A Simple and Reliable Spectrophotometric Method for the Determination of Ascorbic Acid in Pharmaceutical Preparations

In this study, a simple spectrophotometric method based on the reaction between ascorbic acid and the copper(II)–ammonia complex is presented for the determination of the vitamin C content of pharmaceutical preparations. During this reaction, ascorbic acid is oxidized and the copper(II)–ammonia complex is reduced to the copper(I)–ammonia complex, and the absorbance decrease at 600 nm (_max for the copper(II)–NH₃ complex) is measured. Stirring the final solution in the presence of air leads to the primary absorbance again being obtained, which indicates that the copper(I)–NH₃ complex is quantitatively oxidized to the copper(II)–NH₃ complex by O₂. The linear dynamic range of the calibration curve is 0.8–6 mmol with a detection limit of 0.26 mmol. The relative standard deviation for eight repeated experiments is 2.4%, which shows that the proposed method has a good repeatability. Finally, this method was used in the analysis of the vitamin C content of different pharmaceutical preparations, such as multivitamin tablets and syrups, vitamin-C tablets and powders, and effervescent tablets. The obtained results are in good agreement with iodimetric data.     

Experimental and calculated complex formation curves for mixed, dynamic and semi-dynamic, metal–ligand systems.: A differential pulse polarographic study of Cu–sarcosine–OH and Cd–MIDA–OH systems at a fixed ligand to metal ratio and varied pH

The Cu–sarcosine–OH and Cd–MIDA–OH systems have been studied by differential pulse polarography (DPP) at a fixed total ligand to total metal concentration ratio and varied pH at 298 K and μ=0.5 mol dm⁻³ in the background of NaNO₃. Both the metal–ligand systems show initially dynamic (labile), followed by semi-dynamic behaviour on the DPP time scale. It has been shown that the experimental and calculated DPP complex formation curves used previously only for labile metal–ligand systems can be employed for the modelling of all species formed in a solution and optimisation of their stability constants. The stability constants of ML and ML₂ complexes as log β were estimated for Cu^II and Cd^II as 7.75±0.02, 14.49±0.01 and 6.67 ±0.02, 12.00±0.02, respectively (all known hydroxide species of copper and cadmium, including polynuclear species, were incorporated into the metal–ligand–OH systems). The formation of the complex CuL₂(OH) is suggested also and its stability constant as log β has been estimated to be 16.2±0.2. Results reported here seem to be reasonable when compared with the literature data reported at 298 K and different ionic strengths.     

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H₃bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.     

The Effect of Aggregation in Aqueous Aerosil Dispersions on Their Structure and Mechanical Properties

The dynamics of the structure disintegration in the A-175 aerosil dispersions in water and sodium dodecylbenzenesulfonate (SDBS) aqueous solutions was studied by the rotational viscometry. The aqueous dispersions of aerosil were found to be characterized by a high degree of aggregation. At the volume content of the dispersed phase = 0.063–0.082, the flow of the dispersions is described in terms of the theory of highly aggregated systems. The effect of SDBS on the flow behavior of aerosil dispersions is complex. There is a range of SDBS concentrations (C = 2.87 × 10^–6–1.44 × 10^–5 mol/l) where it has a noticeable structuring effect. Aerosil dispersions in micellar SDBS solutions (C > 3.0 × 10^–3 mol/l) demonstrated the behavior of slightly structured systems characterized by the quasi-Newtonian flow.     

