Method for avoiding the interference of formamide with the Karl Fischer titration

 The Karl Fischer titration is based on a specific chemical reaction. Several measures exist to make all the water of insoluble samples accessible for the chemical reactants. The most efficient are the titration at elevated temperatures, the use of a homogenizer in the titration vessel and the modification of the polarity of the working medium (essentially methanol) by the addition of appropriate solvents like chloroform or formamide. It is known however that formamide interferes with the Karl Fischer reaction and so causes more or less false results. This effect increases with higher temperatures. A method is therefore presented to avoid this interference, even when working at the boiling point of the working medium. It takes advantage of the fact that the side reaction has a practically constant velocity, at least as long as usual titrations last. Thus, a constant additional consumption of Karl Fischer reagent is observed. This can be accounted for by measuring this effect before the start of the determination and by deducting the additional reagent consumption, which is proportional to the duration of the titration, from the totally added volume. With certain modern titrators this can even be carried out automatically. They can continuously measure the so-called drift, the titration rate necessary to keep the titration cell dry, and have the capability to use this drift as stop criterion for the titration. This means that the analysis is terminated when the drift existing before the titration is reached again. The additional consumption of reagent, to be deducted from the total volume, can (automatically) be calculated from the drift rate and the titration time. The proposed procedure allows the use of formamide as additional solvent, even at high temperatures, in order to shorten determination times considerably. It avoids false results due to the interference, which has so far prevented its use when exact results were desired and when the duration of the analysis was long. Received: 30 May 1996 / Revised: 26 July 1996/Accepted: 30 July 1996     

Highly selective catalytic determination of ultra trace amounts of rhodium by linear sweep voltammetry

A highly selective and sensitive method is proposed for the determination of rhodium, based on the catalytic effect on the oxidation of Nile blue by periodate. The reaction rate is monitored by measuring the current of Nile blue at –0.37 V vs. Ag/AgCl reference electrode. The linear working range is 5 to 100 ng/mL with a limit of detection of 0.1 ng/mL. The interference effects of more than forty ions were studied. The method was used for the determination of Rh (III) in synthetic samples with satisfactory results. Received: 2 July 1997 / Revised: 23 September 1997 / Accepted: 24 September 1997     

Conductometric determination of quaternary ammonium salts by sodium perchlorate and vice versa

A conductometric method is proposed for the determination of quaternary ammonium salts (chloride or bromide), based on the poor solubility of the relative perchlorates. A comparison between expected and found values shows that the method is suitable, rapid, and easy for quantities in the range from 0.2 to 5 g. The error depends on the solubility of the relative perchlorate. The apparent and tentative solubility product of the studied compounds were calculated. The obtained values allowed to propose benzyltributyl ammonium chloride (BTBA Cl) as precipitant for the conductometric titration of perchlorate. This easy, rapid and accurate determination can be used to analyze the perchlorate ion even in concentrated solutions and to determine stability constants for complex formation in aqueous solutions. Received: 11 June 1997 / Revised: 8 October 1997 / Accepted: 11 October 1997     

Determination of weak acids by indirect potentiometry without titration

The simple determination of the sum of weak (polycarboxylic) acids in aqueous solutions and titratable acidity of wines based on indirect iodate-iodide potentiometry without titration was described. Received: 17 July 1997 / Revised: 20 October 1997 / Accepted: 26 October 1997     

Determination of sulfur in metal sulfides by bromine water-CCl4 oxidative dissolution and modified EDTA titration

The sulfur content in metal sulfides can be determined after oxidative dissolution with bromine water-CCl₄ at 50 °C by modified EDTA titration. The sulfides were quantitatively oxidized to SO₄ ^2– without evolving H₂S, and the metal cations were removed by cation exchange on an H-type column. Sulfate was precipitated as BaSO₄, and the remaining Ba²⁺ was subsequently titrated with EDTA at pH = 10. The standard deviation for the determined sulfur in the metal sulfides, some of which can be hardly dissolved in usual mineral acid media, is about a few %. The method can be carried out with sample amounts containing as little as 1.5–2 mg sulfur. Received: 31 July 1997 / Revised: 5 January 1998 / Accepted: 9 January 1998     

