Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: brmc2tt on January 02, 2015, 04:39:45 PM
-
I have a question that I've been wondering for while. When trying to determine the solubility of something in high school chemistry, I was wondering how much undissolved substance or precipitate can be suspended in solution without the solution becoming visibly cloudy. From my understanding when something is completely dissolved the solution will be perfectly translucent/transparent. So how much precipitate could be present in, say, a 100 mL solution before it will be obviously cloudy?
Thanks for your *delete me*
Brett
-
Depends on the substance.
There is an analytical method based on the scattered light: http://en.wikipedia.org/wiki/Turbidimetry. Rarely used these days, but it was one of the standard methods somewhere in the last century.
-
Not so long ago I used turbidimetry in a 5 cubic meter reactor full of a hot solution to determine the exact temperature when the crystallisation began. Worked like a charm!
-
Interesting but a little expensive for me. Would you say the 100g of suspended precipitate in a 100mL solution would be easily visible by shining a light through the solution. Can you guess about how much it would take? Or does it just vary too much from substance to substance?
-
Would you say the 100g of suspended precipitate in a 100mL solution would be easily visible by shining a light through the solution.
100g in 100 mL is a slurry, not a solution. In typical applications (like determination of chlorides) turbidimetry operates at concentrations in the mg/mL range.
-
Would you say the 100g of suspended precipitate in a 100mL solution would be easily visible by shining a light through the solution.
Visible? You could literally feel that sort of conc. with your finger.
-
There is an analytical method based on the scattered light: http://en.wikipedia.org/wiki/Turbidimetry. Rarely used these days, but it was one of the standard methods somewhere in the last century.
@Borek:
What led to its decline as a technique? Do you know?
-
What led to its decline as a technique? Do you know?
No idea. I guess it was just replaced by easier and more reliable techniques (say, ion selective electrodes).
-
100g in 100 mL is a slurry, not a solution. In typical applications (like determination of chlorides) turbidimetry operates at concentrations in the mg/mL range.
I'm sorry, I mistyped; I meant to say 100mg in a 100mL solution. I'm guessing that if turbidimetry operates in the mg/mL range then that wouldn't be very easy to see with the naked eye?
-
In skim milk (no fat globules), it is the casein micelles that give milk it's turbidity making the milk completely opaque and white. The concentration of protein in cow's milk on average is 3.3% (w/w) with 2.6% being the casein and about 0.7% the whey proteins (which contribute no turbidity).
The casein micelles are on average 200 nm in diameter. On the other hand if you had kappa carrageenan with a similar length of 200 nm (radius of gyration of approx. 100 nm with MW~1,000,000 g/mol and intrinsic viscosity of 15 dl/g) at that same concentration (2.6%), it would be a firm, solid gel yet would be completely transparent while the skim milk at this concentration is a completely turbid low viscosity liquid.
The difference is due to the shape where casein micelles are large but globular in shape which can scatter light well while kappa casein is large but extended & linear which allows light to pass through easier.
So in my opinion there are more variables that make your question difficult to answer without knowing the molecular dimensions (size and shape), not to mention interaction effects/A2 second virial coefficient (does the system prefer polymer-polymer or polymer-solvent interactions).