Automatic coulometric titration of acids

An apparatus is described for the automatic titration of acids by the constant current coulometric technique. The generator electrodes comprise a platinum cathode and a silver/silver bromide anode. The increase in pH resulting from the reduction of hydrogen ion at the cathode is indicated by a glass electrode, in conjunction with a Beckman Automatic Titrator which automatically monitors the titration and interrupts the generating current when the equivalence point pH is reached. Quantities of hydrochloric acid in the neighborhood of 0.12 millimole in 50 ml were titrated with a mean error of ^-0.07% and an average deviation from the mean of ±0.15%. The technique is applicable to any strong or weak acid, and to acid mixtures, provided that no substance is present which is either reducible at the platinum cathode or reactive in any way at the silver anode.     

Acid—base equilibria in the mixed solvent 80% dimethyl sulfoxide—20% water : Determination of pK Values and Investigation of the Conditions for Titration of some Aromatic Carboxylic Acids and their Conjugated Bases

The acid—base properties of some aromatic carboxylic acids of zero charge type HA have been investigated in the mixed solvent, 80% DMSO—20% water, by means of potentiometric measurements with a glass electrode. The pK_a values of the following aromatic carboxylic acids have been determined: benzoic, o-chlorobenzoic,p-chlorobenzoic, m-bromobenzoic, 2,4-dichlorobenzoic, 2,5-dichlorobenzoic, 2,6-dichlorobenzoic, 2,3,5-triiodobenzoic, 3,5-dinitrobenzoic,p-methylbenzoic, m-aminobenzoic, o-hydroxybenzoic 3,4-dihydroxybenzoic, 3,4,5-trihydroxybenzoic, o-methoxybenzoic, 2,3-dimethoxybenzoic, 5-amino-2-hydroxybenzoic, pyridine-2-carboxylic, thiophene-2-carboxylic, 1-naphthylacetic, diphenylglycolic. The mixed solvent investigated offers better titration conditions for the determination of these acids than water and some other non-aqueous solvents.     

Acid-base strengths in 1,2-dichloroethane

The pK_a value of hydriodic acid in 1,2-dichloroethane was determined from conductivity measurements. A glass electrode was calibrated for dichloroethane in the potentiometric titration of hydriodic acid with tetramethylguanidine. From potentiometric titrations, the pK_a values in dichloroethane of hydrobromic acid, hydrochloric acid, picric acid and some sulfonphthaleins as well as some protonated nitrogen bases were determined. In the curves of the titrations of the carboxylic acids and the hydrogen halides with TMG, evidence was found for the formation of the complex B(HX)₂.     

The coulometric titration of acids and bases in m-cresol medium

A method is described for the coulometric titration of acids and bases in the solvent m-cresol. The method is suitable for bases with pK_a values greater than 11 in m-cresol, or for acids with pK_a values below 13 in m-cresol. Amounts of 5–50 μeq of acid or base can be determined with a relative accuracy of ±1%.     

Voltammetric,potentiometric and amperometric studies with a rotated aluminum wire electrode : Amperometric titration of fluoride with aluminum

The rotated aluminum electrode is a suitable indicator electrode in the amperometric titration of fluoride, Fluoride in concentrations varying between 1.10^-4 and 2.10^-3M was titrated with a standard aluminum nitrate solution in aqueous buffer solutions (pH between 3.6 and 46) Equilibrium was established within 15 mim. The fluoride-aluminum ratio at the (true) end-point was found to increase from 2 2 in 10^-4M fluoride to 28 in. 2.10^-3M fluoride. The titration calculated from the stability constants of the various aluminum-fluoride complexes were found to be in excellent agreement with the experimental curves.From a practical point of view titration in an acetate buffer in 50% alcohol in the presence of 0 5 M potassium or sodium nitrate is recommended. The fluoride-aluminum ratio at the end-point was found to be 5 9 ± 0 1 and independent of the fluoride concentration.     

