Sur les indicateurs d'absorption. iv

A new adsorption indicator. Red acid 6B, has been described for argentometric titrations of the halide ions.The great usefulness of the indicator lies in its being applicable for the very diluted solutions of the ions I^-.     

The use of 2,6-dichlorophenolindophenol as indicator in acid-base titrations

2,6-Dichlorophenolindophenol shows acid-base indicator properties. Its colour is red in acidicmedia and blue in alkaline media. 'Ihe transitional pH value is 5.7 and the colour change is sharp and easy to observe. The use of the indicator in various acid-base titrations, as well as the effects of neutral salts, alcohol and temperatures, were checked. The absorption spectra of the acidic and basic forms of the indicator are presented.     

Precise and accurate determination of halogens in organic compounds using an amalgamated gold electrode

Precise and accurate titration of halogens in organic and inorganic materials has been studied. The halogen was titrated with coulometrically generated silver and determined potentiometrically by using an amalgamated gold indicator electrode. An argentometric titration curve using the amalgamated gold indicator electrode showed a large jump in potential at the endpoint as well as in mercurometric determination. Sodium carboxymethyl cellulose (CMC) and aluminum nitrate were added as they are most effective for clarifying the sample solution and eliminating adsorption of the halide ion on the silver halide generated. The electrolytes used were 0.5 M nitric acid containing 0.1% CMC and 0.2 M aluminum nitrate. Approximately 0.2 mmol of chloride, bromide, and iodide ions were titrated with high precision and accuracy. Their standard deviations were 0.03, 0.02, and 0.02%, respectively, with no apparent systematic error. The precise determination of halogens, employing oxygen flask combustion and dissolution of the halide salts in the electrolyte, was carried out with ±0.1% absolute error using samples weighing between 20 and 30 mg.     

Charging of water at inert and hydrophobic surfaces. Effect on interfacial properties of silver halides

Isoelectric point of silver halides depends on pH due to charging of interfacial water layer. The isoelectric point of water at inert and hydrophobic surfaces lies at pH≈3 so that water at surfaces is negatively charged in the pH region above 4. Consequently the electroneutrality point of silver halides pAg_eln in aqueous environment, with respect to adsorption of silver and halide ions, does not correspond to the isoelectric point pAg_iep measured at pH≈6, as previously assumed. The effect of assumed value of pAg_iep was examined. The equilibrium constants for adsorption of silver and halide ions on silver chloride and silver bromide were calculated for the range of assumed pAg_iep values ranging from isoelectric points measured at pH=6 to pH=3. The values of pAg_iep obtained at pH=3 was taken as pAg_eln and the corresponding equilibrium constants of interfacial reactions were obtained.     

Sensitized schiff's reagent as an acid-base indicator

The sensitized Schiff's reagent has been used as an indicator in the neutralization of strong acids by means of strong or weak bases. (1) Since the acid to alkali transition is much sharper than the reverse process, the titration is best done by adding alkali to the acid. (2) The results are compared with those of titrations using methyl orange as indicator. (3) The end-point is very sharp with sensitized Schiff's reagent, the transition taking place from a dull red colour to an abrupt rise in intensity. (4) Acids which are comparable in strengths or weaker than H₂SO₃ cannot be titrated with this reagent as indicator.     

Synthesis of positively charged silver nanoparticles via photoreduction of AgNO3 in branched polyethyleneimine/HEPES solutions

Branched polyethyleneimine (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were used collaboratively to reduce silver nitrate under UV irradiation for the synthesis of positively charged silver nanoparticles. The effects of molar ratio of the ingredients and the molecular weight of BPEI on the particle size and distribution were investigated. The mechanism for the reduction of Ag+ ions in the BPEI/HEPES mixtures entails oxidative cleavage of BPEI chains that results in the formation of positively charged BPEI fragments enriched with amide groups as well as in the production of formaldehyde, which serves as a reducing agent for Ag+ ions. The resultant silver nanoparticles are positively charged due to protonation of surface amino groups. Importantly, these positively charged Ag nanoparticles demonstrate superior SERS activity over negatively charged citrate reduced Ag nanoparticles for the detection of thiocyanate and perchlorate ions; therefore, they are promising candidates for sensing and detection of a variety of negatively charged analytes in aqueous solutions using surface-enhanced Raman spectroscopy (SERS).     

Influence of interfacial water layer on surface properties of silver halides: effect of pH on the isoelectric point

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2相似文献   

Argentometrische titrationen mit redoxindication Bestimmung von halogeniden, cyaniden, rhodaniden und kalium in gegenwart von variaminblau

Variamine-blue can be used with advantage as a redox indicator in the argentometric determination of halide, thiocyanate, cyanide and potassium. At a suitable pH the first drop of silver nitrate in excess can be detected. The procedure may also be used for the determination of silver by means of potassium iodide. Potassium can be determined titrimetrically by argentometric titration of the tetraphenylboron compound in the presence of the indicator.