The [CoN4(OH)(OH2)]2+ (N4 = trpn,cyclen and tren) promoted hydrolysis of the phosphotriester 2,4–dinitrophenyl diethyl phosphate

The [CoL(OH2)2]3+ (L=trpn, cyclen andtren)promoted hydrolysis of the phosphotriester 2,4–dinitrophenyl diethyl phosphate to give diethyl phosphate and 2,4–dinitrophenolate has been studied in detail over the pH range 3–7.5. The pK values of the various complexes have been determined at 25°C and I=0.1moldm–3 by potentiometric titration. The pH-rate profiles for the reactions are consistent with the hydroxoaqua complex [CoL(OH)(OH2)]2+ being the active species in the hydrolysis. No evidence for saturation kinetics was obtained and values of kcat determined at pH 7 and 25°C (I=0.1moldm–3) are 4.24×10–3dm3mol–1 s–1 (tren)<3.05×10–2dm3mol–1 s–1 (cyclen)<7.5 × 10–1dm3mol–1s–1 (trpn). Studies involving the more reactive phosphonate ester 2,4–dinitrophenyl ethyl methylphosphonate suggest that the rate-determining step in the reaction is the decomposition of the ternary complex between the cobalt(III) complex and the phosphotriester. The rate enhancement using 0.01mol dm–3 Co(trpn)3+ is some 4.6×104 fold at pH7 and 25°C.     

Adsorptive Voltammetric Study of Thorium–Alizarin Complexon Complex at a Carbon Paste Electrode

A new sensitive adsorptive voltammetric procedure is described for trace measurement of thorium. It is based on the cathodic stripping peak of the thorium–alizarin complexon (ALC) complex at a carbon paste electrode (CPE). The complex of Th(IV) with alizarin is adsorbed at a CPE in a mixed buffer solution (pH 5.0) which consists of 0.1mol·L^–1 sodium acetate and 0.04mol·L^–1 potassium biphthalate, yielding a sensitive cathodic voltammetric peak corresponding to the reduction of alizarin in the complex at –0.57V (vs. SCE). The second-order derivative peak current of the complex is linearly dependent upon the concentration of Th(IV) over the range of 3.0×10^–9 8.0×10^–7mol·L^–1. The detection limit is 1.0×10^–9mol·L^–1 for 180s accumulation. The molar ratio of each component in the complex was estimated as n_Th(IV):n_ALC=1:1 by a continuous variation method. The electrode processes of the Th(IV)–alizarin complex at a CPE were investigated. The procedure was successfully applied to the trace determination of thorium in ore and clay samples.     

Thermodynamic and kinetic characteristics of one-electron transfer reactions in N-alkylaniline-tetracyanoethylene charge transfer complex systems

One-electron transfer reactions in systems containing an N,N-dialkylaniline (D) and tetracyanoethylene (A) in methylene chloride proceed through the formation of AD and AD₂ charge transfer complexes. The tetracyanoethylene -system and nitrogen unshared electron pair of the N,N-dialkylaniline participate in the formation of the charge transfer complex. The equilibrium constants for complex formation K and rates of formation of the TCE^– radical-anions k were determined for this reaction. The sign of the regression coefficient in the correlation between the thermodynamic (K) and kinetic indices (k) for the one-electron transfer reactions in charge transfer complex systems indicates the formation of shortlived intermediates between the starting compounds and final Reaction products (in the case of increasing K(AD) values with increasing k(TCE^–)) or of several longlived intermediate complexes (in the case of increasing K(AD) values with decreasing k(TCE^–).Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 23, No. 6, pp. 692–699, November–December, 1987.     

Interaction of Methyl Cysteine and Nitrilotriacetate with Transition Metal Ions

In the present work, sulfur-containing amino acid methyl cysteine was studied from the point of view of their coordinating ability with two metal ions, viz. copper(II) and cobalt(II). Solution equilibria of binary (Cu(II)/Co(II)–methyl cysteine and Cu(II)/Co(II)–nitrilotriacetate (NTA)) complex systems are investigated by paper ionophoresis at 35°C, ionic strength I= 0.1 mol/l. In addition to binary complexes, ternary complexes involving nitrilotriacetate and methyl cysteine were also studied. For studying mixed-ligand complexes, the pH of background electrolyte is brought to 8.5 (this pH value is purposely chosen because amino acid and NTA form very stable complexes much ahead of this pH). The stability constants of complexes (Cu(II)–NTA–methyl cysteine and Co(II)–NTA–methyl cysteine) were found to be 4.48 ± 0.07 and 3.55 ± 0.04 (logKvalues), respectively.     

