Ebbe R. Still

Graphic method for the determination of titration end-points

An extension of Grans method for the détermination of equivalence points in potentiometric acid-base titrations is presented. Equations are derived assuming that the stability constants of the reactions are known. The equations make it possible to locate the equivalence point with fair accuracy when acids with stability constants ranging from K(H)(HA) ~ 10(7) to K(H)(HA) ~ 10(10) are titrated. The limit of K(H)(HA) above which Grans equations do not yield satisfactory results is about 10(7). Conditional constants and a-coefficients are used to extend the method to apply to more complex systems, where more than one species reacts with the titrant.

Statistical adjustment of parameters for potentiometric titration data

The paper deals with the statistical problem of adjusting parameters to the experimental data from potentiometric titrations. The parameters to be refined are, first, the value of the stability constant of the titration reaction, and, second, the equivalence volume. The paper discusses the selection of the error variable and the need for weighting in the optimization procedure. The case where both parameters are refined at the same time is also considered. Special reference is made to linearized titration curves for the determination of the equivalence volume.

A Comparison between the Chemistry and Biochemistry of Molybdenum and Related Elements

Publisher Summary This chapter presents a comparison of the chemistry and biochemistry of molybdenum and related elements. The interest in molybdenum as found in biological systems is because of the importance of this special metal ion as an essential trace element participating in a number of enzymatic reactions. The feature common to different types of reactions involving molybdenum is the transfer of an even number of electrons to or from the substrate. The reactions can be interpreted either as formal electron transfer steps or as atom transfer steps accompanied by the release or uptake of protons. Thus, the catalytic site, that is, the molybdenum atom, is required to have the ability to form metal complexes in several oxidation states and to form oxo- or sulphido-ions. The reason why in biology molybdenum has gained importance as a more useful catalyst in particular chemical transformations is that it iss the only metal in the second and third transition series that is essential for life.

Photometric titrations with indicators

Abstract Various types of photometric titration curves are discussed. If a metal M is titrated conipleximetrically using a metal indicator and the absorbance is plotted vs. the titrant consumed, the inflection point appears at a pM value defined by the equation 3 pMinfl = pMtrans + 2 pMeq This expression is valid when M combines in a 1 : 1 ratio with the complexing agent and the indicator and when the indicator concentration is small compared to the total metal concentration. The difference between the pM values at the inflection and equivalence points can be calculated from the equation ΔpM = pMinfl — pMeq = 1 3 (pMtrans — pMeq) = 1 6 log(CMK2MI/KMY) If the inflection point is taken as the equivalence point, the error arising can be calculated from ΔpM, or more simply, read from a diagram. If transmittance, instead of absorbancc, is plotted as a function of the titrant volume, the inflection point depends on the added amount of indicator. However, at high transmittance values, i.e., at low indicator concentrations, the inflection point of a transmittance curve occurs practically at the same volume of added titrant as the inflection point of an absorbance curve. Rules are given for applying an indicator correction for the amount of metal bound to the indicator at the end-point. The derived equations and discussions can also be applied to acid-base titrations.

Solution studies of systems with polynuclear complex formation. 4. Heteronuclear copper(II) citrate complexes with nickel(II) or magnesium(II)

Abstract The existence of heteronuclear complexes formed between citric acid, copper(II) and nickel(II) or magnesium(II) is demonstrated. The stoichiometry and stability of the complexes are reported. A volumetric example demonstrates the unexpectedly high stability of the CuNicitrate complex in comparison with the binary complexes and the interference of heteronuclear complex formation on citrate masking reactions. The paper also discusses the existence of heteronuclear cationic hydrolysis products.

The conditional constant as error variable in the evaluation of stability constants from combined pH and pM measurements

Abstract Combined pH and pM measurements are useful in studies of chelate complexes. A plot of a conditional constant as a function of pH will indicate the species present in the solution and will give good estimates of the unknown stability constants. The values of the constants can be refined by non-linear regression analysis with the conditional constant as error variable. The approach enables programmable pocket calculators to be used in the optimization process.

Theory of complexometric titrations based on extractive end-point detection

Abstract The theory of complexometric titrations, which employ extractive end-point detection, is presented. If dithizone is used as an indicator in the complexometric titrations of zinc, lead and cadmium, high sensitivities can be attained because of the intense colour and low metal-ion concentrations at the colour transition points. It should be possible to titrate metals in very dilute solutions where many other indicators are not sensitive enough, particularly in the pH range around the neutral point.

THE PRECISION OF PHOTOMETRIC AND POTENTIOMETRIC MEASUREMENTS AND TITRATIONS, PARTICULARLY GRAN TITRATIONS

The precision of Potentiometric and photometric measurements and titrations is compared. Special reference is made to the so called Gran titrations and the conditions under which these are better than conventional titrations are discussed.