Résumé

On peut utiliser avec profit le bleu de variamine comme indicateur d'oxydo-réduction dans le dosage argentométrique des halogénures, du thiocyanate, du cyanure et du potassium. Il est possible de déceler, à un pH convenable, la première goutte de nitrate d'argent en excès. Le procédé permet également de déterminer l'argent au moyen de l'iodure de potassium. On peut effectuer le dosage titrimétrique du potassium par le titrage argentométrique du composé tétraphénylbore en présence de l'indicateur.     

Cellulose and chitin nanomaterials for capturing silver ions (Ag+) from water via surface adsorption

The study explores the potential of cellulose nanocrystals (CNC), cellulose nanofibers (CNF) and chitin nanocrystals (ChNC) isolated from bioresidues to remove silver ions from contaminated water. Zeta sizer studies showed negatively charged surfaces for CNC and CNF isolated from cellulose sludge in the acidic and alkaline pHs, whereas ChNC isolated from crab shell residue showed either positive or negative charges depending on pH conditions. Model water containing silver ions showed a decrease in Ag⁺ ion concentration (measured by inductively coupled plasma-optical emission spectrometer; inductively coupled plasma mass spectrometry), after treatment with CNC, CNF and ChNC suspensions. The highest Ag⁺ ion removal was measured near neutral pH for CNC, being 34.4 mg/g, corresponding to 64 % removal. ChNC showed 37 % and CNF showed 27 % removal of silver ions. The WDX (wavelength dispersive X-ray analysis) and XPS (X-ray photoelectron spectroscopy) analysis confirmed the presence of silver ions on the surface of the nanocellulose and nanochitin after adsorption. Surface adsorption on the nanoparticles via electrostatic interactions is considered to be the prominent mechanism of heavy metal ion capture from aqueous medium, with CNC with negative surface charge and negatively charged functional groups being most favourable for the adsorption of positively charged Ag⁺ ions compared to other native bionanomaterials.     

Rapid titrimetric determination of microgram amounts of fluoride ion with spadns -thorium lake

A rapid titrimetric method has been developed for the determination of microgram amounts of fluoride ion in the range from Img-Ioomg, in 50 ml final volume. It involves the adjustment of pH, addition of 1 ml of 0.02% SPADNS indicator, dilution to volume and titration with standard 0.004M Th(NO₃)₄ until the colour obtained matches a blank containing the buffered solution of the indicator with a. trace of thorium nitrate solution. Interference by various ions was also studied. The method described for the determination of fluoride ion is very rapid and the colour change at the end point being sharp, the detection of the end point is very easy. The method is applicable to pure solutions of fluondes.     

Generation of large gas-phase silver cluster ions by laser desorption/ionization of silver-containing salts

This work is the first to demonstrate the direct generation of large silver cluster ions through laser desorption/ionization of silver-containing salts without the assistance of a molecular beam. Both positively and negatively charged silver clusters up to 100-mer have been produced using this approach. It was observed that the silver cluster distributions produced by this method are similar to those produced using the beam-based approaches, namely there are two special features in the mass spectra. The first is that the cluster intensity distribution displays an odd–even alternation pattern. The second is a steep decrease of ion intensities after some clusters called ‘magic numbers'.     

Studies on adsorption indicators : p-ethoxychrysoidine as adsorption indicator

p-Ethoxychrysoidine is studied as adsorption indicator in the direct titration of silver with iodide, bromide, thiocyanate and chloride at pH 4–5. The explanation given by Schulek and Pungor for the colour changes has been modified in the light of compound formation on the surface of the precipitate. The silver compound of p-ethoxychrysoidine has been isolated ; its behaviour provides a suitable explanation for the colour changes as well as the pH changes in the titrations.     

Selective microdetermination of phosphate based on Volhard's titration

A selective method for the microdetermination of phosphate has been developed. The phosphate is precipitated from homogeneous solutions as silver phosphate, which after being filtered and washed is dissolved in dilute nitric acid, and the silver ions set free are determined by Volhard's titration. Whereas the halide ions interfere if the conventional precipitation is adopted, they do not interfere if precipitation is conducted from homogeneous solutions. The method is simple and accurate to a maximum error of ±1%.     

Perbenzoic acid as volumetric reagent

Summary Perbenzoic acid (0.05 M) chloroform solution is used as volumetric reagent for the determination of potassium ferrocyanide, arsenious oxide, tartar emetic, sodium sulphite, stannous chloride, hydrazine sulphate and sodium thiosulphate atph 4.0 to 4.5 in sodium acetate and acetic acid buffer solutions, using iodine as catalyst and preoxidiser. Appearance of faint violet colour in chloroform layer indicates the end point in visual titrations. In potentiometric titrations bright platinum foil is used as an oxidation-reduction electrode and it is coupled with saturated calomel electrode. At the equivalence point there is a sharp jump in potential in each titration.     