Binary and ternary chromium(III) complexes with cellular reductants and DNA components isolated from redox type systems. Part 3. Chromium(VI) – <Emphasis Type="SmallCaps">L</Emphasis>-ascorbic acid – adenine (adenosine,ATP)

The reduction of CrO3 with an excess of L-ascorbic acid and its interaction with DNA fragments (adenine, adenosine) and the ATP nucleotide was studied by the analysis of the isolated solid products. The precipitates were characterised by elemental analyses, FAB-mass spectral data, spectroscopic methods (u.v.–vis., i.r) and magnetic measurements. The Cr^III complexes obtained from ternary systems appeared to be [Cr^III–L-ascorbic acid] and [Cr^III–L-ascorbic acid–adenine (adenosine, ATP)] species. The structure of ternary complexes has been proposed assuming hydrogen bond formation between the [Cr^III–ascorbic acid] complex and the DNA components. The effect of main cellular reductants: cysteine, glutathione and L-ascorbic acid on the composition and structure of isolated products has been discussed.     

Determination of Nickel after Online Sorbent Preconcentration by FI-FAAS Using Dimethylglyoxime as a Complexing Agent

Dimethylglyoxime (DMG) has been tested as a complexing agent for the determination of nickel after online preconcentration on RP-C₁₈ in a microcolumn using flow injection coupled with a flame atomic absorption spectrometry system (FI-FAAS). The Ni–DMG complexes formed online can be adsorbed on the C₁₈ sorbent. Various parameters affecting the online Ni–DMG complex formation and its subsequent adsorption in the microcolumn as well as its elution into the nebulizer of the FAAS were optimized. A 10⁻³ mol/L solution of DMG in 4% ethanol was mixed online with an aqueous sample solution acidified to 0.1% (v/v) nitric acid and flowed for 30 s through the microcolumn. The adsorbed Ni–DMG complex in the microcolumn was eluted with ethanol containing 1% HNO₃ into the nebulizer of the FAAS in 10 s. A good precision (RSD = 1.7%, n = 14), high enrichment factor (21), and high sample throughput (90 h⁻¹) with detection limit (3ς) 3 μg/L were obtained. The method was applied to standard reference materials, i.e., NBS-362, NBS-364 (special low-alloy steel), and mussel (GBW 08571), for the determination of nickel and the results were in good agreement with certified values. Nickel recovery from seawater and high-purity magnesium oxide in the range 98–100% can be obtained by this method.     

Structural and spectroscopic characterization of a cationic aquanitrato copper(II) complex with the tetradentate Schiff-base ligand N,N′-bis[1–(2–pyridyl)ethylidene]ethane-1,2–diamine

Cu(NO3)2·3H2O reacts with 1 equiv. of the tetradentate Schiff base N,N-bis[1–(2–pyridyl)ethylidene]ethane-1,2–diamine (LA) in refluxing acetone to yield the [CuLA(ONO2)(H2O)](NO3) complex in ca. 80% yield. The structure of this salt has been determined by single-crystal X-ray crystallography. The copper is six-coordinated with the LA ligand in the equatorial plane and weakly bonded aqua and nitrato ligands above and below this plane. The complex cation is hydrogen-bonded to the nitrate counter-ion. The complex was also characterized by elemental analyses, molar conductivity, room-temperature magnetic susceptibility and spectroscopic (i.r., far-i.r., u.v.–vis, e.s.r.) studies. The data are discussed in terms of the nature of bonding and the known structure.     

