Gaseous catalysts for end-point indication in titrimetric analysis in the microgram range

The method of end-point detection described is very sensitive. In titrations with sodium sulphide, the first drop of titrant in excess cause evolution of hydrogen sulphide which acts as a catalyst for the indicating iodine—azide reaction. The gas formed at the end-point is transferred by a nitrogen stream to the indicator vessel. Microgram amounts of metal ions (Sb, Ni, Fe, Hg⁺ Cd, Pb⁺ Cu, Ag⁺ Cu) and permanganate can be titrated in acidic solution with standard sulphide solution. In a variation of this method, acid-base titrations are possible.     

Catalytic end-point indication in redox titrations

The applicability of catalytic end-point indication to redox titrations is demonstrated by the determination of 3–30 μmol of ascorbic acid (in 22.5 ml of solution) with standard dichromate solution; the chromium(VI)-catalyzed oxidation of o-dianisidine with hydrogen peroxide serves as indicator reaction. Oxidizing substances, such as vanadium(V), thallium(III) or cerium(IV) can be determined by addition of excess of ascorbic acid and back-titration.     

Gaseous catalysts for end-point indication in titrimetric analysis in the microgram range with iodide and bromide as catalysts

A new technique for end-point detection in titrations with bromate is described. The first drop in excess causes evolution of bromine which is swept by nitrogen into an indicator vessel. There, the bromine reacts with iodide to form iodine which catalyzes the cerium(IV)—arsenic(III) reaction or with sulfite to form bromide which catalyzes the permanganate—iodine reaction. Microgram amounts of antimony(III) or arsenic(III) can be determined.     

Titrimetric analysis with catalytic end-point detection and a spotting technique

In the determinations described, the catalyst is usually titrated with the inhibitor. A semi-automatic spotting method is used for end-point detection. Dilute solutions of metal ions (Cu, Fe, Co, Ni, Zn, Al, Fe + Ni, Fe + Al), usually in the range 3–70 μg ml^?1, are titrated with EDTA solutions. Mixtures of iodide (3–25 mg) and chloride (2–15 mg) are titrated sequentially with 1 M silver nitrate solution to different catalytic end-points. A minute proportion of the solution being titrated is transferred continuously to a maxing chamber where it reacts with suitable indicator solutions and drops onto a moving band, on which the end-point can be recognized by a colour change in successive drops. Calibration graphs are essential.     

Chlorite as an oxidising agent in titrimetric analysis

The possibilities of using a solution of sodium chlorite as an oxidising agent in titrimetric analysis have been investigated. Such a solution, in the absence of light, is chronometrically stable and convenient for the direct titration of sulphite (in applying the titre to a known solution of sulphite); iron, by its reduction to iron^II with tin^II chloride (the result to be read from the difference between the potential-jump corresponding to iron^II × tin^II and the first jump corresponding to tin^II); arsenite; iodide. The titration is generally carried out in an acidic medium, with the occasional application of catalysts such as potassium iodate.     

“Titrations” in the submicro-range with microcrystal end-point detection

Summary Drops with increasing volume of a titrant standard solution are put on a microscope slide. One drop of equal volume of a sample solution is then added to all drops. The drops are allowed to evaporate at room temperature and the residues are observed under a microscope. If the end-point is reached, the characteristic crystals of the titrant appear at the periphery of the evaporated drops. Following substances have been determined by this technique: Ca²⁺ (0.4–4.0g) with (NH₄)₂C₂O₄, SO₄ ^2– (0.48–4.8g) with BaCl₂, Ni²⁺ (0.2–2.0g) with dimethylglyoxime, Fe³⁺ (0.6–6.0g) with K₄[Fe(CN)₆], Cl^– (0.15–1.5g) with silver acetate.

---

Titrationen im Submikro-Bereich mit Mikrokristall-Endpunktsindikation

Zusammenfassung Tropfen mit zunehmenden Volumina eines Titrationsmittels (Maßlösung) werden auf einen Objektträger aufgetragen. Dazu gibt man gleich große Tropfen einer Probelösung. Man läßt diese bei Raumtemperatur eintrocknen und betrachtet die Rückstände unter einem Mikroskop. Nach Erreichen des Endpunkts erscheinen am Tropfenrand charakteristische Kristalle des Titrationsmittels. Mit dieser Technik wurden folgende Ionen bestimmt: Ca²⁺ (0,4–4,0g) mit (NH₄)₂C₂O₄, SO₄ ^2– (0,48–4,8/gmg) mit BaCl₂, Ni²⁺ (0,2–2,0/gmg) mit Dimethylglyoxim, Fe³⁺ (0,6–6,0/gmg) mit K₄[Fe(CN)₆] und Cl^– (0,15–1,5/gmg) mit Silberacetat.