Substituted phenothiaziiies as indicators in the ferrocyanide titration of zinc

Chlorpromazine, promethazine and diethazine hydrochlorides and prochlorperazine maleate have been tested as indicators in the ferrocyanide titration of zinc. Only the first and last are useful, giving a sharp reversible colour change in both the direct and reverse titrations, but the results are negatively biased by 0.2-1.3%.     

Simultaneous determination of halide and thiocyanate ions by potentiometric precipitation titration and multivariate calibration

Halide and thiocyanate ions can be determined by a precipitation titration with silver nitrate as the titrant, and the end-point can be evaluated by a potentiometric method, in which generally a silver indicator electrode is used as the indicator electrode and a double-junction Ag–AgCl electrode as the reference electrode. However, when mixtures of halide and thiocyanate are titrated, it is difficult to determine these components individually for there are overlapping steps in the potentiometric titration curves, especially in the case that there are obvious differences between concentrations of the components. In this paper, the linear equation for the potentiometric precipitation titration of a mixture of halide and thiocyanate ions was developed and it was then used for determining the components in the mixtures simultaneously with the aid of multivariate calibration methods. By application of this model, 27 synthetic mixtures with three- and four-component combinations of chloride, bromide, iodide and thiocyanate with low concentration levels from 1.8×10⁻⁴ to 6.2×10⁻⁴ mol l⁻¹ were analyzed and acceptable results were obtained.     

Electroanalysis with tungsten electrodes : I. The tungsten/platinum bimetallic pair in potentiometry at zero current

The W/Pt bimetallic pair has been applied in various potentiometric titrations. Except for the neutralization of dicarboxylic acids, e.g., oxalic, the pair is suitable for neutralization titrations where ΔE/ΔV values at the equivalence point are higher on W than on Pt. Tungsten oxides increase the inflexion pd. The pair is also suitable for oxidation/reduction titrations using permanganate, dichromate, and ceric sulfate, and for precipitation reactions with silver nitrate. However, it gives a small inflexion pd in the oxidation of thiosulfate by iodine. EDTA titrations of copper, magnesium, and calcium give a sharp inflexion in the case of copper only. This pair may be useful in routine analyses requiring a robust electrode assembly but not a high degree of accuracy.     

Elektroden-probleme in der potentiometrie

The variations of the potential, calculated and observed during a potentiometric titration, are independent of the nature of the electrode used. The influence of changing the rate of agitation upon the potential of silver electrodes of different surfaces is studied. A halide-free reference electrode is described as a suitable simplification in argentometric titrations.     

Sorption properties of freshly precipitated alumogels

The influence of the pH of precipitation and the ionic medium nature on the composition, surface charge, and kinetic sorption characteristics of aluminum oxyhydroxides formed by alkaline hydrolysis of an aqueous solution of aluminum nitrate was studied. The methods of drop titration with alkali, argentometric titration, point of zero charge, and indicator reactions of heterogeneous hydrolysis were used. The composition of the freshly precipitated hydrogels of aluminum(iii) oxyhydroxide, rate constants of heterogeneous hydrolysis of the IrCl₆ ^2– ions as indicator reactions, and the pH of the point of zero charge were determined. The rate of indicator reactions (as for other oxyhydroxide hydrogels) depends strongly on the pH of precipitation. However, it is lower than the rates characteristics of iron(iii) and chromium(iii) oxyhydroxides obtained under similar conditions. The pH of the point of zero charge for the alumogels in a chloride medium is 8.3 and that in a sulfate medium is 9.5.     

Computer Simulation of the Initial Stage of Water Vapor Nucleation on a Silver Iodide Crystal Surface: 1. Microstructure

The nucleation of water vapors on the surface of a fragment of silver iodide crystal is simulated by the Monte Carlo method under the conditions similar to natural conditions in a humid atmosphere. A stable monolayer island of water molecules with clearly pronounced features of hexagonal symmetry and low orientational order is formed at the initial stage, when the vapor pressure is still lower than the saturating pressure. The island readily grows over the surface and, in the unsaturated vapor, does not grow in the direction perpendicular to the surface. The formed monolayer represents a substrate for further growth of a condensed phase and, eventually, is responsible for the mechanism of nucleation on the crystal surface. Water molecules are held by the substrate mainly owing to the directional electrostatic interaction between the negatively charged oxygen atoms and positively charged silver ions. The interaction with iodine ions lowers the binding of the island (nucleus) and the substrate. A point defect in the form of an extra ion on the surface does not change the planar shape of the nucleus and virtually does not distort its hexagonal structure. Indirect experimental data supporting the formation of a water monolayer at the stage preceding nucleation, as well as the data of observations indicating the important role of defects on a crystal surface, are reported.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 548–560.Original Russian Text Copyright © 2005 by Shevkunov.