Water is said to be hard when it contains dissolved minerals especially Ca2+, Mg2+, Fe3+, SO42-, and HCO3-. Hard water can be formed when underground water percolates through limestone (CaCO3) or dolomites (CaMg(CO3)2, or when water dissolves calcium sulfate or magnesium sulfate. The concentration of Ca2+, (and Mg2+) generally exceeds that of any other metal ions, consequently, hardness is expressed in terms of CaCO3.
The determination of water hardness is a useful test that provides a measure of the quality of water for household and industrial uses. Water that contains the above ions has no obvious health implications, but these ions form insoluble compounds with soap. This precipitation interferes with the cleaning action of soap and results in the unsightly "scum" on clothing and the so-called "bath-tub ring". Water hardness is important to industry because hard water, upon being heated, precipitates CaCO3, which then clogs boilers and pipes leading to equipment malfunction or damage and expensive cleaning.
Elimination of water hardness is referred to as "water softening". Hardness due to bicarbonate (HCO3-) can be eliminated by boiling, to expel CO2, as indicated by the following equation:
Bicarbonate hardness is classified as temporary hardness. Hardness arising due to the presence of Ca2+, Mg2+, Fe3+, and SO42-, cannot be eliminated in the same way as temporary hardness. Water containing Ca2+, Mg2+, Fe3+, and SO42-, ions is said to be permanently hard.
Permanent hardness is ordinarily determined by titration with a standard solution of ethylenediamminetetraacetic acid, EDTA. This is called complexometric titration. EDTA is a multidentate (has more than one point of attachment) ligand and complexes most metal ions in aqueous solution. The structure of EDTA is as follows:
EDTA is sometimes abbreviated H4Y, to emphasize the fact that it has four ionizable hydrogen atoms. EDTA is available as the disodium salt, Na2H2Y * 2H2O.
In this experiment, we will determine the ‘total' permanent hardness. Experimental errors in the determined concentrations of about one percent is acceptable due to the "fuzzier" end-points with the metal ion indicators rather than the sudden color changes noted in the acid-base titration experiment. The best advise is to repeat the metal ion determinations as many times as necessary to get a feel for the behavior of the indicators. Eriochrome Black-T (Erio-T) indicator will be used. This is a molecule that has a color change from pink, corresponding to Ca -calcon complex, to blue corresponding to uncomplexed indicator.
Obtain a sample of cold tap water and pipet 25.00 mL into an Erlenmeyer flask. Add 2 mL of pH 10 buffer, 25 ml of distilled water and 4 drops of Erio-T indicator. Heat the solution to about 60 o C to ensure all the metal ions in the water are dissolved ( in solution). Do not heat the solution above 60 o C ; if you do, discard the solution and start over. Titrate at this temperature with the standardized 0.01 M solution of EDTA until a purple color is obtained. Swirl the flask for 20 seconds, and then continue titrating, adding EDTA dropwise with constant swirling until a pure blue color with no tinge of purple is obtained. Repeat titrations until at least two consistent results are obtained (2 results within 0.2 mL).
Using the fact that one mole of EDTA reacts with one mole of cation, calculate the total permanent hardness by substituting in the equations below.
Assume that total hardness is due to the presence of only Ca2+ in the form of CaCO3. The concentration can be calculated and expressed in terms of mg CaCO3 per liter of water solution ( = ppm CaCO3)
(Note: 103 entered on the calculator is 1 EE 3 NOT 10 EE 3)
8/98 EO, 1/2000 DS
1. Why should we be concerned about water hardness?
2. Explain the difference between temporary and permanent water hardness.
3. A student titrates 25.00 mL of tap water using 4.86 mL of 0.02 M EDTA. Calculate the hardness of the water in terms of ppm CaCO3.
|Titration||Intial Buret Reading, mL||Final Buret Reading, mL||Volume used, mL||Mol Ca2+/L||ppm CaCO3|
Send comments to:
last updated: January 7, 2000