Field Methods of Monitoring Aquatic Systems Unit 6 - Hardness Copyright © 2006 by DBS

Major Cations Ca 2+ Mg 2+ Fe 2+ K+K+ Na + Common cations of high enough concentration to be readily monitored are good indicators of pollution events

Hardness Index Hard water contains high concentrations of dissolved calcium and magnesium ions Soft water contains few of these dissolved ions. Hardness = [Ca 2+ ] + [Mg 2+ ] (also Al 3+, Fe 3+, Mn 2+ and Zn 2+ ) Carbonate minerals: limestone - CaCO 3 dolomite - CaCO 3.MgCO 3 sulfates - CaSO 4 Counter ions of alkalinity ions Alkalinity is a good indicator of hardness and vice-versa

Effects Deposition of white solid CaCO 3 or MgCO 3 when water is heated –‘furring-up blocks pipes and lowers efficiency of industrial processes Formation of scum (insoluble ppt) with soap and water Ca 2+ (aq) + H 3 C-(CH 2 ) 10 -COO - (aq) [H 3 C-(CH 2 ) 10 -COO-]2Ca 2+ (s) –detergent action is blocked Staining (due to transition metals) A pipe with hard-water scale build up

Types of Hardness Solid deposit = carbonate hardness or temporary hardness CaCO 3 (s) + H 2 CO 3 ⇌ Ca 2+ + HCO 3 - (removed via boiling) –Causes deposit in pipes and scales in boilers –Temporary hard water has to be softened before it enters the boiler, hot-water tank, or a cooling system No solid = non-carbonate or permanent hardness –Amount of metal ions that can not be removed by boiling Total hardness = temporary hardness + permanent hardness

Beneficial Effects Alkalinity lowers solubility of toxic metals Buffering action of CO 3 2- lessens effects of acidic pollutants Non-toxic essential nutrients Reduces heart disease More pleasant to drink

Analysis Complexometric titration for Ca 2+ and Mg 2+ using ethylenediaminetetraaceticacid EDTA M 2+ + H 2 EDTA 2- ⇌ M(EDTA) H + Buffer to pH 10, indicator is Eriochrome Black T Expressed as mg L -1 (even though not all CaCO 3 ) Conc. CaCO 3 (mg L -1 ) Description Soft Moderately soft Slightly hard Moderately hard Hard > 300Very hard

EDTA Complex 1:1

Total Hardness Titration estimates total divalent (+2) metal as molar concentration Convert to mg L -1 Need precise individual concentrations of Ca, Mg etc. which we don’t have Solution: express as all CaCO 3 e.g. for a 5.00 mL sample titrated with mL of M EDTA C 1 V 1 = C 2 V mL x 1 L x mol L -1 = 5.00 mL x 1 L x C mL 1000 mL C 2 = M M (Ca 2+ + Mg 2+ ) x g CaCO 3 = g L -1 = 824 mg L -1 1 mol (Ca 2+ + Mg 2+ )

Question Which of the following solutions give 50 mg L -1 total hardness? (a) 50 mg L -1 MgCO 3 (b) 21.1 mg L -1 MgCO mg L -1 CaCO 3 (c) 50 mg L -1 CaSO 4 (d) 55 mg L -1 CaCl 2 (a) 0.59 mM, x 100 g mol -1 = 59 mg L -1 CaCO 3 (b) 0.50 mM, x 100 g mol -1 = 50 mg L -1 CaCO 3 (c) 0.37 mM, x 100 g mol -1 = 37 mg L -1 CaCO 3 (d) 0.5 mM, x 100 g mol -1 = 50 mg L -1 CaCO g mol g mol g mol g mol -1

Question Two unpolluted waters have the same pH valu of 7.8 are contaminated by approximately the same amount of acidic pollutant. The pH of one drops sharply, while there is only a small drop in the second. Suggest a reason for the difference and the analysis which could be performed to confirm your suggestion. pH values suggest hardness, perhaps from the presence of limestone. Determine via EDTA titration. Solution having the smaller pH change indicates a greater buffer capacity and so would be expected to have the greatest hardness.

Text Books Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil. Wiley-VCH. Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker. Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods. CRC Press. Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis Publishers. Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books. Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative methods. Taylor & Francis. Reeve, R.N. (2002) Introduction to environmental analysis. Wiley. Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the Examination of Water and Wastewater, 20th Edition. Published by American Public Health Association, American Water Works Association and Water Environment Federation.