Procedure
1.
Submit
a clean 500-mL volumetric flask to receive the unknown, dilute to the mark with
water, and mix thoroughly.
2.
Transfer
50.00-mL aliquots to 250- mL conical flask.
3.
Add
1 to 2 mL of Ph-10 buffer and 3 to 4 drops of Eriochrome Blake T indicator to
each.
Eriochrome Blake T indicator
4.
Titrate
with 0.01M EDTA until the color changes from red to pure blue[1][2].
5.
Express
the results as parts per million of Mg2+ in the sample.
Notes
[1]
The
color change tends to be slow in the vicinity of the end point. Care must be
taken to avoid overtitration.
[2]
Other
alkaline earths, if present, are titration along with the Mg2+ ;
removal of Ca2+ and Ba2+ can be accomplished with (NH4)2CO3.
Most polyvalent cations are also titrated. Precipitation as hydroxides or the
use of a masking reagent may be needed to eliminate this source of interference.
Experimental
Record
|
Concentration
of EDTA
|
0.010
M
|
|
Weight
of Standard MgSO4
|
12.00g
|
|
Consumption
of EDTA solution
|
46.50
mL
|
|
|
46.73
mL
|
|
Average
|
46.62
mL
|
|
|
=
0.4662 mmol
|
|
Concentration
of Mg2+
|
0.4662
mmol /50.00 mL
|
|
|
=
9.32 mM
|
|
Theory
Concentration of Mg2+
|
(12.00g)/(120.31)/1.00
|
|
|
=
9.97 M
|
|
Percentage
of Error
|
6.5%
|
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