Kinetics and mechanism of reaction between aminopolycarboxylatomanganate(III) complexes and cyanide ions: a reinvestigation of the MnCyDTA-CN− reaction

Summary Reactions between CN^– and complexes of Mn^III withtrans-1, 2-diaminocyclohexanetetraacetic acid (CyDTA) and hydroxyethylethylenediaminetriacetic acid (HEEDTA) have been studied spectrophotometrically at the _maxS of their respective hydroxo species under pseudo-first-order conditions. The forward reaction is found to be first-order with respect to both the metal complex and [CN^–].The kinetics of the reverse reaction, i.e. the reaction between [Mn(CN)₆]^3– and CyDTA^4– or HEEDTA^3– taken in large excess) have been followed spectrophotometrically. In both systems, the reactions follow first-order kinetics each in [Mn(CN)₆]^3– and the respective ligand concentration and an inverse first-order dependence in [CN^–]. A six step mechanism is proposed for the forward reaction where the fifth step is the rate-determining one. pH, ionic strength and temperature dependences have been studied for both systems.     

Host/guest complex of β-cyclodextrin/5-thia pentacene-14-one for photoinitiated polymerization of acrylamide in water

β-Cyclodextrin (β-CD) was used to complex the photoinitiator, 5-thia pentacene-14-one (TX-A), yielding a water-soluble host/guest complex. IR, UV–Vis and fluorescence spectroscopy were employed to characterize complexed β-CD/TX-A. Photoinitiated polymerization of acrylamide in water was achieved with β-CD/TX-A in the presence of N-methyldiethanolamine (MDEA). Excellent polymerization yields were observed in air saturated solutions when MDEA was added.     

Adsorptive accumulation voltammetry of copper(II) using complex formation reaction with salicylideneamino-2-thiophenol

Summary The cathodic stripping voltammetry of copper(II) was investigated with a method, based on the adsorptive accumulation of the Cu(II)-salicylideneamino-2-thiophenol (SATP) complex on a hanging mercury drop electrode. The copper(II)-SATP complex could be accumulated on the electrode at –0.20 V in 0.01 mol/l nitric acid. The reduction peak of the copper complex was observed by scanning the potential in a negative direction in the differential pulse mode. The calibration curve for copper was linear over the range 5×10^–9–1×10^–7 mol/l. This method was applied to determine copper(II) in GSJ (Geological Survey of Japan) standard rock reference materials.     

A Laboratory Investigation of the Reduction of Chromium Oxide by a Reverse-Polarity DC Plasma-Driven Molten Oxide Electrolysis Process

A method for producing chromium metal/chromium alloys using a reverse-polarity DC plasma-driven molten oxide electrolysis process was investigated. A laboratory-scale 50 kW DC plasma-crucible system was designed and built to investigate the feasibility of this process. Experiments on molten oxide electrolysis were successfully conducted to produce chromium metal from chromic oxide. Two starting slag systems, SiO₂–CaO–Al₂O₃–Cr₂O₃–Na₂O and SiO₂–CaO–Cr₂O₃–Na₂O, were used in this study. It was found that in each case chromic oxide was successfully reduced to metallic chromium. Aluminum was also reduced with the presence of alumina in the starting slag. Small amounts of carbon monoxide gas were introduced to the electrolysis system to study oxygen evolution rates from the plasma/slag interface. For the SiO₂–CaO–Al₂O₃–Cr₂O₃–Na₂O system, the oxygen evolution rate showed a maximum during the electrolysis process. For the SiO₂–CaO–Cr₂O₃–Na₂O system, the oxygen evolution rates displayed a declining trend with processing time. These two reduction behaviors were apparently controlled by different mechanisms. The significance of this process is that it might be used to produce carbon-free chromium metal/chromium alloys without carbon containing reducing agent and since no carbon based reactants are used for heating or reduction there are no carbon dioxide emissions.     

