Symmetric Potentiometric Cells for the Measurement of Unified pH Values

A unified pH scale of absolute values (pHabs scale) enables the comparison of acidities in different solvents. To date, very few different experimental setups have been used for the measurement of values on this scale. The article describes the design and performance of the different symmetric cells used for unified pH measurement by several institutions. Well-established and reliable standard aqueous buffer solutions are the first step of method validation necessary to achieve a robust metrological level for more complex media. The pH of aqueous standard buffers was measured by differential potentiometry, where the potential between two glass electrodes is measured directly. All the tested electrochemical cells prove to be suitable for unified pH measurements. This validation highlights that the method is, to a large extent, independent of the used equipment, including the cell geometry. The inherent symmetry of the cell design helps to reduce the experimental workload and improve the accuracy of obtained results.

Dispersive liquid-liquid microextraction based preconcentration of selected pesticides and escitalopram oxalate, haloperidol and olanzapine from wastewater samples prior to determination by GC-MS

A multivariate experimental design was used to attain optimum conditions of a dispersive liquid-liquid microextraction (DLLME) method for preconcentration of pesticides and pharmaceuticals for determination by gas chromatography mass spectrometry (GC-MS). Experimental design enabled parameters to be evaluated for their effects on extraction output as well as their interactive effects. The optimum parameters suggested by the design model were 200 µL of chloroform, 1.96 mL of ethanol and 40 s vortexing period. Limits of detection and quantification (LOD and LOQ) were calculated using linear calibration plots of the analytes developed in the standard concentration range of 2.0 µg/L-2.0 mg/L. Enhancement in detection power of the analytes recorded by the optimized method with respect to direct GC-MS determination (based on LOD values) was in the range of 3.6 and 539 folds. Spiked recovery experiments for municipal, medical and synthetic wastewater samples yielded low recovery results when calculated against aqueous standard solutions. Matrix matched calibration standards were used to mitigate interferences from the waste samples, and the percent recoveries obtained were close to 100%. This established accuracy and applicability of the developed method.

Verification of accuracy of standard thermodynamic data of inorganic electrolytes

Accuracy of standard thermodynamic data for some inorganic electrolytes of the types 1-1, 1-2 and 2-1 at 25 °C, in the solid state or aqueous standard solutions, presented in NBS Tables (1982), has been checked. For the purpose, the condition for the pure solid–saturated solution equilibrium and reliable values of solubilities, mean activity coefficients and water activity in saturated solution at the given temperature were used. In most cases, a very good reliability and consistency of all relevant data have been found. On the other hand, some discrepancies have been observed for NaNO3, NaClO4/NaClO4 pair, Na2HPO4·12H2O/Na2HPO4 pair, K2CrO4, K2Cr2O7, (NH4)2SO4 and SrCl2·6H2O/SrCl2 pair the reason of which is not yet known. The verification method can be used for the determination of lacking thermodynamic data. The method has been applied to ΔGşf,cr of NaCNS·2H2O, NaH2PO4·2H2O, Na2CrO4·6H2O, Na2WO4·2H2O, MgI2·8H2O and CaCl2·6H2O at 25 °C. A suitable method has been used for the determination of the most probable solubility in the case when the values scatter. The used methods are applicable to all types of electrolytes and nonelectrolytes.