The equation :(MEBT)- + NaH2EDTA → (MEDTA)-2 + 2 HEBT2-+ H+ + 2Na+
red-wine blue
Consequently, the end-point changes from red-wine colour to blue colour.
Objective
To determine the total hardness in a sample of water by the EDTA titrimetric method
Procedure
Please refer to the lab manual
Result
Standardization of EDTA Solution
Average volume of EDTA used = (38.70+39.40+39.10) / 3 = 39.07ml
Determination of hardness of a tap water sample
(100-ml was used)
As the sample size was too small, 150-ml of tap water sample was used as following.
(150-ml was used)
Average volume of EDTA used = (10.7+10.3+10.2) / 3 = 10.40ml
Determination of hardness of an unknown water sample
(25-ml was used)
Average volume of EDTA used = (19.5+19.5+19.9) / 3 = 19.63ml
Data treatment and Questions:
Standardization of EDTA Solution
Ca2+(aq) + EDTA4-(aq) → [CaEDTA]2-(aq)
Molarity of standard calcium chloride = 0.008 M
Volume of standard calcium chloride used = 25 ml
No. of moles of standard calcium chloride in 25 ml = 0.008 X 0.025
= 2 x 10-4 mole
From the equation above,
mole ratio of EDTA: calcium chloride = 1: 1
No. of moles of EDTA used = 2 x 10-4 moles
Volume of EDTA solution used= 39.07/ 1000 = 0.03907 dm3
The molarity of EDTA solution = 2 x 10-4 / 0.03907 dm3 =
Determination of hardness of a tap water sample:
Molarity of EDTA solution = 0.00512M
Volume of EDTA solution used= 10.40ml
= 10.40/1000 dm3
= 0.0104 dm3
No. of mole of EDTA used = 0.00512M x 0.0104
= 0.0000532 mole
From the equation, mole ratio of EDTA: M2+ = 1:1
The no. of mole of M2+ ions in 150ml of a tap water sample = 0.0000532 mole
The molar mass of EDTA (i.e. CaCO3) = 40 + (16x3) + 12 = 100
The hardness of tap water sample = (0.0000532 x 100 x 1000)/ 0.15
= 35.499 mg/L
Determination of hardness of an unknown water sample
Molarity of EDTA solution = 0.00512M
Volume of EDTA solution used= 19.93ml
= 19.93/1000 dm3
= 0.01993 dm3
No. of mole of EDTA used = 0.00512M x 0.01993
= 0.000102 mole
From the equation, mole ratio of EDTA: M2+ = 1:1
The no. of mole of M2+ ions in 25ml of a unknown sample = 0.000102 mole
The molar mass of EDTA (i.e. CaCO3) = 40 + (16x3) + 12 = 100
The hardness of tap water sample = (0.000102 x 100 x 1000)/ 0.025
= 40.8mg/L
Discussion
We have to determine the molarity of EDTA solution indirectly by titrating it against calcium chloride solution, but not dissolving a known amount of EDTA into water instead. It is because EDTA solution made by the latter method is not stable, it is hygroscopic hence it cannot be a primary standard solution. We have to standardize it by calcium chloride solution.
On the other hand, calcium chloride solution is a bit unstable indeed, hence the way to produce calcium chloride solution would be from calcium carbonate, which is the “real” primary standard as it is stable enough, in order to produce calcium chloride by reaction.
At the second titration, which is EDTA being titrated over tap water, the titrant volume of EDTA is recommended to over 10ml at least. The larger the volume used, the more accurate the result. As error would be minimized due to the following equation:
Use the burette with a narrower meniscus to decrease the observation error.
The larger the sample size, the smaller the relative error.
Precaution:
- While adding indicator, do not put too much amount of indicator into the conical flask, otherwise the too deep colour affects the judgement of end-points of the titration.
