Reaction of Calcium Carbonate in Water with Hydrochloric Acid

[Aezi]

I was told to design an experiment and run it during my chemistry lab period, and I came up with the following idea:

Soil usually is limed with calcium carbonate containing products (agricultural lime) in order to raise the pH of the solution. Thus, I first isolated the amount of calcium from a soil sample made of known agricultural lime and pure soil (meaning no other special ingredients) in order to determine the amount of calcium carbonate given by the agricultural lime (room for error). Then I titrated a dilute soil sample with an automatic industrial titration apparatus with 0.1 M HCl and another solution of simply water and calcium carbonate of the same amount found in the same amount of soil in order to compare the pH curves (pH vs. titrant 0.1 M HCl). I found the pH curves for soil samples to reach the pH of pure HCl as the amount of titrant increased into excess (as I had expected) but the pH curves for the calcium carbonate and water solution indefinitely flat-lined at ~5 pH. Can anyone explain why? I know that calcium carbonate is highly insoluble in water, but when acid molecules "collide" with the calcium carbonate, it should react to form calcium chloride, water, and carbon dioxide.

The purpose of my experiment was to see if the agricultural lime's manufacturer indeed gave the proposed amount of calcium carbonate, and to see if the the pH curves for pure calcium carbonate and soil were different in order to infer if the only buffering mechanism in soil is indeed calcium carbonate (highly unlikely, but it is a conclusion). I know that it sounds simplistic, but this is for a course before organic chemistry.

 

[lightarrow]

I was told to design an experiment and run it during my chemistry lab period, and I came up with the following idea:

Soil usually is limed with calcium carbonate containing products (agricultural lime) in order to raise the pH of the solution. Thus, I first isolated the amount of calcium from a soil sample made of known agricultural lime and pure soil (meaning no other special ingredients) in order to determine the amount of calcium carbonate given by the agricultural lime (room for error). Then I titrated a dilute soil sample with an automatic industrial titration apparatus with 0.1 M HCl and another solution of simply water and calcium carbonate of the same amount found in the same amount of soil in order to compare the pH curves (pH vs. titrant 0.1 M HCl). I found the pH curves for soil samples to reach the pH of pure HCl as the amount of titrant increased into excess (as I had expected) but the pH curves for the calcium carbonate and water solution indefinitely flat-lined at ~5 pH. Can anyone explain why? I know that calcium carbonate is highly insoluble in water, but when acid molecules "collide" with the calcium carbonate, it should react to form calcium chloride, water, and carbon dioxide.

The purpose of my experiment was to see if the agricultural lime's manufacturer indeed gave the proposed amount of calcium carbonate, and to see if the the pH curves for pure calcium carbonate and soil were different in order to infer if the only buffering mechanism in soil is indeed calcium carbonate (highly unlikely, but it is a conclusion). I know that it sounds simplistic, but this is for a course before organic chemistry.

If the curves are very different it should imply that the concentrations of CaCO3 are quite different, but the soil can have been exposed to air CO2 and so be less basic than the pure water + CaCO3.
The fact that the curves for H2O+CaCO3 flat-lined seems due to an high amount of CaCO3 which buffers the acid, until all the carbonate has reacted: CaCO3 + 2HCl --> CaCl2 + H2O + CO2.

 

[Aezi]

If the curves are very different it should imply that the concentrations of CaCO3 are quite different, but the soil can have been exposed to air CO2 and so be less basic than the pure water + CaCO3.
The fact that the curves for H2O+CaCO3 flat-lined seems due to an high amount of CaCO3 which buffers the acid, until all the carbonate has reacted: CaCO3 + 2HCl --> CaCl2 + H2O + CO2.

I first determined the amount of calcium carbonate in the soil sample (with error) before proceeding with the pH curve creations. Thus, assuming that the amount of calcium carbonate in pure water and in the soil solutions are the same (relative to error, once again), the curves should tell us information about the mechanisms of soil acid rain buffering.

My only concern is that the calcium carbonate solution with water flat lined. Indefinitely after immense amounts of strong acid... @_@ The calcium carbonate with water solution was immediately titrated after initial mixing of the two, so the amount of error due to the reaction of calcium carbonate with carbon dioxide should be minimal.

Does anyone else have an explanation for why calcium carbonate suspension in water flatlines when titrated with HCl?

Note: Even if there were calcium carbonate molecules reacting with carbon dioxide to form solid impurities, the curves still should have indicated acidification of the titrate after some time!

 

[chemisttree]

How much of the HCl did you add to your soil sample during the analysis?

 

[Borek]

Stupid question... have you calibrated your pH meter?

 

[chemisttree]

Yeah, can the meter read below 5 the way its set up?