Sciencemadness Discussion Board

Titrating Ca(OH)2

MeshPL - 16-3-2016 at 06:12

I was practising titrations at school and decided to try with Ca(OH)2. It is obviously insoluble. So I was titrating it in a form of a thing suspension, enough to make it opaque, but not enough to see any individual particles at the bottom easily. I used Congo Red as an indicator (has a nice colour:) and transition pount between 3 and 5,2 pH). Although I was adding HCl rather slowly and mixing quite vigorously, I didn't notice any particular transition point in colour of the mixture. What it did is it turned quite gradualy from red to blue, with no particular sharp colour change at any point (as I've seen when titrating hydrochloric acid with NaOH, again using Congo Red).

So, is it my fault, that I didn't notice a particular endpoint or could it be Ca(OH)2 that didn't quite disslve fast enough? At the end some red Ca(OH)2 particles were still visible at the bottom of blue solution, so well... could be the second option.

Or was the stiring insufficient? I obviouslly did it manually, but quite well, especially that theese Erlenmeyer flask allow liquids to be strirred very well without spilling.

blogfast25 - 16-3-2016 at 07:44

Quote: Originally posted by MeshPL  
I was practising titrations at school and decided to try with Ca(OH)2. It is obviously insoluble. So I was titrating it in a form of a thing suspension, enough to make it opaque, but not enough to see any individual particles at the bottom easily. I used Congo Red as an indicator (has a nice colour:) and transition pount between 3 and 5,2 pH). Although I was adding HCl rather slowly and mixing quite vigorously, I didn't notice any particular transition point in colour of the mixture. What it did is it turned quite gradualy from red to blue, with no particular sharp colour change at any point (as I've seen when titrating hydrochloric acid with NaOH, again using Congo Red).

So, is it my fault, that I didn't notice a particular endpoint or could it be Ca(OH)2 that didn't quite disslve fast enough? At the end some red Ca(OH)2 particles were still visible at the bottom of blue solution, so well... could be the second option.

Or was the stiring insufficient? I obviouslly did it manually, but quite well, especially that theese Erlenmeyer flask allow liquids to be strirred very well without spilling.


Ca(OH)2 is much better described as poorly soluble than insoluble. Acc. Wiki its solubility at 20 C is 17.3 g/L or about 0.23 M.

That is high enough to titrate it as a solution, not as a suspension: simply dilute your known sample with a known dilution factor, so you end up with a solution, not a suspension.

Titrating suspensions is difficult and leads to errors and 'spongy' end-points.

Considering CaCl2 is a neutral salt, it would also be better to use a pH indicator with a transition point closer to 7.

[Edited on 16-3-2016 by blogfast25]

MeshPL - 16-3-2016 at 08:32

Oh, yes, sparingly soluble, not insoluble! My bad. I was using Congo Red just for fun, perhaps bromothymol blue would be better next time. And no suspensions!

I wasn't doing it to determine anything, I didn't even measure anything particular... I just was practising operating burette and I was worried that I failed with endpoint determination.

Thanks!

Spongy endpoint... well said.

Congo Red has the nice property of having two distinct and pretty strong colors, when you titrate, you can se the blob of different colour mixing with the rest of solution quite nicely. Yellow form of bromothymol blue is not as strong.
And Congo Red is not phenolophtalein or methyl orange which are the ONLY indicators mentioned in highschool curriculum here where I live, and as a result are pretty boring.

DraconicAcid - 16-3-2016 at 11:23

What you probably want to do is do a back titration. Add a measured amount of hydrochloric acid, heat it gently to make sure it completely reacts, then titrate the leftover acid with standard base to find out how much hydrochloric acid is left.

MeshPL - 16-3-2016 at 11:31

I could do that, if I need to, I do not need to titrate anything like that right now... but when I need in the future I'll certainly do that. As I wrote I was practising operating the burette, not actual determination of Ca(OH)2 content of whatever thing, next time I'll just use NaOH and KOH. NH3 is slightly too volataile to work with in current setup.