A polymeric sulfur nitride, (sn)x,electrode responsive to ph and its application to the aqueous potentiometric titration of weak acids

The characteristics of polymeric sulfur nitride, (SN)_x, electrodes for the sensing of pH in aqueous acid—base systems is reported. The application of this new pH-sensor to titrations of both strong and weak acids with strong base is described. The (SN)_x. electrode has significant advantages for titrations of weak acids (pK_a ? 9) with a strong base, compared to the classical procedure with the glass electrode. Comparative titration curves and quantitative results for hydrochloric acid, perchloric acid, trifluoroacetic acid, boric acid, p-nitrophenol, resorcinol, glycine, phenol, acetic acid, benzoic acid and propanoic acid with glass electrode and (SN)_x electrode are given.     

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.     

Coulometric titrations with mercury(I) and (II) determinations of secondary amines and mercaptans

A study of the titration of aliphatic amines and mercaptans with electrolytically generated mercury(II) reagent is reported. Amines were titrated after conversion to their corresponding dithio-carbamic acids by reaction with carbon disulfide. Using this procedure, a 3.10^-4F solution of secondary amine was titrated with an accuracy of better than 1% and a concentration of 3.10^-6F with an accuracy of better than 2%. Primary amines constituted an interference in this procedure but were removed prior to the titration with salicylaldehyde. Tertiary amines were found not to interfere. Comparable titrations with generated silver ion did not give quantitative results.The application of electrolytically generated rnercury(II) reagents for the titration of another organic functional group, sulfhydryl, was also studied. The titration of a sulfhydryl compound (cysteine) was studied briefly. Amounts of cysteine down to 1. 29 mg in 100 ml of solution (1.3.10^-4F) were determined with an accuracy of better than 1%. Smaller amounts of cysteine down to 0.26 mg (2.10^-5F) were determined with about 3% accuracy.     

Acid—base equilibria in the mixed solvent 80% dimethyl sulfoxide—20% water : Part I. definition of ph scale and determination of pk values of aliphatic monocarboxylic acids

The acid—base equilibria in the mixed solvent, 80% dimethyl sulfoxide—water, have been investigated by potentiometric measurements with a glass—silver—silver chloride electrode couple. The response of the glass electrode is quick and reproducible over the whole pH scale. The autoprotolysis constant (itK_SH) of the mixed solvent has been determined; the value obtained, pK_SH = 18.40, agrees well with reported data. The pK_a values of the following aliphatic monocarboxylic acids are reported: formic, acetic, propionic, n-butyric, iso-butyric, n-valeric, iso-valeric, capric, pentadecanoic, stearic, monochloroacetic. The mixed solvent offers better titration conditions than water for the determination of these acids.     

Coulometric titrations of bases in propylene carbonate using hydrogenpalladium and deuteriumpalladium generator electrodes

The application of hydrogen and deuterium ions obtained by anodic oxidation of hydrogen and deuterium dissolved in palladium, for the coulometric determination of bases (both individual and in mixtures) in propylene carbonate, is described. The current-potential curves at a palladium anode for supporting electrolyte indicator, titrated bases, hydrogen dissolved in palladium and deuterium dissolved in palladium showed that hydrogen and deuterium are oxidized at much less positive potentials than the oxidation potentials of other substances present in the solution. The generated H⁺ and D⁺ ions were used for the titration of bases (pyridine, quinoline, triethylamine, n-butylaniline, 2,2′-dipiridyl and aminopyrine) with visual and potentiometric detection. The oxidation of hydrogen and deuterium proceeded with 100% current efficiency. Two-component mixtures of bases (aliphatic + aromatic amine) were titrated successfully by using two indicators, Eosin and Crystal Violet. The relative error of the determination with respect to each individual base determination, was less than 2.5% for quantities of bases ranging from 1 to 3 mg.     

