CHEMISTRY 221 LABORATORY—QUANTITATIVE ANALYSIS
EXPERIMENT: DETERMINATION OF THE EQUIVALENT WEIGHT OF AN INORGANIC SALT BY CATION EXCHANGE
Introduction: Many common chemical species such as the alkali metal cations and the halide anions are difficult to measure in macro amounts since they do not readily form insoluble compounds or colored complexes which would aid in their analysis. In addition, some metal ions may occur in solution as a stable complex species such as the case of Fe(CN)63-, CoCl42- or in different oxidation states such as UO22+ or U4+. Common techniques for metal analysis such as atomic emission or atomic absorption spectroscopy analyze for the total metal present, but cannot distinguish between the complex species or various oxidation states. Ion exchange may be used to concentrate and separate solution species based on their charges or it may be used to quantitatively determine the concentration of a solution species based on the stoichiometry of a charged species displaced from the solid phase when the analyte binds to the ion exchanger.
An important method of analysis for alkali metal cations and for determining the distribution of complex species is a technique called ion-exchange chromatography. Ion-exchange involves the interaction of a solid, insoluble phase that contains positively or negatively charged sites with a mobile phase (eluent solution) that contains the counterion. The ion-exchange equilibrium can be written as
|B+(aq) + A+Res-(s) A+(aq) + B+Res-(s)||Cation exchange|
|X-(aq) + Res+Y-(s) Y-(aq) + Res+X-(s)||Anion exchange|
where Res- and Res+ represent a solid, porous, polymer phase with specific binding sites. These polymer phases may be inorganic such as zirconium phosphates or aluminosilicates (clays) or, more commonly, an organic polymer that has been cross-linked and reacted to form the active ion-exchange sites.
One of the more common ion-exchange resins is a copolymer of styrene and
4-12% divinylbenzene which has been sulfonated to produce a strong-acid cation
exchanger or aminated to give a strong-base anion exchanger. The cation
exchanger typically has 1.1 sulfonate groups per benzene ring while the anion
exchanger has approximately 1 amine group for every two benzene rings. Typical
structures are illustrated below.
Typical commercial resin names are Dowex 50 (Dow) and Amberlite IR-120 (Rohm & Haas) for cation resins and Dowex 1 or 2 and IRA-400 for anion resins. These resins are stable to about 150 ºC and the hydrogen form of the cation resins typically has a cation exchange capacity of 2-5 milliequivalents (millimoles of charge) per gram while anion resins have an anion exchange capacity of 1-2 meq/g.
CONVERSION OF A SALT TO AN ACID
If a salt solution is passed through a cation exchange column which is in the hydrogen form, a quantitative reaction occurs:
Mn+ + n X- + n H+Res- Mn+(Res- )n + n H+ + n X-
The H+ liberated by the reaction can be titrated with a standard base to determine the amount of M+ in the sample taken for analysis. Note that a salt containing M+2 would give 2 H+ in the reaction. This procedure offers an excellent method for determining total cation charge in complex mixtures, preparing standard solutions of salts that cannot be prepared gravimetrically due to hygroscopic behavior (such as most nitrate salts), or allowing the quantitative analysis of a single cation in a relatively pure salt. It should also be noted that the technique will not give quantitative results if the charged species is involved in a significant acid-base equilibrium, Ka < 10-5, or is involved in a gaseous equilibrium such as CO32-, HCO3-, HSO3-, etc., or if the complex species is quite labile. In this experiment, since the charge on the metal ion is unknown, the determination of the grams of unknown equivalent to one mole of H+ is called equivalent weight. For a +1 ion, the equivalent weight and the molar mass are identical. For a +2 or +3 ion, the equivalent weights are one half and one third of the molar masses, respectively.
|Reagents:||6 M HCl||0.100 M NaOH|
|phenolphthalein indicator||blue litmus paper|
|Dowex 50-X8, 20-50 mesh cation exchange resin|
Summary Procedure: Read the detailed experimental procedure starting on page 391 in the Harris text.
A standard solution of 0.100 M NaOH will be provided. While this solution could be used for direct determination of the unknown, it is strongly advised that the NaOH should be standardized against reagent grade potassium hydrogen phthalate, KHP.
Standardization of 0.10 M NaOH:
A standard 0.1 M NaOH solution will be supplied for the experiment. Fill a
clean plastic 1 L bottle with approximately 500 mL of the NaOH solution.
to never store NaOH in a glass stoppered bottle. Save
this solution for the next laboratory. The general standardization procedure
involves the titration of known, solid, primary standard potassium hydrogen
phthalate, KHP. The reaction involved is
You will need 3 resin columns packed with Dowex 50W-X8, a strongly acidic cation exchange resin with 8% divinylbenzene cross-linking (X8). This resin has a total cation exchange capacity of 1.77 meq/mL, wet volume, and 4.83 meq/mL, dry volume. These figures indicate that there is considerable swelling of the resin in solution, a characteristic of most ion-exchange media. The resin size to be used is 100 mesh which will insure an adequate flow rate for equilibration. If your column is already packed with the correct resin, omit the next step.
If you need new resin, add some water to your column, shake, and pour the resin-slurry into a beaker. When your column is empty, clamp your ion-exchange columns to a column support and place waste beakers underneath the columns. Add some distilled water with the column cap on. Make a slurry of the new resin in distilled water and pour into the column until the resin settles into a layer that just reaches the neck bottom of the top reservoir. Take off the cap to help drain the column.
Clamp your ion-exchange columns to a buret support and place waste beakers underneath the columns. Try to keep solution over the resin at all times to prevent "channeling" in the column, a process that creates paths for the solution to pass through the column without equilibrating with the resin phase. Also note that there will be a considerable "shrinkage" of the resin in the concentrated HCl solution.
Preparation of the Unknown
You will be given an unknown salt solution and its approximate equivalent weight. Weigh out enough sample to be dissolved in a 100 mL volumetric flask so that a 10.00 mL aliquot (a 1/10 portion of the total sample) contains approximately 3.5 milliequivalents of salt so that approximately 35 mL of 0.10 M NaOH would be required for titration of each aliquot.
The Ion Exchange Process:
The cation exchange resin is converted to the H+Res- form by washing with 3 M HCl. Prepare the 3 M HCl solution by adding 50 mL of concentrated 6 M HCl to 50 mL of distilled water in a beaker. For maximum efficiency, run several columns at once.
If your equivalent weight is not within 10 g/mole of the value given to you by the instructor consider the following sources of error. Assuming that the correct size sample was weighed out and the 10 mL aliquot was applied to the column, if your equivalent weight is too large, then the volume of titrant used was too small. This can occur if the sample aliquot does not adequately equilibrate in the column. If your equivalent weight is too small, then too many mL of titrant were used. The most likely source of this error is contamination of the sample by HCl fumes or glassware that was in contact with the 3 M HCl. If either of these errors occur, run two more samples using precautions to prevent a repetition of the error.
Another problem encountered in quantitative ion-exchange is ensuring that your sample does not exceed the exchange capacity of your column. This can be quickly checked by pipetting a 5 mL aliquot sample on your column and following the procedure outlined above. This sample should require exactly one-half the mL of NaOH that the 10 mL samples take. If this is not observed, see the instructor.
Report the equivalent weight of your unknown. The range on unknown values should be between ±10 gm/mole from the estimate given by the instructor.
SAVE YOUR STANDARDIZED NaOH FOR THE NEXT EXPERIMENT
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