[Alberto_Kravina]

Does somebody of you guys know how I colud perform a quantitative analysis of Ca²⁺ in milk?
A titration or a gravimetric analysis would be great...

Thanks for any suggestion.

[Albert]

Generally speaking, gravimetric analysis is always more accurate but time-consuming.
Well, I'd say an ISE for Ca²⁺ to be the best way of carrying out your determination. Or you can titrate it using EDTA or EGTA.

[Alberto_Kravina]

Generally speaking, gravimetric analysis is always more accurate but time-consuming.
Well, I'd say an ISE for Ca2+ to be the best way of carrying out your determination. Or you can titrate it using EDTA or EGTA.

Yes-gravimetric analysis eats a lot of time. I thought about using EDTA (complexometry) but...doesn't the matrix interfer in this case? ISE is also a smart idea, by the way.
If the other methods fail I'll perform gravimetry..

My 200th post!

[Albert]

I'd try to find information about EGTA: I think that the matrix shouldn't interfer. However, I've never used it.

Congratulations!

[Alberto_Kravina]

Well, I'll try tomorrow...EDTA or EGTA should work...and if not...I'll find a solution (hopefully not gravimetry :ermm: )

Thanks for your useful advice!

[Alberto_Kravina]

Just finished performing the analysis:

Result: 1.205 g/L Ca²⁺

It worked perfectly with EDTA...

[Borek]

Can you post procedure?

[Alberto_Kravina]

I'll post it ASAP - I left the sheet where I wrote down the procedure in the lab- I should get it on monday afternoon...

[Alberto_Kravina]

OK, here is the procedure:

Take 25 mL of milk, add 5-10 mL of conc. HCl, filtrate it (this took quite a while in my case...this goddam solution didn't want to go through the funnel...it took more than 20 minutes... ).
Make sure that 100% of the milk goes thoruogh the funnel!
Pour the filtrate in a 100 mL volumetric flask and fill it.

Now take 20 or 25 mL with a transfer pipette and put it in an Erlenmeyer flask, add ammonia (c=0.1 mol/L) until you reach a pH of 9-10 and titrate with EDTA (c=0.01 mol/L)

Indicator: Murexide

That's it!

[Borek]

I wonder how much Ca²⁺ is lost with filtrate

[Alberto_Kravina]

I wonder how much Ca²⁺ is lost with filtrate

Yeah - As I said - I improvised....maybe the filtration process interfers in this case because Ca²⁺ ions could be lost... but you can't titrate the milk directly..
I don't think that vacum filtration is a solution, byw. Maybe centrifugation, but in this case you have to work with very small amounts of sample.

[mike]

(this took quite a while in my case...this goddam solution didn't want to go through the funnel...it took more than 20 minutes...  ).

Acidic solutions tend to swell filter paper and make filtering very difficult.

[Alberto_Kravina]

Acidic solutions tend to swell filter paper and make filtering very difficult.

True! But I in this case the acidic conditions are required to precipiatate the substances that could interfer...but maybe another separation method would be nice....