Coulometric generation of molybdenum(v)

Molybdenum (V) was generated at a platinum cathode from 0.7 M molybdenum(VI) in 4 M sulfuric acid. A current efficiency of 99.9% was attained. A limiting current density of 0.05 mA/cm²/mM was found. The formal potential of the Mo(VI)–Mo(V) couple in 4 M sulfuric acid was determined to be ca. 0.55 V vs. N.H.E. Chromium(VI) solutions were titrated over a wide range of sample size and generating current. Amperometric titration curves were interpreted from current-voltage curves. Titrations could be performed in the presence of oxygen at the 1μeq. level. The effect of nitrate, perchlorate, orthophosphate, and chloride ions on the titration was determined.     

Coulometric generation of H+ ions by anodic oxidation of esters of gallic acid,phenols and ascorbic acid in propylene carbonate

The application of esters of gallic acid (methyl, ethyl, propyl, butyl, octyl and dodecyl gallate), phenols (pyrocatechol and pyrogallol) and ascorbic acid for the coulometric generation of H⁺ ions in propylene carbonate, is described. The current-potential curves recorded for these depolarizers, titrated bases, indicator and the solvent used showed that the investigated depolarizers are oxidized at lower potentials than the oxidation potentials of other components in the solution H⁺ ions formed by the oxidation of these depolarizers were used for the titration of organic bases (pyridine, piperidine and quinoline) with visual and potentiometric end-point detection. The current efficiency was 100%.     

A kinetic study of the Zn2+/Zn(Hg) system in dimethylsulfoxide and water + dimethylsulfoxide mixtures at various concentrations of NaClO4

The kinetics of the Zn²⁺/Zn(Hg) system have been studied at the hanging mercury drop electrode in dimethylsulfoxide (DMSO) and in H₂O + 50 % vol. DMSO mixtures at various concentrations of NaClO₄.The diffusion coefficients, the formal potentials, the standard rate constants and the parameters αn_α and βn_β have been determined.The influence of the concentration of electrolyte on Zn²⁺ reduction in DMSO has been explained by the Frumkin double-layer effect.The mechanism of the cathodic and anodic processes has been discussed.     

Solute-solvent interactions in acid-base dissociation: nine protonated nitrogen bases in water-DMSO solvents

A titration method utilizing glass electrodes and silver-silver chloride electrodes in a cell without liquid junction has been used to determine the acidic dissociation constants at 15, 25, and 35°C of nine protonated nitrogen bases in mixtures of water and dimethyl sulfoxide (DMSO). The mole fraction of DMSO in the mixed solvents was 0.2, 0.4, 0.6, and 0.8. The cell was calibrated with HCl (molality=0.01 mole-kg^?1) in the mixed solvents, and the ionic strength and chloride molality remained substantially unchanged during the titration with added base. This method minimizes the errors resulting from the formation of AgCl ₂ ^? in the media rich in DMSO. The pK _a of all the protonated bases passes through a minimum at a solvent composition close to that at which H₂O-DMSO mixtures display a maximum solvent structure. The results are discussed in terms of the preferential solvation of ions by the two types of solvent molecules. They are consistent with the hypothesis that increased solvent structure is accompanied by increased desolvation of the cation acids.     

Coulometric generation of hydrogen ions by oxidation of mercury in methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone

Mercury(II)-chloride reacts with anhydrous methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone in a precise stoichiometry ratio (1:2), and weakly ionized compounds of mercury with ketones are formed and equivalent quantity of HCl is released. The application of a mercury anode for the quantitative generation of H⁺ ions in 0.25 M sodium perchlorate in anhydrous methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone has been investigated. Current/potentials curves for the solvents, titrated bases, indicator and mercury showed that in these solvents mercury is oxidized at potentials much more negative than those for the titrated bases and other components present in the solution. The protons generated in this way have been used for the titration of some organic bases, with either visual or potentiometric end-point detection. The oxidation of mercury in methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone and the reaction of mercury ions with these solvents have been found to proceed with 100% current efficiency.     