Determination of sulfur in metal sulfides by bromine water-CCl4 oxidative dissolution and modified EDTA titration

The sulfur content in metal sulfides can be determined after oxidative dissolution with bromine water-CCl₄ at 50 °C by modified EDTA titration. The sulfides were quantitatively oxidized to SO₄ ^2– without evolving H₂S, and the metal cations were removed by cation exchange on an H-type column. Sulfate was precipitated as BaSO₄, and the remaining Ba²⁺ was subsequently titrated with EDTA at pH = 10. The standard deviation for the determined sulfur in the metal sulfides, some of which can be hardly dissolved in usual mineral acid media, is about a few %. The method can be carried out with sample amounts containing as little as 1.5–2 mg sulfur. Received: 31 July 1997 / Revised: 5 January 1998 / Accepted: 9 January 1998     

Determination of the stoichiometry of mixed microcrystals KxCsyZnCl4 using instrumental neutron activation analysis

Instrumental neutron activation analysis (INAA) has been employed as an absolute method for the determination of the stoichiometry of mixed microcrystals K_xCs_yZnCl₄ with a weight ranging between 20 and 50 μg. The reliability of the method has been checked with the pure substances KCl, NaCl, CsCl and RbCl, for which the mean value of the ratio Cl/X was found to be 1.04 (3). Received: 26 May 1997 / Revised: 1 July 1997 / Accepted: 2 July 1997     

Determination of the stoichiometry of mixed microcrystals KxCsyZnCl4 using instrumental neutron activation analysis

Instrumental neutron activation analysis (INAA) has been employed as an absolute method for the determination of the stoichiometry of mixed microcrystals K_xCs_yZnCl₄ with a weight ranging between 20 and 50 μg. The reliability of the method has been checked with the pure substances KCl, NaCl, CsCl and RbCl, for which the mean value of the ratio Cl/X was found to be 1.04 (3). Received: 26 May 1997 / Revised: 1 July 1997 / Accepted: 2 July 1997     

Determination of total water content in inulin using the volumetric Karl Fischer titration

A new sample preparation method for the water content determination of inulin by volumetric Karl Fischer (KF) titration was developed and compared to the usual method of introducing the sample directly in the methanol-based working medium, modified or not by formamide (1:3, v/v) in order to increase sample solubility. In the proposed method, inulins were externally prepared by dissolving them in pure formamide (2.5:7.5, w/w). The time of analysis of the liquid/liquid reaction of the new method between the dissolved sample and the reaction medium is about 1–2 min, while the usual KF method is stopped after the 10 min delay time. The developed method permits the determination of water included in the crystals of the sample, confirmed by analysing both crystalline and amorphous inulin samples. Another advantage of this new method is its applicability for the water content determination of other polysaccharides that are not readily soluble in the working medium. Moreover, water content determination can be done by any type of volumetric KF titrator, as this proposed technique is not dependent on any additional tools such as a built-in homogeniser or a heatable titration beaker.     

Conductometric and potentiometric investigations of ionic surfactant– gelatin interaction

 The interaction between gelatin and ionic surfactant is relevant to many biological and industrial processes. Therefore, knowledge of the mechanism of ionic surfactant–gelatin interaction is an important factor influencing practical application of such systems. In this paper, conductometric and potentiometric titrations were used to study the interaction of sodium dodecyl sulfate as an anionic and cetyltrimethylammonium bromide as a cationic surfactant with gelatin solutions of different concentrations. Titrations were carried out at 40 °C by adding surfactant to the gelatin solutions. The titration course was followed by measuring specific conductance and pH changes. On the basis of the titration curves the prevailing mechanisms of surfactant–gelatin interaction, as well as the characteristic concentrations at which they are changed, were determined. From the linear relationship established between the characteristic surfactant concentrations and gelatin concentration, maximal amount of surfactant bonded per gram of gelatin was calculated. Received: 22 July 1997 Accepted: 11 December 1997     