Cesium dimethyltetrachloroplatinate,a complex of dimethylplatinum(IV). Synthesis and some reductive elimination reactions involving the monomethylplatinum(II) complex as an intermediate

The reduction of Pt(IV) complexes followed by the oxidative addition of dimethyl sulfate to Pt(II) affords Cs₂PtMe₂Cl₄, a complex of dimethylplatinum(IV). On treatment with such nucleophiles as Cl^–, Br^–, I^–, and PtCl₄ ^2– in aqueous solutions at 368 K this complex undergoes reductive elimination to give MeX and Pt^IIMe as a transient species. The latter is further converted to methane upon protolysis, whereas in the presence of an oxidant (Na₂PtCl₆) it gives rise to the Pt^IVMe species. The kinetics of decomposition of Cs₂PtMe₂Cl₄ in aqueous HCl-KCl systems (2M or 3M in Cl^–; [Pt^IVMe₂][Cl^–]) were studied. The reaction takes place as anS _N 2 attack of X^– on the carbon atom of a methyl group located with thetrans position with respect to the aqua-ligand of the [PtMe₂Cl₃(H₂O)]^– complex.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 389–395, February, 1993.     

Extending the dynamic range of the determination of copper by adsorption differential pulse stripping method using a principal component artificial neural network

A principal component artificial neural network (PC-ANN) has been applied for the analysis of the voltammogram data of Cu(II) and Cu(II)–PAN complex for extending the dynamic range of the determination of Cu(II) (5–550 μg/l). A three layer back-propagation network (6:5:1) was used with learning rate (r=0.09) and momentum (m=0.9), with sigmoidal transfer function in the hidden layer and one bias node in the input layer. Overall, the application of PC-ANN enables the extension of the dynamic range of the determination of Cu(II) from its narrow linear range (5–50 μg/l) to the high dynamic range (5–550 μg/l).     

Mixed metal complexes in solution. Part 4. Formation and stability of heterobinuclear complexes of cadmium(II)-citrate with some bivalent metal ions in aqueous solution

Summary The systems, Cd-Ni-citrate, Cd-Mn-citrate and Cd-Zn citrate have been investigated pH-metrically at 25°C and I = 0.1 mol dm^–3 (KNO₃).As previously found for analogous citrate systems (namely for Cu-Ni-, Cu-Zn- and Ni-Zn-citrate) the existence of mixed metal complexes of the type [MM(cit)₂H_–2]^4– has been shown. In addition, the species [MM(cit)₂H_–1]^3– was also found to be present for Cd-Ni- and Cd-Zn-citrate systems The significance of the formation of such species is discussed.The existence of mixed metal complexes is also discussed in connection with the transport and the absorption of metal ions in biological systems.     

Kinetics of the destruction of cyclic complexes of nickel(III) and their participation in oscillating chemical processes

A spectrophotometric method has been used to study in sulfuric acid solutions the chelate reduction of nickel(III) compounds Ni^IIIL, where L are l4-membered tetraazamacrocyclic ligands with a different number of substitutents (from 0 to 0) and double bonds (from U to 4). Based on the comparison of the kinetic characteristics of the redox reactions of the nickel complexes with data on the participation of such compounds in oscillating chemical reactions in the systems NiLBrO^– ₃-H₂SO₄ it was concluded that a necessary condition for the creation of OCR in this case is, besides the autocatalytic regime of the oxidation of the [NiL]²⁺ complex with bromate, the ability of nickel(III) to undergo rapid reduction (k > 10^–4 sec^–1) by the oxidation of the coordinated macrocyclic ligand.L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of Ukrainian SSR, Kiev. Translated from Teoretichesakaya i ÉksperimentaI'naya Khimiya, Vol. 27, No. l pp. 51–55, January–February, 1991. Original article submitted May 29, 1990.     

Very-high-speed liquid column chromatography the system and selected applications

Summary Instrumentation needed to achieve very high performance (plate/s) liquid chromatography is discussed. With recently available systems using relatively short (100–125 mm) columns of conventional internal diameter (4–6 mm) and packed with 3–5 m particles, analysis in very short times (usually 1–3 min) is possible. Coupling such columns in series provide very high performance in a time still significantly less than previously encountered. Performance of such liquid chromatographic systems is illustrated with a number of practical examples including analgesic tablets, cosmetics, soft drinks, antioxidants and polyaromatic hydrocarbons.Enlarged text of a paper presented at the 32nd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 9–13, 1981, Atlantic City, New Jersey (Paper No. 462).