High throughput continuous titration based on a flow ratiometry controlled with feedback-based variable triangular waves and subsequent fixed triangular waves

We propose a new approach for high-throughput continuous titration based on a flow ratiometry. The method was applied to potentiometric titrations of acids and bases. A base solution, the flow rate F_B of which was varied in response to controller output voltage V_c, was merged with an acid solution under constant total (acid + base) flow rate. Downstream, the pH of the mixed solution was measured with a flow-through glass electrode. Initially, V_c, and thus F_B was increased linearly. At the instant the equivalence point (EP) was sensed, the ramp direction of V_c was reversed from upward to downward. The direction was reversed to upward when EP was sensed again. Such the feedback-based operation gives a triangular waveform of V_c, because there is a delay corresponding mainly to the transit time of merged solutions to reach the sensor. The value of V_c that gives EP composition, V_E, was estimated by averaging the most recent maximum and minimum values of V_c. Next, fixed triangular waves of V_c was used to control F_B. The amplitude and the scan rate of the waves were fixed narrower and faster, respectively, than those in the feedback-based operation in order to improve the throughput rate. The EP can be located as long as the scan range covers V_E. These automated processes limited the titration to just the narrow range around EP, and thus realized extremely high throughput rate of maximally 17.6 titrations per minute (=3.4 s per titration) at R.S.D. = 0.35%.     

Ethyleneglycol-bis(aminoethyl)tetraacetic acid in the amperometric titration of cadmium in the presence of zinc

Cadmium can be titrated amperometrically with EGTA in the presence of zinc; three modifications of the titration are described. The diffusion current of cadmium may be used to follow the titration at —0.90 or —0.70 V vs, the S.C.E, in ammoniacal or acetate-buffered medium respectively The titration may also be performed in ammoniacal solution at —0.30 V vs. the S.C.E., using copper as the amperometric indicator. The influence of iron, aluminium, magnesium, alkaline earths and lead was studied. Ascorbic acid could be used to mask iron and as an effective remover of oxygen. Under appropriate conditions cadmium could be titrated in the presence of 500-fold amounts of zinc. The titration error is approximately 0.002–0.003 ml of a 0.1 M titrant solution.     

I.M. Kolthoff,P.J. Elving and F.H. Stross (Editors), Treatise on analytical chemistry. Part III. Analytical chemistry in industry : Vol. 2, Wiley-Interscience,New York, 1971, xxiv+597 pp., price £ 12.00

The titration of some substituted azoles in non-aqueous solution has been studied and their relative basic strengths determined. Differentiating titration of mixtures of substituted imidazoles is only possible when one of the imidazoles is substituted with a powerful electronegative substituent, e.g. a nitro group. Azolium halides may be titrated with perchloric acid in anhydrous acetic acid after addition of mercury(II) acetate, or dissolved in acetic anhydride without addition of mercury(II) acetate; dissolved in methanol, they may be titrated as acids with tetrabutylammonium hydroxide in benzene/methanol. The acidic and basic properties of some substituted pyrazol-5-ones have been compared.     

Titration of Potentiometric NH4VO3, KH2PO4, K2HPO4, and their Mixture in Molten NaNO3 at 350°C Using a Novel Solid‐State Glass/TiO2 pH Indicator Electrode

A solid‐state Glass/TiO₂ electrode was fabricated using a transparent conductive titanium oxide film on a glass substrate. The coating of the glass substrate was achieved by a novel simple chemical vapor deposition (CVD) procedure. This electrode can be used as an indicator electrode in potentiometric acid‐base titration. This electrode behaves reversibly and responds to the oxide ion concentration in molten nitrate. NH₄VO₃, KH₂PO₄, K₂HPO₄, and their mixture were titrated with Na₂O₂ at 350°C using the glass/TiO₂ electrode in molten NaNO₃.