[lemonoman]

solubility of calcium phosphate in gastric juices

Sounds like a Ph.D. thesis to me!

[Organishe]

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Yeah, but I made sure that I did all the titrations before the milk's best-before-date.  And other students in my class have been doing the same type of investigation (though using different brands of milk); out of the three I spoke to, two have gotten similar trends to mine, and one has gotten no trend.
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Yeah, that last dude...he's the organic chemist of the bunch.

So, what I've to say doesn't deal with your titrations directly, but does answer some of the tangential quetions raised.  The presence of phosphate along with calcium will hinder its absorbtion, and also calcium is absorbed by the body exclusively in the stomach.  My assumption about the decreased absorption in the presence of phosphate is the insolubility of calcium phosphate, but in an acidic environment, one might expect to see solubilization. The other thing I know is that the presence of citrate with calcium aids in its absorbtion (the brandname of calcium suppliment Citrical utilizes this) compared to either calcium carbonate or calcium phosphate.   So, drink some orange juice with your tums as opposed to pop 

So in retrospect, this post is overall not really contributing to the thread, but whatever. It's neat stuff, perhaps you will find it interesting, even as an aside.

[Borek]

solubility of calcium phosphate in gastric juices

Sounds like a Ph.D. thesis to me!

Assuming gastric juice is mostly HCl and its pH is about 2 that's just an equlibrium question - K_so for Ca₃(PO₄)₂ and CaHPO₄ are known as well as all three dissociation constants for H₃PO₄. Not an easy question, but not that difficult too.

[lemonoman]

solubility of calcium phosphate in gastric juices

Sounds like a Ph.D. thesis to me!

Assuming gastric juice is mostly HCl and its pH is about 2 that's just an equlibrium question - K_so for Ca₃(PO₄)₂ and CaHPO₄ are known as well as all three dissociation constants for H₃PO₄. Not an easy question, but not that difficult too.

I suppose you're right...I figured there were so many matrix effects that it couldn't be calculated.  But alas, chemists and their assumptions save the day

[mike]

Do you know the accuracy and precision of your results? What are the errors associated with them? Could it be that all of your concentrations are the same within errors?

[ivychen1989]

Do you know the accuracy and precision of your results? What are the errors associated with them? Could it be that all of your concentrations are the same within errors?

I haven't calculated the percentage or absolute error of my result, but if the concentrations were all the same within errors, I would not have gotten a similar trend with both bottles of milk that I analysed (?).  And I don't think my results were all that inaccurate, as I got three concordant titre volumes for each sample (heated to a certain temperature), and these ranged from 12.49 mL to 12.97 mL (which is quite a large range?).  I tried to make everything as accurate as I could in my investigation.

Of course, if you can think of anything I might have done to make my results so inaccurate, please tell me and I'll include that in my report.

[Borek]

Plot your results with the error bars to see if the trend holds or not.

[ivychen1989]

Plot your results with the error bars to see if the trend holds or not.

How do I go about finding the error?  Do I start by saying, for example, for making the magnesium sulfate solution used to standardize the EDTA, 0.01g error from the electronic balance used to measure the solid, and 0.5% error (?) for the volumetric flask used?  And find all the uncertainties that I can think of from the equipment, and from parallax error, etc.?

Wow, that would be so complicated!

[Borek]

Have you used the same EDTA solution all the time? If so, even if your results are not correct in terms of absolute numbers, their ratios are OK. Start assuming that your accuracy of volume measurement is half of the scale. Alternatively - you did three titrations for every point, so you can estimate both average and deviation for every measurement.

[ivychen1989]

Have you used the same EDTA solution all the time? If so, even if your results are not correct in terms of absolute numbers, their ratios are OK. Start assuming that your accuracy of volume measurement is half of the scale. Alternatively - you did three titrations for every point, so you can estimate both average and deviation for every measurement.

Yes, I used the same EDTA.

Plot your results with the error bars to see if the trend holds or not.

When I said my titrations were concordant, the ranges for each point were mostly 0.04 mL.  If I only use that to calculate my uncertainties then plot everything on a graph, I have a trend.  But I still can't explain it.  I only know that the calcium hydroxyl phosphate in the casein micelles (a milk protein) is affected by heating, but it doesn't support my trend.  When heated above 75 degrees Celsius, calcium orthophosphate is made, which doesn't dissolve easily and precipitates with a pH higher than 7 (my titration was done at about pH 13).

[evoy6]

can sum1 post how the calcium concentration were worked out?

[ivychen1989]

can sum1 post how the calcium concentration were worked out?

This is a back titration.  Figure out the concentration of the Ca²⁺ in the conical flask that you put in (your standard solution).  Then change that into moles (you know the volume you used).  From the volume of EDTA titrated, figure out how many moles of Ca²⁺ reacted with this (a 1:1 ratio).  Subtract this from the original moles of Ca²⁺, and you'll come up with the moles of Ca²⁺ actually in the milk.  Use the volume of milk you put in to the conical flask to figure out the concentration of Ca²⁺.