ION EXCHANGE METHODS

Background

Some of the more common cations, such as Na⁺ and K⁺, are not readily analyzed by simple volumetric or gravimetric procedures. These cations can be analyzed quantitatively by the technique of ion exchange. In this case the Na⁺ and K⁺ is titrated with a standardized strong base solution. The reactions involved are as follows:

Ion exchange reaction

Na⁺ + H(Resin) ó Na(Resin) + H⁺

Titration reaction

H⁺ + OH^- ó H₂O

The situation is complicated, however, if a cation such as Ca²⁺ is present, since it also will react with the resin to liberate H⁺, which would be titrated in the titration reaction. Furthermore, any strong acid in the original solution will pass through the resin unchanged and can be titrated. To take care of a problem such as this, a person has to resort to the use of multiple techniques to analyze the system.

Consider starting with a solution containing H⁺, Ca²⁺, and Na⁺. H⁺ can be titrated with strong base and found directly. Ca²⁺ can be titrated with ethylenediaminetetraacetic acid (EDTA) and found directly. Na⁺ can only be found, however, by exchanging both the Na⁺ and Ca²⁺ for H⁺ on the ion exchange process and arriving at the amount of Na⁺ in the original sample by differences.

If an aliquot of the original sample contains X mmoles of Ca²⁺, Y mmoles of Na⁺, and Z mmoles of H⁺, one can see that after the ion exchange step there are (2X + Y + Z) mmoles of strong acid to be titrated with the strong base. Titration of another aliquot of the sample with EDTA will allow one to find the number of mmoles of Ca²⁺ (X) in the sample. Titration of still a third aliquot with the standardized strong base will give the number of mmoles of strong acid (Z) in the sample. The number of mmoles of Na⁺ (Y) is easily found from these data.

Preliminary Calculations

1. Determine the amount of 18 N NaOH to use to prepare 1 liter of 0.1 N NaOH.
2. Calculate the normality of a NaOH solution if 828.6 mg of KHP is found to be equivalent to 40.20 mL of the NaOH.
3. The capacity for exchange of an ion exchange resin is given in terms of meq/mL resin. Wet Dowex 50 in the acid form has a capacity of 1.7 meq/ml resin. How many 10 mL aliquots of a sample that is 0.15 F in Na⁺, 0.15 F in H⁺, and 0.025 F in Ca²⁺ can be passed through 30 mL of resin before it needs regeneration?

Procedure

In this analysis it will be necessary to use both a standardized 0.1 N NaOH solution and a standardized 0.01 F EDTA solution as titrants. The 0.1 N NaOH is the same that was used in the soda ash titration and is already standardized. The 0.01 F EDTA solution is the same that was used in the hardness of water experiment and is also already standardized. The ion exchange column has been previously prepared and can be checked out from the laboratory instructor.

Preparation and Standardization of 0.1 N NaOH.

NOTE: Retain the standardized NaOH solution for use in subsequent experiments.

1. With a graduated cylinder, measure out enough 50% NaOH (density = 1.535 g/ml) to prepare 1 liter of 0.1 N NaOH. Dilute to 1 liter with freshly boiled distilled water. Store in a stoppered polyethylene bottle.
2. Into 250 mL Erlenmeyer flasks weight precisely triplicate 0.7 - 0.8 g portions of reagent-grade KHP (potassium acid phthalate) that has been dried at 100-110 ^oC for 2 hours.
3. Add about 50 mL of freshly boiled distilled water (room temperature) and 2 drops of phenolphthalein indicator.
4. Titrate with the 0.1 M NaOH to the first pink color that persists for a half-minute.

Preparation of the Ion Exchange Column

Check out a prepared ion exchange column. Regenerate the resin by pouring a 100 mL portion of 3 N HCl through the column. Run at a rate of about 1 drop a second. Do not allow the level of the solution to fall below the surface of the resin; should it do so, backwash the column (see instructor). After regeneration, pass distilled water through the column to wash off excess H⁺. Test the effluent by collecting 30 mL and adding 1 drop of methyl red indicator. At neutrality, 1 drop of 0.1 N NaOH is sufficient to change the indicator to the alkaline color.

Analysis of the Unknown Sample

1. Turn in a clean 250 mL volumetric flask, clearly labeled with your name, section number, and the experiment number. To the sample given to you in the flask, add enough distilled water to dilute to the mark. Mix well.
2. Titration of the strong acid: Transfer a 10.00 mL aliquot of the diluted sample to a 250 mL Erlenmeyer flask, add about 50 mL of water, add 2-3 drops of methyl orange indicator and titrate to the end point with your standardized NaOH. Run at least three of these titrations.
3. Titration of the Ca²⁺: Titrate a 10.00 mL aliquot of the diluted sample with standardized 0.010 F EDTA titrant. Perform analyses on three separate aliquots.
4. Ion exchange: Transfer a 10.00 mL aliquot of the diluted sample to the ion exchange column. Allow the solution to drain to near the top of the resin. Collect all of the effluent in a 250 mL Erlenmeyer flask. Add 5 10-ml portions of water to the top of the column allowing each portion to drain to near the top of the resin before adding the next portion. After adding the 50 mL of wash water, add an additional 10 mL of water and collect in a separate container, treat with 2 drops of methyl red and 1 drop of .10 N NaOH. Take the NaOH from your buret, which has been previously leveled. When the effluent is neutral, add several drops of methyl red to the effluent in the Erlenmeyer. Add to the effluent those portions of wash that were tested for neutrality. Titrate with 0.10 N NaOH. Perform this analysis on three aliquots.

Calculations

Calculate the concentrations of H⁺, Na⁺, and Ca²⁺ in units of N, :g/ml, and ppm CaCO₃ respectively.

Questions

1. Suppose your sample contained H⁺, Na⁺, Ca²⁺, and Zn²⁺. Can you analyze for all of these cations by using the same procedure as outlined in the experiment? Why?
2. If you cannot analyze for all the above cations, suggest a modification of your procedure that would allow you to determine each of the four cations.
3. Which of the three cations, H⁺, Na⁺, Ca²⁺, would have the greatest amount of uncertainty associated with its analysis using the procedure of this analysis? Why?
4. A sample known to contain only Na⁺, Ca²⁺, and H⁺ was analyzed as follows. One 10.00 mL aliquot was titrated with 0.1000 N NaOH to the methyl orange end point; 13.00 mL of the titrant was required. A second 10 mL aliquot was titrated with 0.0100 F EDTA, 30.00 mL being required to reach the Eriochrome Black T end point. A third 10.00 mL aliquot was passed through a Dowex 50 ion exchange column in the acid form, the sample was collected and titrated with 0.1000 N NaOH to the methyl orange end point; 31.00 mL of the NaOH was required. Calculate the molar concentration of Na⁺, H⁺, and Ca²⁺ in the original sample.
5. Determine the weight of KCl that is equivalent to 30 mL of 0.1 N NaOH if the K⁺ is exchanged for H⁺ on an ion exchange column and the H⁺ titrated with the NaOH.

Results

Report concentrations and precisions of H⁺, Na⁺, and Ca²⁺ in units of N, mg/ml, and ppm CaCO₃ respectively. Your lab score will be based 100% on these values.

Reference

• D. C. Harris Quantitative Chemical Analysis 4th Ed., W. H. Freeman and Company, New York 1995 Chapters 5, 22, and 23

Friday, August 25, 2006