Separation of Cations
Purpose
The Purpose of this lab is to use masking and ion exchange (two types of techniques used for selectivity
in analytical chemistry) to determine the concentration of H+, Zn2+, Mg2+, and Na+ in a sample. Also,
another purpose is to divide and conquer as a lab section.
Procedure
1. 0.01 M EDTA was prepared and standardized with CaCO3
2. A 0.1M solution of NaOH was made from 6M NaOH
3. NaOH was standardized against KHP.
4. Use 0.5 g of KHP was massed out three separate times and added 25 mL water in three separate
Erlenmeyer flasks
5. A pH meter was calibrated with known buffers (4,7,10 pH)
6. Glass wool was placed above the stopcock of a 50mL burette and 10mL of dowex 50WX4-200 ion
exchange resin slurry was added to the burette. This was washed with 0.1M HCl and distilled water. 15
mL of the unknown was run through the cation exchange resin column and collected in an Erlenmeyer
flask. The column was washed with distilled water again. The exchange that went into the Erlenmeyer
flask contained H+ that exchanged with the metal cations. A blank was run first and then the samples
were run.
7. An aliquot of the sample was taken and titrated with 0.1M NaOH to the equivalence point.
Phenolphthalein was added to each sample.
8. Three samples of 1 mL diluted with 20 mL of water were titrated with 0.1M NaOH until the samples
went clear.
9. The H+ concentration was determined through a direct titration with standardized NaOH
10. Three 10mL samples of unknown were obtained and titrated with EDTA to find Mg and Zn
concentrations. This was done by adding 5mL of ammonia buffer and 1mL CaCl2 to the solution and
titration with 0.1M EDTA until the calmagite indicator turned blue
11. The pH of three 10 mL samples of the unknown was adjusted to be 10 by adding ammonia buffer.
Then 1 gram of KCN was added. Then they were titrated with EDTA to find the Mg concentration. CaCl2
did not need to be added because the indicator detects Mg and Ca. The solution was titrated until the
indicator turned blue
Data
0.9306 grams of EDTA
KCN (grams)
1.0000
1.0000
1.0008
mL Unknown
10
10
10
pH
10.03
10.06
10.01
mL Unknown
10
10
10
mL Buffer
5
5
5
*mL EDTA estimated since the titration did not work
mL EDTA
50
50
50
mL EDTA
44.13
46.20
45.80
mol Mg2+
0.004413
0.004620
0.004580
[Mg2+]
4.413
4.620
4.580
Total mol metal
0.005
0.005
0.005
Mol Zn2+
0.000462
0.000462
0.000462
[Zn2+]
0.462
0.462
0.462
Cation Exchange Titration:
Trials
Unknown (mL)
NaOH (mL)
[H+]
Mol [H+] in 2 mL aliquot
1
15
-
2
4
-
3
4
3.8
0.0924
0.000185
Direct NaOH Titration of H+
NaOH used (mL)
12.20
12.08
12.80
[H+]
1.187
1.175
1.245
Moles NaOH
0.001187
0.001175
0.001245
*used 1 mL of unknown and diluted down with 20 of water
Calculations
Moles Mg 2+
[Mg 2+ ] ∶
0.100 mol EDTA
1L
1 mol Ca2+
= mL EDTA ×
×
×
L
1000 mL 1 mol EDTA
Moles Mg 2+ = 44.13 mL EDTA ×
0.100 mol EDTA
1L
1 mol Ca2+
×
×
= 0.004413 mol
L
1000 mL 1 mol EDTA
2+
moles Mg
[Mg 2+ ] =
L unknown
[Mg 2+ ] =
0.004413 moles Mg2+
0.001 L unknown
= 4.413
[Zn2+ ] ∶
0.100 mol EDTA
1L
1 mol Ca2+
Mol metal = 50 mL EDTA ×
×
×
= 0.005
L
1000 mL 1 mol EDTA
mol Zn2+ = mol metal − average mol Mg 2+
0.000462 mol = 0.005 − .004538 𝑚𝑜𝑙
moles Zn2+
L unknown
0.000462
2+
[Zn ] =
= 0.462
0.001
[Zn2+ ] =
𝐿𝑖𝑡𝑒𝑟𝑠 𝑜𝑓 𝑁𝑎𝑂𝐻 𝑎𝑑𝑑𝑒𝑑 × 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 𝑁𝑎𝑂𝐻 ×
0.0038 𝐿 × 0.09727 𝑀 ×
1 𝑚𝑜𝑙 𝐻 +
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1 𝑚𝑜𝑙 𝐻 +
1
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
×
0.004 𝐿
×
1
𝑙𝑖𝑡𝑒𝑟𝑠 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
= 0.0924 M
Average [H+]: 1.202
[H + ] ∶
Molarity of NaOH × Liters NaOH used = mol NaOH
0.09727 M × 0.0122 = 0.001187 mol
mol NaOH
Liters of acid(ammonia buffer)
= M of H+
0.001187
= 1.187 M H+
0.001
= [H+]
[Na+ ] = total mol of ions in solution − moles of all other determine ions
*The moles of other determined ions were greater that the total moles of ions due to an error with the cation exchange
column, which did not give back the required results.
Conclusion
The goal of using masking and ion exchange was met, but finding the concentrations was not. Some
parts of the experiment didn’t work properly, which threw off data and other all other part later in the
experiment that relied on parts before them.