CVEG 3240 L
Hardness and Alkalinity Lab
Introduction
Hardness is an important water quality indicator, formally defined at that sum of soluble divalent metal cations in solution. By far, the
most common divalent cation in solution is calcium ion, Ca++, with magnesium ion, Mg++, the second most common. Other cations are
usually negligible, so we have calcium hardness and magnesium hardness. Hardness is commonly expressed in units of mg/L as
CaCO3, i.e., the concentration of CaCO3 with the same number of equivalents as the hardness.
Alkalinity is the capacity to neutralize acids. The primary source of alkalinity is the bicarbonate ion, HCO 3-. Alkalinity is also
typically expressed as mg/L as CaCO3. The hardness that has alkalinity to “match up” with it is the carbonate hardness. The
remainder is the noncarbonate hardness. If the alkalinity is greater than the total hardness, then all the hardness is carbonate hardness
and there is no noncarbonate hardness.
Total hardness = calcium hardness + magnesium hardness
Total hardness = carbonate hardness + noncarbonate hardness
Carbonate hardness = alkalinity
noncarbonate hardness = total – alkalinity (if alkalinity < total hardness)
(if alk > total hardness, carbonate hardness = total hardness, noncarbonate hardness = 0)
In this laboratory exercise, we will measure total hardness, calcium hardness, and alkalinity. From the results we will calculate
magnesium hardness and carbonate and noncarbonate hardness.
Procedure
A. Total hardness
1) Place 50 mL of sample water in beaker.
2) Add 2 mL of buffer solution.
3) Add a few grains of Eriochrome Black T indicator. This will turn the solution purple.
4) Titrate the solution with 0.01 M EDTA (ethylenediaminetetracetic acid).
5) Record the mL of EDTA titrant that changes the purple solution to blue.
B. Calcium hardness
1) Place 50 mL of sample water in beaker.
2) Add 5 mL of Sodium Hydroxide (NaOH, “caustic soda”) solution to sample.
3) Add a few grains of Murexide indicator. This will turn the solution pink.
4) Titrate with EDTA until the solution turns purple. Record the mL of EDTA used.
C. Alkalinity
1) Add 50 mL of sample to beaker.
2) Add Methyl Orange to sample. This will turn the sample yellow.
3) Titrate sample with 0.02 N sulfuric acid (H2SO4) until the sample turns orange. Record the mL of H2SO4 titrant used.
D. Calculations
1) total hardness as mg/L of CaCO3 = A*B/(L of sample)
A = mL of EDTA titrant from step A.
B = mg of CaCO3 per mL of EDTA (convert 0.01 M EDTA to mg/mL as CaCO 3) (EDTA has 2 eq/mole)
= MEDTA*(EDTA eq per mole)*(eq wt of CaCO3)
= NEDTA*(eq wt of CaCO3)
note: that’s L of sample (=mL/1000)
2) calcium hardness as mg/L of CaCO3 = A*B/(L of sample)
A and B like total hardness calculation but use data from step B calcium hardness.
3) Alkalinity as mg/L of CaCO3 = A*N*B/(L of sample)
A = mL of acid titrant used in step C alkalinity.
N = normality of acid used
B = eq wt of CaCO3 = (mol wt of CaCO3)/(eq/mole) = mg/eq
note: this is really just like the hardness calculations
4) from the results, calculate
magnesium hardness
carbonate and noncarbonate hardness
E. Report: total, calcium, magnesium, carbonate, and noncarbonate hardness. Alkalinity. Report as CaCO3 and as meq/L. Draw a
bar-graph.