Potentiometric back-titration method

The TBN values obtained for the fresh, unused, lubricating oil additive package components show results from conductometric and IP 177 (potentiometric) methods being 90% to 98 % and 85% to 90%, respectively, of the corresponding IP 276 (potentiometric) values. The TBN values for some selective products such as zinc dialkyl-dithiophosphate (ZDDP) was observed as an inflection point using the IP 276 (potentiometric) back titration method and also the conductometric method. 

A potentiometric back-titration method of excess barium ions following precipitation as BaS04 (Mucci, 1991) is outlined in Section 11.2.5.6. 

In classical potentiometric titration, soluble species produced by dissolution of the solid may contribute to the consumption of an acid or base and induce an error in the calculated Oq. This error is not corrected for in back titrations of an inert electrolyte (Figure 2.6). The back-titration method was designed to avoid the error induced by the dissolution of the solid phase, and it has been used for solids that show appreciable solubility. For solids that are practically insoluble, the... 

The back titration method can be used to measure total alkalinity with simpler equipment than that required for the potentiometric titrations described above, but at the cost of a poorer precision. The method described is that of Gripenberg (1936) as modified by Koroleff (1972). The sample is acidified with hydrochloric acid to a pH of about 3.5, and the carbon dioxide is driven off by boiling. The solution is then back-titrated to pH 6 with sodium hydroxide solution using bromothymol blue as indicator. The alkalinity is estimated simply as the difference between the amounts of hydrochloric acid and sodium hydroxide added. 

Sulphate concentrations may also be determined accurately by potentiometric back-titration of excess Ba + with a mercury electrode following the precipitation of BaS04 Mucci, 1991). The seawater sample is freed from most seasalt cations with an ion-exchange-column. Then the eluate is reacted with an excess of barium, and after filtration of precipitated BaS04, the solution is titrated potentiometrically with an EGTA solution (see also Section 11.2) to the endpoint. The method applies over a wide range of salinities and sulphate concentrations in 1 mL or less of seawater and marine pore water samples, however, it is somewhat less precise (c.v. of about 0.6 %) than the simple gravimetric procedure described. 

Considering the related Mettler DL 40 and DL 40 RC MemoTitrators, the DL 40 can be used for ten different volumetric and potentiometric methods titration to a pre-selected absolute (EPA) or relative (EPR) end-point equilibrium titration (EQU)-recording titration (REC)-incremental titration (INC) Karl Fischer water determination (KF)-controlled dispensing (DOSE) pH and pX measurements (pX/E) multi-level titrations and back-titrations with automatic calculation (CALC) and manual titration (MAN)-automatic calibration of electrodes (CAL). 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

Titrimetric Methods. Frequently, the reduction of gold solutions leads to finely divided precipitates which are difficult to recover quantitatively. In such cases, the reduction of Au(III) to Au(0) by, eg, hydroquinone, can be followed potentiometrically (49). The end point in such titrations also can be determined with indicators such as benzidine (50) or 0-anisidine (51). Alternatively, the reduction can be effected with excess hydroquinone which is then back-titrated with Ce(IV) (52). Iodometric determination of Au(III) also is useful (53). 

The conductometric titration method has several advantages over the potentiometric titration method. It is applicable in a straight forward manner, without back titration or other modifications, to the determination of TBN for a wide range of petroleum products including fresh and heavily used oils. The conductometric method is quick and easy to perform, with two intersecting lines at the equivalent point, also contamination of electrodes is eliminated. The... 

Such methods are wet oxidation of pulp followed by estimation of sulfate by precipitation of barium sulfate (Canadian Pulp and Paper Association Standard G28 1970), X-ray fluorescence spectroscopy (Rivington 1988, Kibblewhite et al. 1987), and combustion of pulp followed by analysis of sulfur as sulfur dioxide or as sulfate. The sulfur dioxide evolved is determined by iodometric titration (Canadian Pulp and Paper Association useful method G.7U, March 1959). Sulfate can be determined by titration with barium chloride (Ora 1960), back-titration with sulfuric acid after addition of barium perchlorate (Aldrich 1974), potentiometric titration with lead perchlorate using an ion-selective electrode (Ross and Frant 1969), or ion chromatography (Douek and Ing 1989). 

The hydroxyl number is very important for the synthesis of polyurethane. It is defined as the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of one gram of the sample. The principle of the analytical method is that the hydroxyl group is esterified with a solution of phthalic anhydride in pyridine. The excess reagent is back-titrated with standard sodium hydroxide solution, and a blank is run on the reagents to determine the amount of anhydride consumed. The phenolphthalein is an indicator for the titrate. Because the color of recycled polyol is dark brown, the faint pink endpoint is not clear, so a potentiometric titrate can be applied. 

In another procedure, a halogen acid is generated by the reaction of an ionic halide salt, eg, tetraethylammoniiun bromide in acetic acid with perchloric acid with subsequent formation of a halohydrin the epoxy group is determined by back-titration with perchloric acid nsing crystal violet indicator (83). The end point can be determined visually or potentiometrically. A monograph on epoxide determinations was published in 1969 (84). This is the method adopted by ASTM and is currently used by most resin producers. 

The aim of this chapter is to describe both the high precision methods for the determination of total alkalinity At (potentiometric titration) and total dissolved inorganic carbon Ct (coulometry) that are the methods of choice today, and also a simpler method for total alkalinity (back titration) which does not require sophisticated computerised equipment. Thermodynamic calculations of carbonate speciation are also covered. 

Ester groups occur in a wide range of polymers (e.g., polyethylene terephthalate) and in copolymers such as, for example, ethylene vinyl acetate. The classical chemical method for the determination of ester groups, namely, saponification, can be applied to some types of polymer. For example, copolymers of vinyl esters and esters of vinyl esters and esters of acrylic acid, can be saponified in a sealed tube with 2 M sodium hydroxide. The free acids from the vinyl esters were determined by potentiometric titration or gas chromatography. The alcohols formed by the hydrolysis of the acrylate esters were determined by gas chromatography. Vinyl acetate ethylene copolymers can be determined by saponification with 1 N ethanolic potassium hydroxide at 80 C for 3 hours and back titration with standard acid or by saponification with p-toluene sulfonic acid and back titration with standard acetic acid [49, 50]. 

Using potentiometric titrations for the determination of hydrolysis constants presents several draw backs. The method is only indirect, where the validity of the assumptions made can only be justified by the goodness of fit together with coordination chemistry considerations (a model should make chemical sense). Thus, there can often be doubt raised with respect to which species actually exist in any specific case. 

The determination of partition caefcients by the potentiometric method is part of the function of the Sirus PCA101 (Avdeef, 1993). Typically, a preacidiLed solution of a weak acid is alkalimetrically titrated to some appropriately high pH the partition solvent such as octanol is then added, and the dual-solvent mixture is acidimetrically titrated back to the starting pH. Analysis of the two titration curves will yield two fKgS pl< and pol, where pdQ, is the apparent constant derived from... 

Traditional methods to measure ionization constants are based on potentiometric or spectrophotometric titration. These methods are inherently slow due to the equilibration time needed after each addition of titrant typically one titration takes 20 to 40 minutes, and as most samples are poorly soluble in water, one needs to perform three titrations in water-co-solvent mixtures and extrapolate back to aqueous. 

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