     

Redox-potentiometric/titrimetric analysis of aqueous iodine solutions

Redox-potential, titrimetric oxidation capacity and pH were used to analyze aqueous iodine solutions over a concentration range of five decades. By a single operation the equilibrium concentrations of I^–, I₂, I₃ ^–, I₅ ^–, I₆ ^2–, HOI and OI^– were assessed. Accuracy was most influenced by the redox-potential showing a repeatability of < ± 0.1 mV in solutions containing appreciable iodide (Lugol’s solution) which increased to ±1.5 mV at a 1:100 000 dilution. Together with the uncertainty of calibration this equals a calculated total error which ranges for “free iodine” ([I₂] + [HOI]), from ± 0.5% to ± 4.2% and for total iodide from ± 0.7 to ± 5.3%. Accuracy was assessed by a comparison of the evaluated equilibrium concentrations and parameters derived therefrom, such as total iodide and redox-potential, with measured resp. calculated values. The deviations at ionic strength < 0.1 M were below ± 3% and revealed a satisfactory accuracy of the method. As a unique feature it was possible to monitor the disproportionation in a iodide-free iodine solution with an initial concentration of 16% hypoiodic acid (HOI) at pH 7.6. Showing a half-life time of only ≈ 75 s it was confirmed that stability largely depends on HOI concentration. Received: 26 February 1998 / Accepted: 15 March 1998     

Improvement of end-point detection in the non-aqueous titration of sulphacetamide sodium

The sluggish end-point in the non-aqueous titration of sulphacetamide sodium in glacial acetic acid can be improved by addition of acetic anhydride to the titration medium, and selective determination of sulphacetamide sodium in presence of phenylephrine hydrochloride in eye drops then becomes possible. A mixture of sulphacetamide sodium and the antihistamine drug phenyltoloxamine dihydrogen citrate can also be analysed.     

Flow injection titrimetric analysis of vitamin C in pharmaceutical products

A flow injection titrimetric method for the assay of vitamin C using cerium(IV) was developed. A 157 mul vitamin C solution was injected into a carrier stream of 7.48 x 10(-3)M cerium(IV) prepared in 0.10M sulfuric acid flowing at a rate of 5.0 ml/min. A 1.3 ml mixing chamber was employed for dispersion of the resultant solution which was propelled through a flow-through cell and detected using the spectrophotometer at a wavelength monitored at 410 nm. The method was applied to the determination of vitamin C in pharmaceutical preparations and a high accuracy was attained when compared with the British Pharmacoepeia (BP) method. Excellent precision indicated by a relative standard deviation of less than 0.9% was always obtained. No interferences were encountered and as many as 36 samples per hour were reached for throughput.     

Potentiometric study of end-point detection in the coulometric determination of carbon dioxide

The coulometric determination of carbon dioxide is based on the alkalimetric titration of the product of absorption of carbon dioxide in an organic solution of monoethanolamine. The processes occurring in various solvents have been investigated and optimized for analytical application. The protolytic reactions of 2-hydroxyethylcarbamic acid have been investigated in 2-propanol + 2, 5 and 10% water, dimethylformamide + 2% water, dimethylsulphoxide +5, 10 and 20% water. In coulometric generation of the base, 0.1M solutions of tetraethylammonium bromide in these solvents were used. The course of the titration was followed potentiometrically with glass and antimony indicator electrodes. From the titration curves the autoprotolysis constants of the mixed solvents and protonation constants of monoethanolamine in them were calculated. These constants and the concentration of monoethanolamine influence the size of the end-point break for the titrations. On this basis, optimal conditions for analysis have been selected and the total carbon content in samples of natural waters has been determined.     

