FINAL REPORT

and 5ml into a 25ml volumetric flask and making up to the mark with water. Analyze in the
same way as samples. Prepare sample blanks by adding all the reagents except barium
chloride to 25ml of sample and measure the absorbance. Substract from each sample reading
the blank reading obtained using the same sample to compensate for sample color and
turbidity. Prepare a calibration graph of absorbance against mg SO 2- 4 . Read off the amount of
sulfate in the samples using the corrected absorbance reading and calculate the concentration
in the sample as:
Mg SO 2- 4 /l= 1000*mgSO 2- 4 /V
Where V is the volume of the sample (ml)
The volume of sample is 10ml in the above procedure. If the sample contains more sulfate
than the highest calibration standards, dilute the sample to fit on the curve and correct for the
dilution when calculating the result.
9. Determination of content Ca + Mg and Ca
Methodology
A method commonly used for hardness determination is the direct complexation titration with
ethylenediaminetetraacetic acid (EDTA).
EDTA forms 1:1 complexes with divalent metals such as calcium:
Ca 2+ +EDTA→{Ca⋅EDTA} complex
Erochrome Black T can be sued as indicators. If a small quantity of indicator is added to water
simple containing Ca and Mg ions at pH 10, the solution becomes wine red. The indicator
forms complexes with Ca and Mg ions which give the solution a wine-red color:
Ca 2+ +Eriochrome Black T→{Ca⋅Eriochrome Black T} complex
wine-red
As EDTA is added it displaces the cations from the cation-indicator complex by forming more
stable complexes with the cations. When all of the Ca and Mg is complexed EDTA, (at the
end point), the solution turns from wine red to blue due to the free Eriochrome Black T
indicator. In order to obtain a sharp end point a small amount of magnesium ions must be
present. This is generally not a problem with natural water samples which tend to contain
some Mg, but it is a problem when standardizing EDTA solutionswith pure CaCO 3 .
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