Reactions of MgO in the acidic solutions - Buffer reaction? (Jul/02/2007 )

Dear all,

I have done the following experiments and hope you guys can help me out to explain what have happened.

I have prepared 3 kinds of acidic solution:
#1 - 1.0M acetic (CH3CooH) solution
#2 - 0.5M phosphoric (H3PO4) solution
#3 - 1.0M KDP (KH2PO4) solution

Then I put 5 grams of MgO particles (purity of about 97%, particle size of about 0.02 mm) into 100 mL of each of the above solutions under gentle stirring. The pH value of the solution was monitored until it reached pH=7.0, ie. the neutralization process was recorded.

The time for reaching pH=7.0 for the different solution are as below:
#1 - 5g MgO in 100mL 1.0M acetic solution is about 90sec
#2 - 5g MgO in 100mL 0.5M phosphoric solution is about 460sec
#3 - 5g MgO in 100mL 1.0M KDP solution is about 690sec

I suppose phosphoric solution would be the solution to have a shorter reaction time, but the measurements show a different result. I am also don't know what exactly reactions have taken place in the different solutions. May be the different salts produced also affect the neutralization process.

Besides, a buffering effect was observed for the phosphric and KDP solution as there was quite a long period of time the solution was kept at a constant pH value after just 30 sec of the reaction.

Can anybody kindly help. Thanks in advance!

look at the number of ionizable hydrogens, the respective pKs (and buffering effect) and the concentration of each solution.

attached to that pKa effect, what do you think the pH of the various solution were before the reaction started?
also look up the Solubility costant of the reaction products.
also consider that MgO will react with water.

You will find Mg(OH)2 and MgPO4 being quite insoluble. What do you think an insouble coat on the MgO particles would do?

Thanks for both of your kind comments and hints. The record of the tests is shown above, where

#1- MgO in 1M acetic acid,
#2- MgO in 0.5M phosphoric acid, &
#3- MgO in 1M KDP solution.

For the #1 test, the possible salts are Mg(OH)2 and Mg(CH3COO)2.4H2O, Mg(OH)2 would not precipitate when pH<9, and magnesium acetate is water soluble, so the salts would not affect or delay the neutralization.

For the #2 test, the possible salts are Mg(OH)2, and the 3 forms of magnesium phosphate, I guess most are the tribasic Mg3(PO4)2, right. The point is the tribasic phosphate is practically insoluble. As the reaction between MgO and phosphoric solution would be rather quick (compared with acetic acid), so a coating of the tribasic on the MgO particles would be formed and prohibit the further neutralization. As a result, a diffusion controlled slow phase of the neutralization process dominate the lengthy reaction time, that's the "buffer effect" I thought before. Now I don't think that's a buffer reaction anymore (??).

For the #3 test, the possible salts are Mg(OH)2, magnesium phosphate and also possibly KMgPO4.6H2O (??). The coating effect of the magnesium phosphate also play a role here. Moreover, the [H+] is higher than #2 test. There are 2 moles (1Mx2) of H+ in #3 test while there is about 1.5 moles (0.5Mx3) of H+ in #2 test, so test #3 takes the longest time for the neutralization process.

I think I would have something missed in my analysis. Any comments would be highly appreciated.

Thank you very much.