Hydrogen-cation exchange, titration

Figure 1. Titration curves for monovalent cation-hydrogen exchange. (A) Na at 0.11 (B) Na at 0.3 I (C)Na at 0.5 I (D) K" at 0.11. Every third data point is shown. Interpolation is by a cubic spline method.

As a corollary to the above it should be pointed out that the exchange is in some instances stoichiometric and therefore the amount of cation in solution can be estimated by passage through a hydrogen exchanger as above and subsequent titration of the acid in the effluent. 

The methylene blue test can also be used to determine cation exchange capacity of clays and shales. In the test a weighed amount of clay is dispersed into water by a high-speed stirrer. Titration is carried out as for drilling muds, except that hydrogen peroxide is not added. The cation exchange capacity of clays is expressed as milliequivalents of methylene blue per 100 g of clay. 

Soil acidity is usually characterized by the pH of the soil solution or by exchanging the hydrogen ions by a cationic (e.g., sodium, potassium, or calcium) salt (Sevink et al. 1998) and the titration of the dissolved hydrogen ions by a basic solution. In this way, exchangeable acidity can be determined. 

When a solution of standard base is used only for titration of strong acids, a small amount of carbonate is not a serious source of error provided the end point is taken with an indicator that changes color at a pH of about 4 or 5. For standardizing such carbonate-containing solutions, potassium hydrogen phthalate is an unsuitable primary standard. An alternative is pure potassium chloride, which is passed through a cation-exchange column, converted to hydrochloric acid, and titrated with the sodium hydroxide. 

For a cation exchanger the available capacity is determined by converting the resin to the hydrogen form, then using a neutral solution of a sodium salt to displace H" " ions which are then titrated as free acid by a standard solution of sodium hydroxide. Then if wt of dry resin = Wg NaOH titre = Tml molarity of NaOH = A dry weight capacity = T.A/Wm eq gm . ... 

An acid form of a cation exchange resin, H(exchanger), can be used in place of the sodium form described in part E. Hydrogen ions, H+, are then displaced by the +2 cations. Suppose that 25.00 mL of permanently hard water are eluted through an acid cation exchange resin followed by washing of the resin. The eluent then requires 11.26 mL of 0.1038 M sodium hydroxide, NaOH, solution for titration to a phenolphthalein endpoint. 

If 100 ml of water is subjected to this ion exchange process, the value yielded by titration of the effluent with 0.1-m NaOH provides the sum of the anions less the mmol value for HC03 . If the latter value, taken from hydrogen carbonate determination, is added, this yields the total mmol of the anions and hence also the cations. After the ion exchange process, the undissociated substances (e.g. H2Si03> are contained in the effluent in... 

The sample is ignited and the ash dissolved in acids. Hydrogen peroxide is added and the solution passed down a cation exchange column. Vanadium, present as a pentavalent peroxide complex, passes unadsorbed down the column and is determined photo-absorptiometrically. Aluminium is removed from the column by elution with a more concentrated acid and determined by a complexometric titration. 

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