DETERMINATION OF THE ION EXCHANGE CAPACITY
Successful performance of ion exchange techniques is largely based on proper preparation of the exchanger. This preparation is essentially the conversion of an exchanger from the form provided by the manufacturer to a form that can be used.
There are four basic procedures involved in preparing an exchanger:
1.Removal of impurities that result from incomplete purification of the exchanger by the manufacturer or slight decomposition of the exchanger during storage.
2.Swelling the exchanger (termed precycling) so that a greater percentage of the charged groups are exposed to the suspending solution.
3.Removal of the fines (very small particles of exchanger).
4.Conversion of the counter ion of the exchanger to that desired for a given application.
- Washing procedures for polystyrene ion exchangers include washes with water, hydrochloric acid, ethanol and a high concentration of the counter ion to which the exchanger is being converted.
- The purpose of precycling an exchanger is to expose the charged groups that are bound to the matrix.
- After the exchanger has been thoroughly swelled, the fines should be removed. Fines are particles of exchanger generated during manufacture of the exchanger or as a result of excessively vigorous stirring during washing and precycling procedures.
- This process should ALWAYS performed after the matrix is fully swollen to prevent the trapping of fines in the unswollen portion of the exchanger.
Removal of these fine particles is accomplished by repeatedly suspending the exchanger in large volumes of water, allowing approximately 90 to 95% of the exchanger to settle, and decanting off slow sedimenting material. Failure to remove the fines usually results in decreased flow rates and poor resolution.
PROCEDURE: Preparation of the ion exchanger:
1.Suspend the ion exchange resin in deionized water.
2.When the resin has almost completely settled, decant off the liquid containing the fines.
3.Repeat the above steps three to five times.
4.Rinse the resin twice with 95% ethanol to remove ethanol soluble impurities.
5.Suspend the resin in 2N HCl and heat to 100°C. Repeat this operation three to four times or until supernatant is clear and colourless, using fresh HCl each time. Allow the resin to cool for one hour each time before repeating the heating step.
6.Wash the resin with deionized water until the pH of the effluent is the same as that of the water added.
PACKING OF THE COLUMN:
1. Take a clean 10 x 15 cm column
2. Attach a 5cm piece of 0.5cm i.d. rubber tubing to the bottom of the column.
3. To the other end of the tubing insert a glass pipette tip.
4. Fix a stopcock on the rubber tubing.
5.Tamp the column with glass wool ( the glass wool should be tight enough to prevent any of the resin particles from passing through and clogging the stopcock or tip, but loose enough and small enough so as not to impede the flow of liquid through the column).
6.Position the column vertically on a stand.
7.Fill the column 1/3 full of water and sufficient resin to make a bed height of 10cm (the important condition for packing of a chromatographic column with the ion exchange resin suspension is careful control to prevent the suspension introduced from carrying air with it and to prevent drying of the column- pour the slurry down a glass rod or funnel whose tip touches the inner wall of the column).
8.Wash the resin off the sides of the column with water. Pack the resin by placing a 5-10 cm column of water on top of the column and forcing the water through the resin.
9.Wash the resin column with water and adjust the flow rate of effluent to 3ml/min.The column is ready for use.
CAPACITY OF ION EXCHANGER:
1.Pass 100ml of 0.5N sodium sulphate solution through the column at 3ml/min.
2.Collect the eluate in a conical flask.
3.Measure the volume of the eluate.
4.Take 5ml of the eluate and titrate it against the sodium hydroxide which has been standardized.
5.Calculate the milliequivalent of H ion per milliliter of eluate and then the amount of total exchangable H ions in the column bed.
6.Calculate the meq H ions per millilitre of ion exchange resin.