. Rh(III) complex waves are shown to respond to six different proteins, albumin, α-, β-, γ-globulin, mercaptoalbumin, and glycoprotein at trace concentration levels. The effect gives linear calibration for all proteins in the range 0.1–0.8 μg ml^?1 with sensitivity in the order glycoprotein >albumin>γ-globulin > β-globulin > α-globulin > mercaptoalbumin at pH 8. Important factors controlling sensitivity are buffer composition and pH, and Rh complex structure and concentration, and at pH 9.2, almost equal response of proteins is obtained. The method for protein analysis is compared to the Brdicka method and found to have several advantages in terms of sensitivity, pH range, response of differing proteins and wave resolution.     

On the use of sulfonated tolyl and xylyl stearic acids as polarographic maxima suppressors

Summary Sulfonated tolyl and xylyl steario acids were used as maxima suppressors in the reduction of simple and complex metal ions. It was found that these compounds can be employed as maxima suppressors for the cathodic waves of Pb²⁺, Co²⁺, Ni²⁺, copper glycine complex and copper biuret complex. In the case of difficulty suppressible maxima like those of iodide-cadmium complex and Ni²⁺-Co²⁺ mixture these were found to be more effective than other maxima suppressors. The P. M. P. and M. S. P. values were calculated in presence of these ions. The P.M.P. values of STSA were found to be greater than those for SXSA. This behaviour may be attributed to greater +I effect of xylyl group.     

The polarographic determination of aluminium : The oscillographic polarography of aluminium in calcium chloride solution

A study of the polarography of aluminium using the linear-sweep cathode ray polarograph is described. The conditions nccessasary for obtaining satisfactory aluminium waves in this medium have been established and the relationship between aluminium peak height and concentration examined. The occurrence of unexpected phenomena relating the change of step shape with varying start potential values is described and discussed and a method for overcoming the effects, by buffering, is presented     

超微量铂丶铑连续催化极谱测定及其催化波机理的探讨

For simultaneous determination of ultramicro amounts of platinum and rhodium the optimum condition has been described as 1.5N H2SO4-1.2% NH4Cl-0.0012M (CH2)6N4-0.003% N2H4XH2SO4. Both platinum and rhodium produce hydrogen catalytic waves with peak potential at-1.03 V and - 1.27 V (vs. S. C. E.) and the peak height of differential wave in single-sweep polarograph is directly proportional to the concentration of the metals in the range from 0.05 ppb to 1.0 ppb for platinum and from 0.0025 ppb to 0.1ppb for rhodium, respectively. The influence of other platinum group metals and some base metals on the height of catalytic waves has been examined. It has been shown that the method is very selective. It is applied even at 200:1 or at 1:10 (Pt:Rh). The mechanism of the catalytic waves has been discussed. The catalytic waves of both platinum and rhodium are due to "surface catalytic wave of hydrogen". The wave of rhodium can be attributed to catalytic discharge of hydrogen ion by the complex (Rh(CH2O)2Cl4)-. One of the ligands, formaldehyde, is the product of hydrolysis of hexamethylenetetramine. The wave of plainum can be attributed to catalytic discharge of hydrogen ion by the complex (PtACl5)-, where a denotes intermediate product (a substance containing CH2=N group) formed during the hydrolysis of hexamethylenetetramine. The role of hydrazine sulfate in catalytic system has been shown. Hydzazine can react with formaldehyde to from (CH2=N)2 which promotes the growth of platinum catalytic wave and in this way the concentration of formaldehyde in the system can be controlled.     

Catalytic polarographic wave of cobalt(II) in sodium azide with hydrazoic acid

Extensive catalytic waves of Co(II) are observed in N₃^?/HN₃ buffers, in which hydrogen discharge is not the dominant process, but rather HN₃ reduction to NH₄⁺ and N₂. Cobalt(I) was shown to be the active intermediate of the catalytic process. The rate-determing step was found to be the Co(I) reduction to the metal. Heterogeneous rate constants were calculated and relationships with average ligand number were derived.