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Complexometric Determination of Water

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Complexometric Determination of Water



In this experiment, the standardization of EDTA will be completed and the calcium in water will be determined by titration. From a calcium concentration solution, the hardness of the water of an unknown sample will be determined. An indicator, Eriochrom Black T will be used to capture the change from pink to blue hue. The average mollify found was 0.0029 M. The average calcium concentration for the unknown water sample was 156 ppm.


Water hardness is defined as the concentration of Calcium and Magnesium cations present in water.  In this lab, chelation will be used in order to detect water hardness in a standardized ethylenediaminetetraacetic acid (EDTA) solution and assigned an unknown water sample code #3. The process of chelation is one in which polyatomic ligands form a complex with a metal ion.  

        As such, water hardness can be determined by performing a complexometric titration using a standard ethylenediaminetetraacetic acid (EDTA) solution.  EDTA complexes with calcium and magnesium in a one to one molar ratio because they come one complex ion.  The endpoint in this experiment will be determined using an indicator.  This indicator makes the solution red while there are calcium and magnesium ions that have not complexed with EDTA; when the endpoint has been reached, the indicator will turn blue.[1]


To begin, a disodium EDTA solution was prepared by weighing out .7677 g of Na2EDTA and adding it to 500 mL of deionized (DI)  water  in a plastic bottle.

In a 250 mL Erlenmeyer flask, 10 mL of standardized calcium ion stock solution was added; then 30 mL of DI water. This flask was then taken to a fume hood where 3 mL of ammonia buffer pH10 was added; the buffer is an inhalation irritant, hence the precautionary measures. On a magnetic stirrer, the flask was placed with a magnetic stir bar so that the solution could still for about 30 seconds.  Prior to the titration, about 6 drops of Eriochrome Black T indication solution was added in order to detect the color change when it occurred. The volume of the starting EDTA solution was recorded; then, towards the endpoint of the titration, when the solution turned from pink to blue, the volume delivered was recorded. This was repeated two more times.

        A similar procedure was done with the unknown water sample code #3.  25 mL of the prepared water sample was transferred to a 250 mL Erlenmeyer flask, 20 mL of DI water.  The magnetic stir bar was added an placed on the stirrer for about 30 seconds.   Then, inside of the fume hood, 3 mL of ammonia buffer pH 10 was added and stirred for 30 seconds. 6 drops of Eriochrome Black T were added as indicator for this solution.  This process was repeated two more times.


Data 1

Standardized EDTA solution concentration: 0.004 M

Actual Mass of Na2EDTA: 0.7677g        Na2EDTA Mean: 2.09x10^-3 M

Estimated Precision: 31.9 ppt

Calcium Ion stock

Trial 1

Trial 2

Trial 3

Initial buret reading

0 ml

0 ml

0 ml

Final buret reading

 48.3 ml

 49.5 ml

45.5 ml

Volume delivered

48.3 ml

49.5 ml

45.5 ml


2.07x10^-3 M

2.02x10^-3 M

2.2x10^-3 M


Volume of Titrations

Final Volume-Initial Volume= Volume used to titrate

48.3-0= 48.3 mL   *Repeat for trial 2 and 3


(10 ml CaCO3/48 mL Na2EDTA Solution) x (1 g CaCO3/ 1L CaCO3 sln) x (1 mol CaCO3/100.1 g CaCo3) x (1 mol Na2EDTA/1 Mol CaCO3)= 2.07 x 10^-3M



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