Before attempting this experiment, you may need to consult the section in SWHC dealing with complexometric titrations. In this experiment a stock solution of EDTA will be standardized against primary standard CaCO3. This standardized EDTA solution is then used to determine water "hardness".
Standardization of EDTA
Prepare 0.01 M EDTA solution in ~ 0.16 M Sodium Glycine Buffer: Add approximately 0.88 g of dry disodium EDTA salt and ~ 4g Sodium Glycine to about 200 mL of deionized (DI) water in a 400 mL beaker and swirl periodically. (Water used in the preparation of standard EDTA solutions must be totally free of polyvalent cations.) Dissolution may take 15 min. or longer. Test the pH of the solution using a calibrated pH meter (see Fisher Accumet AB15 pH meter manual). Ideally, pH should be above 10.3.
When all of the solid has dissolved, transfer to a 250mL volumetric flask. Dilute to the mark with water and mix well. In calculating the expected molarity of the solution, correct the mass of the salt for the 0.3% moisture it ordinarily retains after drying.
Dry primary standard calcium carbonate for two hours at 110 ºC. (Note 1) Weigh out (to the nearest ± 0.1 mg) about 0.3 grams of dry primary standard calcium carbonate into a 250-mL Erlenmeyer flask. Add 25 mL of calcium free water, then slowly add 6 M HCl with a dropper until all of the calcium carbonate dissolves. (Note 2a) Be careful to avoid loss of solution due to rapid foaming or splattering as CO2 is evolved. Make certain dissolution is complete. Undissolved solid may stick to the upper wall of the flask and very small particles are difficult to see. Warm the solution gently. (Note 2b) Cool and transfer quantitatively to a calibrated 250-mL volumetric flask. Fill to the mark with water. Calculate the molar concentration of calcium.
Pipet 25.00 mL aliquots of the standard calcium solution into three 250-mL Erlenmeyer flasks. Right before titrating, add 1.00 mL of 0.001 M magnesium solution and one small scoop (Note 3 and Note 4) of Calmagite indicator.
Titrate with EDTA until the
color changes to a clear blue. There should be no reddish or purple tint at the
end point. (Note 5)
In the above titrations, magnesium was added in order to obtain a sharper color change at the end point. However, EDTA reacts with magnesium in exactly the same way as it does with calcium. It is therefore necessary to subtract the volume of EDTA required to titrate the added magnesium. This is called an indicator blank. Repeat the titration with EDTA exactly as above, however substitute 25 mL of DI water for the standard calcium solution. Repeat the blank determination two or three times until you are confident of its value. Subtract the mean of the blank volume measurements from the titrations in which calcium was present.
Calculate the molarity of the EDTA solution.
Determination of Water Hardness
Measure out 100-mL aliquots of the hard water sample into three 250-mL Erlenmeyer flasks. To each sample, add the magnesium solution, and the Calmagite or Eriochrome Black T indicator as above. Titrate with the standardized EDTA solution. Correct the volume of titrant by subtracting the indicator blank. Calculate the molar concentration of calcium in the hard water sample.
It is common practice to report water hardness in terms of the number of milligrams of CaCO3 per liter. That is, regardless of the form of the calcium (or whether it even was calcium, since magnesium would be indistinguishable) we calculate the result as if all of the material that reacted with EDTA was calcium carbonate. This is a common analytical practice. For example, you will see the phosphates in fertilizer reported as phosphoric acid. (Note 7)
You must present the results for the standardization of the EDTA and for the calcium content (hardness as mg CaCO3 per liter) of the water to your lab TA before leaving. As usual, the results include the mean and relative standard deviation of your replicate determinations of the concentrations of the solutions.
1) Standard operating procedure is to dry for one hour at 110 C. However, in a lab with many students opening and closing the oven the temperature may drop.
2a) When adding HCl to the CaCO3, add a few drops and then swirl the solution. Ideally, you should add a minimal amount of HCl to dissolve the CaCO3. If you add too much HCl, then the glycine buffer that you created for the EDTA will not work properly – i.e. we have to keep the to keep the pH at or slightly above 10. This is important for two reasons: (a) all reactions between metal ions and EDTA are pH dependent, and for divalent ions, solutions must be kept basic (and buffered) for the reaction to go to completion; (b) the eriochrome black T indicator requires a pH of 8 to 10 for the desired color change.
2b) "Warm" means just that. Not hot, and certainly not boiling! The acidification and heating step is to remove carbonates, which if present, will precipitate CaCO3 when the solution is made basic. The precipitate obscures the end point.
3) Always use magnesium solution from the same bottle. EDTA also reacts with the added magnesium and a blank correction will be applied.
4) The Calmagite indicator is a granular solid composed of one part indicator and 300 parts sodium chloride. The sodium chloride is an inert filler. The size of a small scoop should be kept consistent throughout. You can pour a small amount on to a watch glass and then use your lab scoop to deliver the indicator to the flask.
5) Titrations must be performed swiftly (but carefully) because ammonia will evaporate and thus the pH of the solution will change. In general, the faster the titrations are performed the better the results will be (as long as the endpoint is not overshot due to the speed). It is advantageous to perform a trial titration to locate the approximate endpoint and to observe the color change. In succeeding titrations, titrate very rapidly to within about 1 or 2 mL of the endpoint, then titrate very carefully (dropwise) to the endpoint. If the color change is not sharp, check the pH with a narrow range test paper or probe to make sure it is close to 10.
6) Eriochrome Black T exists as a wine-red complex when Mg2+ is present in solution at pH =10. When the EDTA has chelated all the Mg2+ present in solution, the indicator (free and uncomplexed to Mg2+) will be robin’s egg blue. This color change marks the endpoint. Add only very small quantities of the indicator are needed. Do your best to keep the intensity of the indicator color relatively weak and consistent from sample-to-sample. Sometimes the Eriochrome black T solution goes bad because of air oxidation. If the endpoints seem very indistinct to you, try a fresh bottle of indicator. Alternatively, try adding a small amount of solid Eriochrome black T mixture (1 g indicator ground with 100 g NaCl); a small amount on the end of a spatula is sufficient.
7) When completely done with the experiment mix any remaining EDTA titrant and any remaining Ca stock solution together in a large beaker. Pour down the drain with copious amounts of cold tap water. The two solutions are slightly basic and slightly acidic, respectively; when mixed, they will be near neutral. There are also no toxic chemicals present, so disposal directly down the drain is allowable and safe.