Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: cc19 on August 23, 2012, 12:01:23 PM
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I am trying to figure out the possible effects of saline (0.9% NaCl solution) on chelation of Ca2+ ions by citric acid and trisodium citrate in solution.
I am wondering if the Na+ ions prevent the citrate from chelating the Ca2+ ions that it's supposed to chelate.
(In my work, it's not chelating all of the Ca2+, even though there should be enough)
My questions:
1) Would the citrate chelate Na+ ions in the saline? (therefore, there isn't enough available to chelate the Ca2+ ions?)
2) Does trisodium citrate disassociate into 3 Na+ ions and a citrate? (thus putting more Na+ ions in the solution)
3) Or, does the citrate form more stable complexes with Ca2+ than Na+, therefore, the sodium in solution doesn't matter?
Thank you for your time in advance.
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(In my work, it's not chelating all of the Ca2+, even though there should be enough)
How high is the Ca2+ concentration?
What is pH of your solution?
Have you checked the stability constant?
Is the concentration of Ca2+ higher than one should expect from equilibrium calculations?
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Borek,
Thank you for the help,
To be more specific, I am looking at the anti-coagulation of whole blood vs saline-diluted blood, wherein anti-coagulation of the saline-diluted blood with citrate-based anticoagulant does not work well for some reason. (don't worry, biochemistry shouldn't be much of an issue here).
I would like to rule out the saline as a possible reason, therefore the questions in the original post.
I am using citrate phosphate dextrose anticoagulant with the diluted blood, in the same ratio (1:7) as whole blood. (unknown how diluted the blood is)
The mechanism of action seems basic- citrate in the anticoagulant solution chelates the Ca2+ ions in blood, which are critical to the coagulation cascade.
Ca2+ exists in blood in the concentration "6.7-10.7 ... (mg/dL) or 1.90-2.75 ... (mmol/L)", and blood pH is around 7.4
The anticoagulant contains 15.5mmol/L citric acid and 89.9mmol/L trisodium citrate. (2.99g @ 192.124 g/mol (anhydrous) and 26.3g @ 294.10 g/mol (dihydrate), for citric acid and trisodium citrate (sodium citrate (dihydrate), respectively, per liter).
Therefore, combined in the recommended 1:7 ratio, this means 15.5mmol citric acid and 89.9 mmol trisodium citrate per 13.3-19.25 mmol Ca2+ in blood (for whole blood). There should be even less Ca2+ for diluted blood.
Seems more than enough, unless the the Na+ from the saline somehow had more of an effect... (Na2+ concentration would be 154mmol/L for 0.9% saline (9g NaCl into 1 litre).
(Na+ concentration in blood is "136-145 ... (mEq/L) or 136-145 ... (mmol/L)" which is not too different.)
I am still searching for stability constants, as they are difficult to find for sodium, calcium, and citrate...
A previous forum post on an unrelated subject (difficulty finding now) mentioned something about "I know that Na+ doesn't [affect] chelation" but I'm not sure if it applies in this case.
(I suppose there could be other factors involved as well too, I am looking into these)
Therefore, the questions in the original post.
Any help or thoughts would be appreciated, again, thanks in advance.
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Looking into this further,
I believe I have eliminated saline as a possible reason for the failure to chelate Ca2+ ions.
Na+ seems to be unstable enough with citrate that it likely has negligible effect in preventing Ca2+ chelation by citrate.
Question answered, topic is concluded.
Moderator, please remove this topic or last post to save people's time of reading now-moot information.
Thanks all,
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Na+ compounds are highly soluble and has very less chelation effect due to large size and small charge .Ca2+ will form more stable chelating complexes because of its small size and high charge.
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Uma,
Indeed, I had suspected such, however is only the second time I have seen it mentioned anywhere online.
Thank you for your input!