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Each instrument used in analytical chemistry has a useful working range. This is the range of concentration (or mass) that can be adequately determined by the instrument, where the instrument provides a useful signal that can be related to the concentration of the analyte. [1] All instruments have an upper and a lower working limit.
The Davies equation is an empirical extension of Debye–Hückel theory which can be used to calculate activity coefficients of electrolyte solutions at relatively high concentrations at 25 °C. The equation, originally published in 1938, [ 1 ] was refined by fitting to experimental data.
The most common method of eliminating the liquid junction potential is to place a salt bridge consisting of a saturated solution of potassium chloride (KCl) and ammonium nitrate (NH 4 NO 3) with lithium acetate (CH 3 COOLi) between the two solutions constituting the junction. When such a bridge is used, the ions in the bridge are present in ...
The data below tabulates standard electrode potentials (E°), in volts relative to the standard hydrogen electrode (SHE), at: . Temperature 298.15 K (25.00 °C; 77.00 °F); ...
In this case, raising the pH of the phase mobile above 4–5 = pH (which is the typical pKa range for carboxylic groups) increases their ionization, hence decreases their retention. Conversely, using a mobile phase at a pH lower than 4 [31] will increase their retention, because it will decrease their ionization degree, rendering them less polar.
Therefore, the difference in potential between the two electrodes gives an assessment of the sample's composition. In fact, since the potentiometric measurement is a non-destructive measurement, assuming that the electrode is in equilibrium with the solution, we are measuring the solution's potential.
If this convention is used, then the values are in the same range as monovalent ions, e.g. 59.5 S cm 2 mol −1 for 1 / 2 Ca 2+ and 80.0 S cm 2 mol −1 for 1 / 2 SO 2− 4. [4] From the ionic molar conductivities of cations and anions, effective ionic radii can be calculated using the concept of Stokes radius.
The difference between successive pK a values is sufficiently large so that salts of either monohydrogen phosphate, HPO 2− 4 or dihydrogen phosphate, H 2 PO − 4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pK a values.