Determination of [Ca2+] and [Mg2+] in
Natural Waters Using a
Compleximetric Titration
GW Bush
Chem 1212L-124
January 29, 2040
______________________(new
page below)
Procedure:
The procedure for the experiment is from the lab manual referenced below. There were no significant deviations from that procedure.
Block/McKelvy/Denniston/Silverstein (2006). Laboratory Experiments for Chem 1211L and 1212L. Thomson Custom, p 225-26 (Determination of Ca2+ and Mg2+ in Natural Waters Using a Compleximetric Titration).
Data:
Mass of EDTA: 0.3040 g.
|
Standardization of
EDTA sol’n |
Trial #1 |
Trial #2 |
Trial #3 |
|
Final buret reading, EDTA |
33.91ml |
33.87ml |
34.03ml |
|
Initial buret reading, EDTA |
0.01ml |
0.09ml |
0.02ml |
|
Volume used, EDTA |
33.90ml |
33.79ml |
34.01ml |
|
|
|
|
|
|
|
|
|
|
|
[Mg2+] in standard sol’n |
|
0.010M |
|
|
Determination of
[Ca2+
+ Mg 2+] |
Trial #1 |
Trial #2 |
Trial #3 |
|
Volume of seawater |
1.00ml |
1.00ml |
1.00ml |
|
Final buret reading, EDTA |
20.94ml |
18.50ml |
22.59ml |
|
Initial buret reading, EDTA |
0.10ml |
0.23ml |
0.12ml |
|
Volume used, EDTA |
20.84ml |
18.27ml |
22.47ml |
|
Measurement of water blank |
|
|
|
|
Final buret reading, EDTA |
|
0.64ml |
|
|
Initial buret reading, EDTA |
|
0.22ml |
|
|
|
|
|
|
|
|
|
|
|
|
Determination of
[Ca2+] |
Trial #1 |
Trial #2 |
Trial #3 |
|
Volume of seawater |
1.00ml |
1.00ml |
1.00ml |
|
Final buret reading, EDTA |
4.10ml |
8.09ml |
11.81ml |
|
Initial buret reading, EDTA |
0.64ml |
4.49ml |
8.09ml |
|
Volume used, EDTA |
3.46ml |
3.60ml |
3.72ml |
|
Measurement of
water blank |
|
|
|
|
Final buret reading, EDTA |
|
4.49ml |
|
|
Initial buret reading, EDTA |
|
4.10ml |
|
|
|
|
|
|
|
|
|
|
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Sample Calculations:
Expected molarity:
M = [(0.3040g EDTA) / (372gNa2EDTA/mol)]/0.250L sol’n=3.27*10-3M
Molarity of EDTA solution:
M = (0.010L Mg2+) * (0.010M Mg2+) = 1.0*10^-4 mol Mg2+ = 1.0*10-4 mol EDTA
1.0*10-4 mol EDTA/0.0339L EDTA = 2.95*10-3M EDTA
Determination of [Mg2+
+ Ca2+]:
20.84ml EDTA – 0.42ml blank = 20.42ml EDTA = 0.02042L EDTA
(0.02042L EDTA) * (2.95*10-3M EDTA) = 6.02*10-5 mol EDTA = 6.02*10-5 mol metal ions in seawater
(6.02*10-5 mol Mg2+ + Ca2+) / (0.001L seawater) =6.02*10-2M (Mg2+ + Ca2+)
Determination of [Ca2+]:
3.46ml EDTA – 0.39ml blank = 3.07ml EDTA = 0.00307L EDTA
(0.00307L EDTA) * (2.95*10-3M EDTA) = 9.06*10-6 mol EDTA = 9.06*10-6 mol Ca2+
(9.06*10-6 mol Ca2+) / (0.001L seawater) = 9.06*10-3M Ca2+
Determination of [Mg2+]:
(5.93*10-2M (Mg2+ + Ca2+)) – (9.70*10-3M Ca2+) = 4.96*10-2M Mg2+
Density of seawater:
(22.4781g pycnometer+seawater) – (12.2677g pycnometer) = 10.2104g seawater
(10.2104g seawater) / (10.0ml seawater) = 1.02 g/ml seawater
ppm Ca2+:
(9.70*10-3M Ca2+) * (0.001L seawater) = 9.70*10-6 mol Ca2+
(9.70*10-6 mol Ca2+) * (40.078 g/mol Ca2+) = 3.89*10-4g Ca2+
(1.02 g/ml seawater) * (1.00ml seawater) = 1.02g seawater
[(3.89*10-4g Ca2+) / (1.02g seawater)] x1E06 =3.81*102 ppm Ca2+
Results:
The average molarity of the EDTA solution was 2.95*10-3M.
The average concentration of Mg2+ and Ca2+ was 5.93*10-2M.
The average concentration of Ca2+ was 9.70*10-3M or 3.81*102ppm.
The average concentration of Mg2+ was 4.96*10-2M or 1.18*103ppm
The density of seawater was 1.02g/ml.
Error Analysis:
The most important source of error came from the ************
These errors had the following impact on my results:
1
2
…
Discussion:
We determined the concentration of calcium and magnesium ions in seawater using a standardized EDTA solution. This is an important useful important procedure because…
The expected value for the molarity of the EDTA solution based on the molar mass was about 3.27*10-3M. My experimentally determined value from standardization was 2.95*10-3M, about a 10% deviation.
Principles of Geochemistry, Mason (1966), p.195 states that seawater contains about 400 ppm Ca2+ and 1350 ppm Mg2+; these values are quite close to my own.