Experiment 4: Introduction To Chromatography - Quantification Of Copper In Solution

Question 1

Chromatography definition

Answer 1

A technique that is used the separate the components of a mixture based on physical characteristics like molecular size, shape, charge, solubility, etc

Question 2

The basic components of any chromatographic system:

Answer 2

Stationary phase, chromatographic bed, mobile phase, delivery system, detector

Question 3

Stationary phase

Answer 3

A solid, gel or immobilized liquid

Question 4

Chromatographic bed

Answer 4

A column (glass or metal) or sheet (glass, plastic or paper) used to support the stationary phase

Question 5

Mobile phase

Answer 5

A solvent (liquid or gas) which carries samples through the stationary phase

Question 6

Delivery system

Answer 6

A force which moves the mobile phase through the stationary phase (gravity, pressure)

Question 7

Ion exchange chromatography (in depth)

Answer 7

utilizes the reversible interchange of ions between a solid stationary phase and a liquid mobile phase with no change to the solid to separate mixtures based on the charges of the components

Question 8

How is ion exchange chromatography used in the experiment?

Answer 8

Removes cations from a sample to determine the concentration of the original sample

Question 9

What is the exchange of ions based on?

Answer 9

Relative binding affinity of each ion to the resin
E.g. calcium is more strongly attracted to the resin than sodium, and this displaces it from the resin binding sites

Question 10

What is the stationary phase for the ion exchange chromatography experiment?

Answer 10

Cation exchange resin that is packed into a glass column, this type of resin is composed of a cross-linked polymer matrix with ion-active sites throughout its structure
- the ion sites are negatively charged and will attract positively charged cations

Question 11

How are the cations in the solution quantified?

Answer 11

• the sample that you will analyze is an aqueous cation solution
• in order to quantify the cations in the solution, the resin in the column is prepared by saturating with H+ cations
• soaking the resin in a strong acid ensures that all binding sites are occupied by H+
• excess unused acid is decanted from the resin, which is then poured into a glass column and rinsed with DI water to ensure that all unbound H+ are removed from the column

Question 12

How does the cation exchange resin work in this experiment?

Answer 12

The cation in the sample, Cu2+ has a higher binding affinity for the resin than H+, as the sample runs through the column, it will displace H+ ions
- to determine how many Cu2+ ions are in your sample volume, you will collect the displace H+ ions in solution and titrate with a standardized solution of NaOH

Question 13

After running sample through column, what should you do?

Answer 13

• after running your sample through the column, you will rinse the column with DI water and collect this rinse water into your sample solution as well
• this rinse ensures that all displaced H+ ions are removed from the column and collected in your sample

Question 14

Resin definition

Answer 14

Small insoluble particles, usually a polymer, with a charged coating, which can reversibly bind ions of the opposite charge

Question 15

Binding affinity meaning

Answer 15

A measure of the strength of the attraction between two particles

Question 16

Eluate

Answer 16

The sample leaving the column

Question 17

Eluent

Answer 17

The sample loaded into the column

Question 18

Elute

Answer 18

The separation process itself

Question 19

Part A: running samples on the ion exchange column

Answer 19

• allow liquid inside resin the drain until liquid is 1-2 mm above the top of resin
• test pH of eluate and add 10 mL of sample solution to top of column without disturbing resin bed
• place Erlenmeyer flask (125 or 250 mL) under column to collect eluate
• let eluate drain until liquid is 1-2 mm above resin
• rinse column with DI water until 50 mL of DI water has been used
• if pH is neutral (same pH as DI water, 5.5 - 6), move onto titration and repeat two more times

Question 20

What does the DI water rinse do?

Answer 20

Acts like a plunger to push the mobile phase through the column to be collected with the eluate solution

Question 21

Part B: analyzing sample using acid-base titration

Answer 21

• titrate each of the samples from the column with standardized NaOH

Question 22

How can the resin be used/re-used?

Answer 22

Fill the reservoir will DI water and cover column with Parafilm — keeps the resin from drying out

Question 23

What is the white material in the bottom of the chromatography column and what is its purpose?

Answer 23

• glass wool, keeps the resin beads from falling out

Question 24

When do we consider that the sample is loaded onto the column?

Answer 24

When the sample has been drained into the resin bed

Question 25

Why is the rinse added in small portions rather than all at once?

Answer 25

If all rinse were added at once, it would combine with the Cu2+ sample and make a large, dilute sample, which would them require additional rinsing to push it off the column

Question 26

Why does the use of tap water cause an error in this experiment?

Answer 26

Tap water contains ions (typically Ca2+ and Mg2+) that can displace H+ from the resin binding sites, the eluate drop will test as acidic for a very long time (until the resin is completely depleted of H+ ions)

Question 27

What would happen to the sample if only 10 mL of DI rinse water were added after the sample solution?

Answer 27

The displaced H+ ions would still be in the mobile phase inside the resin bed (on the column) rather than in the eluate collection flask

Question 28

What could cause an outlier?

Answer 28

• the column contained sample from a previous user before the new sample was added
• Cu2+ sample solution was not measured accurately
• either the initial or final volume reading of the burette was incorrect
• the column was not rinsed to a neutral pH before the sample was added

Question 29

Detector

Answer 29

A method of identifying or quantifying substances as they elute (leave the stationary phase)