Comparing Calcium Carbonate in Shells: An Experiment

[jan]

what experiment should i conduct to compare the amounts of calcium carbonate in 2 different shells...

i believe this might be of use:
acid + carbonate = salt + water + carbon dioxide

my email is janice_ling88@yahoo.com.sg

 

[Monique]

I would do a titration reaction, where the added acid gets neutrolized until there in no more carbonate available, this can be monitered by pH-sensitive colors added to the solution..

 

[jan]

hmmm r there any other options
the shells r solid, and i had been thinking of producing calcium sulphate by adding sulpheric acid to the shells. but I'm unsure if whether i will see clear results

 

[Monique]

Of course you'll first need to grind up the shells to a powder in order for it to work.. a reaction you could use is the following:
CaCO₃ + 2HCl -> CaCl₂ + CO₂ + H₂

If you know exactly how much HCl you put in and moniter the point at which the reaction is saturated, you can calculate the amount of CaCO₃ from that..

 

[thunderfvck]

When it stops bubbling CO2, it's done, you've reached the end point. You must be careful not to exceed this point because there are no further warnings.

Phenolphtalein is a beautiful thing (sadly, not as beautiful as some...Monique ), it will tell you when you've reached pH of 7 by turning pink...Just restating what Monique had said, yes. Remember, you must know the molarity of the acid you are using so you can see exactly how much H+ was required to react with the CaCO3.

And yes, you must grind the shells..mortar and pestle, yes? Dissolve them in water, if you can't get them dissolve it's fine...Just make sure you stir the solution when you're adding the acid in this case...Well in both cases but more so if there's no dissolution...

 

[Monique]

Right, you must know exactly the molarity of the acid, but also exactly how much of the powdered shell you are bringing in suspension/solution. The thing I wonder about.. how do you know how much shell to put in and with how much acid to start the titration.. I guess trial and error is only going to learn.. :)

 

[thunderfvck]

I think the shell is mostly CaCO3...

But yeah, trial and error is often the way to go. I'd start with a gram or so, or whichever molar equivalent which may be easy to work with...Dissolve it, and titrate it with a moderately strong acid...2 M should be okay I guess. Phenolphtalein would be best, I don't trust the gas evolution thing, that holds too much room for personal error...

 

[Monique]

You know.. CaCO3 actually needs acid to dissolve, that is why limestone statues slowly dissolve in acid rain. I think if you do an acid titration with the suspension you are not going to get accurate results.

Rather you could dissolve your powdered shell in a known and recorded amount of acid, and then do a titration with a base to see what the amount of excess acid is.

I think it is safe to assume that 80% of the shell is CaCO3, and use that to approximate the amount of acid you'll add. 2 M sounds really strong though, I think 0.2 M would be better..

 

[thunderfvck]

You're right. CaCO3 is poorly soluble, that's what I was thinking but not saying...Most carbonates are insoluble.

I think you've developped a perfect method for determining the CaCO3 content in shells. Dissolve it in acid and check how much acid reacted. If you're using HCl you could even see how much CaCl2 is produced by evaporating the acid; this is another way of doing it I suppose, but not as easy as Monique's. I hope you've presented a speech for the prize ceremony.

 

[jan]

thanks for your answers. i was thinking about methods for a week already