非终点滴定法与其应用

为了实现滴定分析的快速和自动化,本文借鉴以往研究的经验针对其不足[1~8],提出了一种新的分析方法,滴定中只要加二次标准液,将采集到的数据通过适当的数学计算,即可得到样品液的浓度,本法不需要已知反应常数、电极转换系数等,对中和度没有严格要求[9~11],故具有可操作性和实用性。本法不需要确定终点,不需要全过程滴定,为滴定分析的自动、快速化提供了充分的条件,特别适合冶金、环境、食品、药品的大批量快速分析和在线分析。根据该方法的原理即可开发出一种新的自动分析仪。     

Biamperometric end-point detection with quinhydrone electrodes in coulometric neutralization titrations in nonaqueous media

Some new possibilities for continuous coulometric titration of bases, alone and in mixture, as well as of mineral acids, alone and in mixture, were investigated with acetic anhydride-anhydrous acetic acid as solvents. The titration end point was determined biamperometrically. It was established that quinhydrone electrodes with platinum, palladium, and gold as the metal phase can be equally satisfactorily applied to the end-point determination of both the titrations of bases or acids alone, and of their two-component mixtures. Amounts of 6–10 μequiv. of bases, i.e., 20–47 μequiv. of mineral acids, were determined with the maximal average deviation of 1.4%. Results of biamperometric determinations of bases are compared to those of catalytic thermometric determinations, while those of acids are compared with the results of photometric titrations. The agreement between the methods is satisfactory.     

Compleximetric titration of chromium (III) with catalytic end-point detection

Chromium (III) (2–20 mg) is determined by reaction with excess of EDTA and backtitration with a standard copper(II) solution to a catalytic end-point. The indicator reaction is the copper (II) -catalyzed autodecomposition of hydrogen peroxide, which is observed biamperometrically. For 10 mg of chromium (III) , the relative standard deviation was < 0.5 % (n=10).     

Selective titration of microgram amounts of copper with photometric end-point detection

The reaction of copper ions with pyridylazonaphthol is very suitable and sensitive for the photometric titration of copper. The titration is self-indicating, and at the 0.1-ppm level the standard deviation is 2.5%. The main interferences are caused by bismuth, iron, titanium and vanadium but can be overcome by suitable masking.     

Potentiometric iodimetric titrations with an interfacial voltaic cell for end-point detection

The simple cheap interfacial (antimony scratch) voltaic cell provides sharp sensitive end-point signals in titrations of inorganic and organic compounds directly with iodine or in backtitrations with thiosulfate. Recoveries were in the range 98.3–101.7% for 2–10 mg of sample. The results were the same as those obtained by classical iodimetric titrations.     

Oscillopolarographic titration — a promising volumetric method with visual end-point detection

Oscillopolarographic titration is more straightforward to use than other physico-chemical titrimetric methods and is more sensitive than classical indicator methods. It may prove particularly useful in the determination of drugs.     

Biamperometric neutralisation titrations in non-aqueous solvents applying tin electrode pair for the end-point detection

Summary Tertiary amines and salts of organic acids in acetic anhydride with 5% of acetic acid were titrated applying a biamperometric indicating system consisting of a polarised or unpolarised pair of tin electrodes by means of 0.1 N perchloric acid in glacial acetic acid. Amounts of 15.4–29.8 mg were analysed with average deviations of <0.6%, the results obtained being in a good agreement with those of potentiometry. The biamperometric end-point detection using an indicating system of polarised pair of tin electrodes can be applied for coulometric determinations of these bases with satisfying accuracy. By means of anodic generation of hydrogen ions in the presence of hydroquinone amounts of 0.5–1.0 mg were titrated with average deviations of < 1.3%. The results obtained were compared with those obtained of catalytic thermometric and photometric titrations.

---

Biamperometrische Neutralisationstitrationen in nichtwärigen Lösungsmitteln unter Anwendung eines Zinn-Elektrodenpaares als Indicatorsystem

Zusammenfassung Tertiäre Amine und Salze organischer Säuren werden in Acetanhydrid mit 5% Essigsäure durch Titration mit einer 0,1 N Lösung von Perchlorsäure in Essigsäure bestimmt. Das verwendete biamperometrische Indicatorsystem besteht aus einem Paar polarisierter oder nichtpolarisierter Zinnelektroden. Mengen von 15,4–29,8 mg wurden mit einer mittleren Abweichung von <0,6% analysiert. Die Ergebnisse stimmen gut mit denen von potentiometrischen Titrationen überein. Die biamperometrische Endpunkttechnik mit polarisierten Zinnelektroden kann für coulometrische Bestimmungen dieser Basen mit zufriedenstellender Genauigkeit benutzt werden. Durch anodische Erzeugung von Wasserstoffionen in Gegenwart von Hydrochinon wurden Mengen von 0,5–1,0 mg mit einer mittleren Abweichung von < 1,3% titriert. Die Ergebnisse wurden mit denen von katalytisch-thermometrischen und photometrischen Titrationen verglichen.