Preparation of doubly 13C-labelled benzylpenicillin as internal standard for residue analytical use with GC/MS and LC/MS

A simple and rapid method is described for the preparation of doubly ¹³C-labelled benzylpenicillin for use in isotope dilution analyses of benzylpenicillin residues. The product is characterized by IR, NMR, GC/MS and LC/MS. Tissue analyses demonstrate its advantage with residue analytical methods using mass spectrometric techniques for measurement. Different fragmentation patterns are observed when using GC/EI-MS or LC/electrospray-MS. The consequences for residue analysis are discussed. Received: 28 May 1997 / Revised: 23 July 1997 / Accepted: 25 July 1997     

Study of ionization of tyrosine residues in proteins by second-derivative UV spectroscopy

Four spectrally different forms of tyrosine residues were shown to be present in proteins, namely, nonionized residues, either buried or exposed to solvent, and ionized residues buried or exposed to solvent. A method for determining the pK _a values of the tyrosine residues in proteins was proposed. It is based on the decrease in the absorption intensity in the second derivative of the UV spectrum at 284.2 nm, which is the wavelength of the isobestic point corresponding to the transition of the nonionized tyrosine residues from the buried to the exposed state. Several proteins were studied by this method; the results obtained were found to be close to the corresponding published data. This method is simpler than the conventional UV titration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1394–1398, July, 1997.     

In-situ methylation of strongly polar organic acids in natural waters supported by ion-pairing agents for headspace GC-MSD analysis

Strongly polar organic substances like halogenated acetic acids have been analyzed in surface water and groundwater in the catchment area of the upper Elbe river in Saxony since 1992. Coming directly from anthropogenic sources like industry, agriculture and indirectly by rainfall, their concentrations can increase up to 100 μg/L in the aquatic environment of this catchment area. A new static headspace GC-MSD method without a manual pre-concentration step is presented to analyze the chlorinated acetic acids relevant to the Elbe river as their volatile methyl esters. Using an ion-pairing agent as modifier for the in-situ methylation of the analytes by dimethylsulfate, a minimal detection limit of 1 μg/L can be achieved. Problems like the thermal degradation of chlorinated acetic acids to halogenated hydrocarbons and changing reaction yields during the headspace methylation, could be effectively reduced. The method has been successfully applied to monitoring bank infiltrate, surface water, groundwater and water works pumped raw water according to health provision principles. Received: 17 July 1997 / Revised: 29 September 1997 / Accepted: 2 October 1997     

Supercritical fluid extraction combined with solid phase extraction as sample preparation technique for the analysis of β-blockers in serum and urine

A method is developed for the determination of β-blockers in serum and urine at levels of 0.5 μg/mL. The technique uses a combination of solid phase extraction (SPE) with in situ derivatization and supercritical fluid extraction (SFE) with subsequent gas chromatography mass spectrometry. The optimization of the SFE step shows that a static extraction period can be eliminated. The method gives good linearity (r = 0.991–0.999) and repeatability in the concentration range of 0.5 to 5 μg/mL. Relative standard deviations for oxprenolol, propanolol and metoprolol were less than 5% in serum and 5–11% in urine. Received: 23 May 1997 / Revised: 28 July 1997 / Accepted: 5 August 1997     

Is traceability an exclusive property of analytical results? An extended approach to traceability in chemical analysis

The need to adapt the definition of traceability in the Metrological Dictionary of ISO to the growing use of this concept in Analytical Chemistry aroused the broader, more flexible proposal expounded in this paper which aims to be closer to the bench level. The traceability concept is addressed in a hierarchical manner by ranking the different notions to which the qualifier “traceable” applies (results, standards, equipment and samples) in such a way that it is compatible with the ISO definition. Relationships among them and with classical analytical properties are also exposed. Received: 12 May 1997 / Revised: 22 July 1997 / Accepted: 25 July 1997     

Is traceability an exclusive property of analytical results? An extended approach to traceability in chemical analysis

