Chemlab: Chemistry 3/5


Natural Salt Solutions 1: Ion Exchange

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Question 1


My column seems really slow. Could it be clogged? What should I do?


Question 2


My column doesn't look uniform. The resin has vertical ridges and the eluant runs through very quickly. Is there something wrong with it?


Question 3


What is a milliequivalent? Is it the total amount of positive charge that a column can exchange in mL?


Question 4


Prelab problem 1 asks for a balanced chemical reaction describing the displacement of Al3+ by Ca2+ from a cation exchange resin. I am confused because this is different from the given example with RSO3H and Mg2+. That example replaces H+ with Mg2+. How do I make an exchange, when the resin is not in the acid form, RSO3H?


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Questions & Answers

Question 1


My column seems really slow. Could it be clogged? What should I do?


Professor Milde


A slow column could be the result of too much resin or a problem with the ground glass filter at the end of the column. The best thing to do is make a new column, since it is quicker than trying to fix one with a problem.


Question 2


My column doesn't look uniform. The resin has vertical ridges and the eluant runs through very quickly. Is there something wrong with it?


Professor Milde


Your column has developed channels and the solution can run through without passing by beads of resin. This may be the result of allowing the column to run dry. The best thing to do is make a new column, since it is quicker than trying to fix one with a problem. To avoid this problem, always keep the resin in the column wet, with flowing water, flowing eluant solution, or by capping the bottom with water or solution in the column, so nothing is flowing through.


Question 3


What is a milliequivalent? Is it the total amount of positive charge that a column can exchange in mL?


Professor Milde


A milliequivalent is 1/1000 of an equivalent. An equivalent is one mole of charges. So one equivalent could consist of one mole of Cl- (chloride) ions or one mole of K+ ions, for example. It could also be 1/2 a mole of Ca2+ ions or 1/3 of a mole of Fe3+ ions. You can express the ion exchange collumn capacity in units of milliequivalents. Next week you will determine the total concentration of cations in seawater, measured in milliequivalents per mL of seawater.


Question 4


Prelab problem 1 asks for a balanced chemical reaction describing the displacement of Al3+ by Ca2+ from a cation exchange resin. I am confused because this is different from the given example with RSO3H and Mg2+. That example replaces H+ with Mg2+. How do I make an exchange, when the resin is not in the acid form, RSO3H?



Professor Milde


The key thing in this prelab problem is the stoichiometry of the exchange process. For example Mg+2 ions will exchange on the resin, reacting with TWO RSO3H groups, to give TWO H+ ions. In the problem, the Al+3 is already bound to the resin, so on the reactants side you will have to figure out the correct number of RSO3- groups that will bind a Al+3 ion. This will react with the replacing ion to make the bound form of this second ion as a product.

If this doesn't help clarify, please go to TA office hours. The TAs there should be able to help you with this and the other prelab problems.

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