Lab 8: Separation of Cations (Group Lab)
Purpose:
Two techniques commonly used for selectivity in analytical chemistry will be tested. The analyte
of interest is in a sample containing other materials that could potentially interfere with
analytical determination. Different selectivity methods include: masking, trapping, conversion,
extraction, gas diffusion, dialysis, and chromatographic separation. Masking and ion exchange
conversion will be explored in this lab.
Procedure:
EDTA (0.9061 g) was massed and placed in a volumetric flask ( 250 mL). DH2O was added to the
dilution point making an approximate 0.002M EDTA solution.
Glass wool was placed in a 50mL burette just above the stopcock. A resin slurry (~10mL) was
made and added to the burette. The resin slurry was washed 3 times with HCl (~0.01M) and
then an additional 3 times with dH2O. An unknown sample (20 mL) was pipetted and loaded
onto the column and run through the cation exchange resin column into an Erlenmeyer
flask(250mL). Distilled H2O was then pushed through the column and collected in the same
flask. The solution was transferred into a volumetric flask (100 mL) and diluted to the mark.
The solution from the cation exchange column was titrated with NaOH (0.095 M) until it reached
the equivalence point.
An NaOH solution (~100mL) was made to adjust the pH of the samples. The pH meter was
calibrated using 4.01, 7.00, and 10.01 buffer solutions. The pH of the unknown samples was
adjusted to approximately a pH of 10.
The unknown sample (10 mL) was pipetted into three Erlenmeyer flasks (250mL). The pH of
each sample was adjusted to approximately 10. KCN (~1g) was added to each sample.
The concentration of the [H+] was found by titrating an unknown sample using standardized
NaOH. To standardize the NaOH, KHP (0.0996 g) was dissolved in dH2O and titrated with NaOH.
The unknown solution (~20mL, pH 10) was titrated using a standardized EDTA solution.
Calmagite indicator (pink/ violet to blue) was used to determine the end point of the titration.
Data:
Moles of EDTA
Molarity EDTA
Standardization of NaOH:
Start of titration (mL)
3.55
End of titration (mL)
8.45
Total (mL)
4.90
Titration with Zn2+ masked with KCN:
Sample
KCN Added (g)
1
1.0040
2
1.0048
3
1.0029
Average: 0.0174 M
Example:
Titration of H+:
pH
10.40
9.70
9.90
mL EDTA Added
17.3
17.4
17.4
[Mg2+]
0.0173
0.0174
0.0174
Start
Volume
8.50
10.00
13.00
mL Unknown
10.0
20.0
20.0
End
Volume
10.00
13.00
16.10
Total
1.50
3.00
3.10
[H+]
0.0149 +/- .0003
0.0149 +/- .0003
0.0154 +/- .0003
Example:
Sample
1
2
3
mL Unknown
20
20
20
Volume EDTA
48.1
45.4
45.2
Average:0.0231 M [Mg2+] +[Zn2+]
Example:
Cation Exchange:
Trial 1 Trial 2 Trial 3
mL Unknown 20
20
20
mL NaOH used 3.0
2.98 3.05
+
[H ]
0.0143 0.0142 0.0145
Average [H+]: 0.0143 M
Zn2+ Concentration:
[Mg2+] + [Zn2+]
0.0241 M
0.0227 M
0.0226
Average: 0.0151 M
[H+]
[Zn2+ +Mg2+]-[Mg2+]=[Zn2+]
0.0231-0.0174=0.0057 M Zn2+
Na+ Concentration
[H+]-[Zn2+ + Mg2+]=[Na+]
Inconclusive
Conclusion:
The purpose of this experiment was to demonstrate two types of techniques commonly used for
selectivity in analytical chemistry: masking and ion exchange resin conversion. This experiment
was separated into several parts and each was completed by different lab groups. Due to this
separated nature, the procedures were difficult to follow, and the data was difficult to analyze
to form conclusions. The titrations of Mg2+, Zn2+, and H+ ions were successful. However, when
the solution was passed through the cation exchange column in an attempt to determine the
total metal ion concentration (replacing the metal ions with H+ ), a lower concentration of H+
than the original solution was found. It is believed that some H+ was lost in the cation exchange
resin. This error prevented an accurate calculation of the Na+.