Page 238 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE
Concentration of silver nitrate solution = 0.019 36M
Cl -: total titres (less blanks) = 89.54 mL = 61.47 mg
Br-: total titres (less blanks) = 83.65 mL = 129.4 mg

7.12 DETERMINATION OF THE TOTAL CATION CONCENTRATION IN WATER

Theory. The following procedure is a rapid one for the determination of the
total c.ations present in water, particularly that used for industrial ion exchange
plants, but may be used for al1 samples of water, including tap water. When
water containing dissolved ionised solids is passed through a cation exchanger
in the hydrogen form al1 cations are removed and replaced by hydrogen ions.
By this means any alkalinity present in the water is destroyed, and the neutral
salts present in solution are converted into the corresponding minera1 acids.
The effluent is titrated with 0.02M sodium hydroxide using screened methyl
orange as indicator.
Procedure. Prepare a 25-30cm column of Duolite C225 in a 14-16mm
chromatographic tube (Section 7.8). Pass 250mL of 2M hydrochloric acid
through the tube during about 30 minutes; rinse the column with distilled water
until the effluent is just alkaline to screened methyl orange or until a 10mL
portion of the effluent does not require more than one drop of 0.02M sodium
hydroxide to give an alkaline reaction to bromothymol blue indicator. The resin
is now ready for use: the level of the water should never be permitted to drop
below the upper surface of the resin in the column. Pass 50.0 mL of the sample
of water under test through the column at a rate of 3-4mL per minute, and
discard the effluent. Now pass two 100.0 mL portions through the column at
the same rate, collect the effluents separately, and titrate each with standard
0.02M sodium hydroxide using screened methyl orange as indicator. After the
determination has been completed, pass 100-150 mL of distilled water through
the column.
From the results of the titration calculate the millimoles of calcium present
in the water. It may be expressed, if desired, as the equivalent minera1 acidity
(EMA) in terms of mg CaCO, per litre of water (i.e. parts per million of CaCO,).
In general, if the titre is A mL of sodium hydroxide of molarity B for an aliquot
volume of V mL, the EMA is given by (AB x 50 x 1000)/K
Commercial samples of water are frequently alkaline due to the presence of
hydrogencarbonates, carbonates, or hydroxides. The alkalinity is determined
by titrating a 100.0 mL sample with 0.02M hydrochloric acid using screened
methyl orange as indicator (or to a pH of 3.8). To obtain the total cation content
in terms of CaCO,, the total methyl orange alkalinity is added to the EMA.

7.13 SEPARATION OF CADMIUM AND ZINC ON AN ANION EXCHANGER
Theory. Cadmium and zinc form negatively charged chloro-complexes which
are absorbed by a strongly basic anion exchange resin, such as Duolite A113.
The maximum absorption of cadmium and zinc is obtained in 0.12 M hydrochloric
acid containing 100 g of sodium chloride per litre. The zinc is eluted quantitatively
by a 2M sodium hydroxide solution containing 20 g of sodium chloride per
litre, while the cadmium is retained on the resin. Finally, the cadmium is eluted