The need to adapt the definition of traceability in the Metrological Dictionary of ISO to the growing use of this concept in Analytical Chemistry aroused the broader, more flexible proposal expounded in this paper which aims to be closer to the bench level. The traceability concept is addressed in a hierarchical manner by ranking the different notions to which the qualifier “traceable” applies (results, standards, equipment and samples) in such a way that it is compatible with the ISO definition. Relationships among them and with classical analytical properties are also exposed. Received: 12 May 1997 / Revised: 22 July 1997 / Accepted: 25 July 1997     

Calibration of titration methods

Summary A calibration method is proposed which makes it possible to use titration techniques in the presence of systematic errors, even if these errors depend on the concentrations of the analytes. The approach uses an empirical calibration model which approximates the relationship between apparent (found) and true concentrations of the analytes. Also a calibration of the physical model of the titration process is proposed, as well as a method of determination of the model parameters, which are useful when analytes are determined by fitting the model to experimental data of titration. Both approaches, empirical and based on the physical model, may be applied jointly. The example presented reveals high efficiency of the proposed approach in cases when a deficient physicochemical model of the titration process is used in the determination of an analyte concentration (simulated titration data applied). The calibration proposed may be considered as a generalization of the titrant standardization used in the conventional volumetric analysis. It may be applied to all titration techniques and for all methods of end-point detection and determination of the concentration of analytes. It opens new possibilities for the development of titration methods. Permanent address: Department of Analytical Chemistry, Jagiellonian University, Karasia 3, PL-30-060 Krakow, Poland     

An open microwave digestion of oxide materials continuously analyzed by ICP emission spectrometry

A device for microwave digestion in an open system was designed, with which a slurry automatically produced from solid sample powders can be processed continuously and then be transported directly into the ICP spectrometer by an online method. The success of this procedure is dependent to a high degree on the behavior of the material to be analyzed and the instrumental device used. In principle, it can be easily installed in a laboratory by combining single units like a mill, a pump, a microwave oven, and an ultrasonic bath in front of an ICP spectrometer. As examples, converter sludge and sinter were processed. In contrast to the sludge, the sinter material was difficult to homogenize sufficiently well. This led to recovery problems, especially for oxides of Si, Al or Ti being separated from the matrix by long-time milling. To overcome the blocking of valves, an open microwave digestion system is applied here (as usual by acid attack). Furthermore, the ICP source is able to excite the particles of a slurry much more efficiently than any other flame. The sufficient results obtained for the online method described here showed it to be a fast method for routine analysis. Received: 16 June 1997 / Revised: 29 July 1998 / Accepted: 31 July 1998     

Mercurous nitrate as a reductimetric reagent: The potentiometric titration of ferric iron in the presence of ions forming highly coloured thiocyanates

The reduction of ferric iron by means of mercurous salts in the presence of an excebs of ammonium, thiocyanate can be predicted on theoretical grounds. By consideration of the oxidation potentials involved it has been shown by calculation that quantitative reduction of the ferric iron is to be expected.—The titration may be effected potentiometrically using a bright platinum indicator electrode in conjunction with a silver/silver chloride electrode or a saturated calomel reference electrode. Ferric iron can be titrated in the presence of ions such as cobalt which form intensely coloured thiocyanate complexes.A possible method for the titration of molybdate by means of mercurous nitrate has been examined. The sensitivity of the method suffers from the instability of potential of the system under titration, but particularly since tungstate does not interfere, the method is worthy of further study.     

Infrared spectroscopic characterization of the buried interface and surfaces of bonded silicon wafers

Basic investigations have been carried out on the characterization of different processing steps in bond preparation using etched <111> faces of silicon wafers for the incidence of the infrared beam to a multiple internal reflection geometry. The method is very sensitive to the surface coverage and interface. Surface activation by RCA cleaning yields an increase of water coverage and a decrease of SiH and CH groups. The detection limit for an oxide layer between silicon wafers has been found to be about 3 nm. Received: 18 July 1997 / Revised: 15 August 1997 / Accepted: 19 August 1997