Determination of Calcium-Magnesium by EDTA Titration
The purpose of this experiment is four-fold:
o improve proficiency with quantitative titrimetric determinations.
o introduce complex-formation titrations for determinations of metal ions using a widely applicable chelating agent, EDTA. The skills learned with this experiment will be applied later (sections D.2, D.3) for certification of laboratory metal reagents.
o provide an example of determining each of two analytes in a mixture, implementing a physical separation of analytes by selective precipitation.
o provide one (of three) proficiency test(s) designed to certify each student as qualified to perform instrument and reagent validation operations required by Good Laboratory Practice guidelines, and to conduct more complex analytical studies.
Synopsis of the Analytical Problem
The analytical study conducted here involves the determination of both the "total hardness" and the relative concentrations of Mg and Ca in a brine sample containing only salts of these two ions. This is accomplished by a titration method using the chelating agent, EDTA (ethylenediamine tetraacetic acid). First, the titration is conducted under conditions where both ions react with the EDTA, and the end point is detected after both are titrated. Then, one of the ions (Mg) is precipitated as Mg(OH)2 by increasing the pH. A subsequent titration of the solution determines only the remaining Ca ion, which remains soluble. The Mg content can be determined by difference.
Significance of the Ca-Mg Determination
"Hard" water typically contains high concentrations of Ca, Mg (and other) cations, which interfere with the use of the water for many applications. (For example, these ions diminish the effectiveness of soaps and detergents for cleansing operations; they diminish the drinking quality of water; and they contribute to the accumulation of insoluble salt deposits in storage vessels or plumbing.) The determinations of levels of these ions in untreated (natural) water are very common procedures in commercial laboratories. For some purposes it is only necessary to know the "total hardness"; this is related to the total amount of Mg and Ca in the water, and is usually expressed in terms of "ppm CaCO3". (That is, the total moles/liter of Ca and Mg is converted into the equivalent weight of CaCO3, in ppm units.) Sometimes, however, it is desired to know the relative amounts of each ion, Ca and Mg, as one may be a more serious interference in the planned application for the "hard" water.
Preparation of titrant. See Appendix 2 of Lab Manual to prepare 500 mL of about 0.01M EDTA solution (modify the Appendix procedure specified for one liter of 0.01M EDTA). Do not store EDTA in glass stoppered flasks. (The solution tends to "freeze" stoppers.)
nknown sample. Submit a clean 500 mL volumetric flask to receive the unknown. Dilute to the mark with distilled water, and mix thoroughly.
Procedure for "Hardness in Water". Acidify 50 mL aliquots of unknown with a few drops of dil HCl, and boil gently for a few minutes to expel carbon dioxide. Cool, add two drops of methyl red and 0.3M NaOH dropwise until the indicator changes color (see NOTE 2). Introduce 5 mL of special pH 10 buffer, 2 drops of eriochrome black T indicator (very viscous), and titrate with standard EDTA until the color changes from red to pure blue (see NOTE 3). Calculate total hardness, expressed as ppm CaCO3.
(1) The acidification and boiling step is to remove carbonates, which if present, will precipitate CaCO3 when the solution is made basic. The precipitate obscures the end point.
(2) The excess HCl must be neutralized carefully. If too much NaOH is added, Mg(OH)2 precipitates, giving a poor end point.
(3) As the end point is approached, the color goes from purple to blue. When you think it is blue, read the buret, then add another drop of titrant; if there is a perceptible color change, again read buret and add another drop. Continue until a drop of titrant gives no color change, then record the preceding buret reading. If CaCO3 or Mg(OH)2 is present, the purple color returns; add more titrant until the blue color persists for at least one minute.
(4) Trace amounts of many metals, such as Cu or Cd, react with the indicator and interfere seriously with the end point. If these metals are present, they can be masked with a few drops of 0.1M KCN. When KCN is used, review the hazards and the requirement for disposal.
(5) It is necessary to keep the pH at about 10 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.
Procedure for Calcium Only. If the pH of the sample solution is raised to about 13, magnesium will precipitate as the very insoluble hydroxide which will not react with EDTA, and calcium alone can be titrated using calcon indicator. Since the total hardness titration gives the sum of both Ca and Mg in the sample, and titration at pH 13 gives Ca only, Mg can be obtained by difference.
Perform as follows a preliminary titration to find the approximate volume of EDTA solution needed to react with the calcium. To 50 mL of the sample solution, add 3-4 drops of polyvinyl alcohol, 20 drops of 50% NaOH, and heat to 60-70o. Add 2 shots of hydroxy naphthol powder indicator (caution) and titrate with EDTA to a clear blue. Titrate rapidly without trying to get an exact end point.
To another aliquot of the sample solution, add 3-4 drops of polyvinyl alcohol, 2 shots of hydroxy naphthol powder indicator, and accurately add (carefully measure) ~95% of the volume of EDTA used in the preliminary titration. (See NOTE 1.) Heat to 60-70o, add 20 drops of 50% NaOH, mix, and continue the titration to a clear blue end point. Repeat with two additional portions of sample.
(1) The precipitated magnesium hydroxide obscures the end point somewhat. The precipitate also adsorbs and coprecipitates calcium ions, and frequently causes the calcium results to be low. By adding 95% of the volume of EDTA obtained in the preliminary titration, the calcium ion concentration is reduced to a very low value before precipitating magnesium hydroxide and thus little is adsorbed or coprecipitated. Also polyvinyl alcohol and heating to 70o reduces the adsorption of calcium.
(2) Eriochrome Black T indicator does not give a satisfactory color change at high pH. Hydroxy naphthol indicator is similar to Eriochrome Black T, and functions in the same manner, but at high pH.
For the brine sample, report:
(1) Total hardness, expressed as ppm CaCO3
(2) Calcium, expressed as weight/volume percent
(3) Magnesium, expressed as weight/volume percent
While the EDTA solution is freshly prepared, it is recommended that the student perform one of the reagent certification procedures of the Series D lab studies. (Select one from Series D.2; procedures at end of Lab Manual).
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