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Topic: 4 part ? What is the amount of sodium disassociated from tri sodium citrate?  (Read 2682 times)

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Offline born2dive00

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Ok I am trying to understand this with a definitive answer. There are 4 parts to this question, and I am no chemist (i have a very very basic understanding of chemistry so please be as simple as possible in the answers)

Part 1
If i understand my readings correctly Trisodium citrate will disassociate X amount of sodium as the PH decreases.

So to start i took water that had a pH of 7.2 and added trisodium citrate to it till it could not dissolve any more 100% saturation the pH measured 8.1
So using this as my starting point, I assume that at a pH of 8.1 i have 100% Na3C6H5O7 with Zero disassociation of any of the sodium would this be correct?

Which scenario is correct

Scenario 1
Ph/ pKa: 6.401 i will have it will have (0 amount of) Na3C6H5O7, and (100%) Na2C6H6O7 as the sodium will disassociate, at pH /pKa 4.76, there is (Zero amount of) Na2C6HO7 and (100%) NaC6H5O7 and at A Ph/pKa of 3.138 (Zero amount of) NaC6HO7 and (100%) C6H5O7 is my assumption correct?  OR DO I HAVE THIS IN REVERSE?

Scenario 2
So if the Ph/ pKa: is at 3.138 it will have (0 amount of) Na3C6H5O7, and (100%) Na2C6H6O7, at pH /pKa 4.76, there is (Zero amount of) Na2C6HO7 and (100%) NaC6H5O7, and with a pH/pKa of 6.401 there is (Zero amount of) NaC6HO7 and (100%) C6H5O7 and all the sodium will be disassociated.

My gut reaction is this is opposite of a titration of sodium hydroxide and Citric acid where more disassociation occurs as the pH increases, as this is a salt not an acid.

Part 2. If scenario 1 is correct, at what pH will the Na3C6H5O7 start to dissociate sodium, it would have to have a pH/pKa much lower than the 3.138

Part 3. Assuming Scenario 1 is accurate, How much sodium will dissociate at the following pH of 2.85, 2.90, 2.95, and 3.0 3.05, 3.1? I need a way to calculate what percentage of sodium is dissociated

I know that there is mid points so for example if we take 4.76 and subtract 3.138 we get 1.622 and if we divide that by 2 we get .811 so if we add 3.138 + .811 we would get a pH/pKa of 3.949 so at this point we should have a mixture of 50% NaC6HO7 and 50% C6H5O7. But since it is not linear, and there is a curve as you get closer to the end points, how do i calculate the amount of disassociation at a given pH

Part 4. If I am using say phosphoric acid to adjust the Ph, does the dissacioated sodium combine to form sodium phosphates?

Please let me know
Thank you all for your help.

Offline Borek

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You can safely assume salt is 100% dissociated into Na+ and whatever anion is there, then pH is effect of hydrolysis of the anion.
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Offline born2dive00

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So you are saying that All the sodium is disconnected from the citrate when in an aqueous solution? regardless of the pH?
can you explain this more for clarity

You can safely assume salt is 100% dissociated into Na+ and whatever anion is there, then pH is effect of hydrolysis of the anion.

Offline chenbeier

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Part 1: No,

sodium citrate in water is dissociated, like other salts do if dissolved in water. So you have sodium ions and citrate ions. You have 3 equilibrium between all species of ions.

Cit3-  + H2O. <=> HCit2-  + OH-
HCit2- + H2O <=> H2Cit- + OH-
H2Cit2- + H2O <=> H3Cit  + OH-

The negative charges compensated by Na+.

Calculations can be done with Henderson Hasselbalch equation.



Offline Vidya

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Ok I am trying to understand this with a definitive answer. There are 4 parts to this question, and I am no chemist (i have a very very basic understanding of chemistry so please be as simple as possible in the answers)

Part 1
If i understand my readings correctly Trisodium citrate will disassociate X amount of sodium as the PH decreases.

So to start i took water that had a pH of 7.2 and added trisodium citrate to it till it could not dissolve any more 100% saturation the pH measured 8.1
So using this as my starting point, I assume that at a pH of 8.1 i have 100% Na3C6H5O7 with Zero disassociation of any of the sodium would this be correct?

Which scenario is correct

Scenario 1
Ph/ pKa: 6.401 i will have it will have (0 amount of) Na3C6H5O7, and (100%) Na2C6H6O7 as the sodium will disassociate, at pH /pKa 4.76, there is (Zero amount of) Na2C6HO7 and (100%) NaC6H5O7 and at A Ph/pKa of 3.138 (Zero amount of) NaC6HO7 and (100%) C6H5O7 is my assumption correct?  OR DO I HAVE THIS IN REVERSE?

Scenario 2
So if the Ph/ pKa: is at 3.138 it will have (0 amount of) Na3C6H5O7, and (100%) Na2C6H6O7, at pH /pKa 4.76, there is (Zero amount of) Na2C6HO7 and (100%) NaC6H5O7, and with a pH/pKa of 6.401 there is (Zero amount of) NaC6HO7 and (100%) C6H5O7 and all the sodium will be disassociated.

My gut reaction is this is opposite of a titration of sodium hydroxide and Citric acid where more disassociation occurs as the pH increases, as this is a salt not an acid.

Part 2. If scenario 1 is correct, at what pH will the Na3C6H5O7 start to dissociate sodium, it would have to have a pH/pKa much lower than the 3.138

Part 3. Assuming Scenario 1 is accurate, How much sodium will dissociate at the following pH of 2.85, 2.90, 2.95, and 3.0 3.05, 3.1? I need a way to calculate what percentage of sodium is dissociated

I know that there is mid points so for example if we take 4.76 and subtract 3.138 we get 1.622 and if we divide that by 2 we get .811 so if we add 3.138 + .811 we would get a pH/pKa of 3.949 so at this point we should have a mixture of 50% NaC6HO7 and 50% C6H5O7. But since it is not linear, and there is a curve as you get closer to the end points, how do i calculate the amount of disassociation at a given pH

Part 4. If I am using say phosphoric acid to adjust the Ph, does the dissacioated sodium combine to form sodium phosphates?

Please let me know
Thank you all for your help.
All sodium compounds are soluble and it will undergo 100% dissociation and citrate ions are hydrolyzed as they are stronger base than water.
Your question is how much amount of sodium ....one mole of trisodium citrate gives 3 moles of sodium ions .

Offline born2dive00

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Thank you for this explination Chenbeier I now get this part with the citrate and the OH.

Now what happens to the Sodium that is "floating" around when phosphoric acid is added? does it form Trisodiumphosphate Na3PO4? and or why does the sodium bind with the OH- to form sodium hydroxide?


Part 1: No,

sodium citrate in water is dissociated, like other salts do if dissolved in water. So you have sodium ions and citrate ions. You have 3 equilibrium between all species of ions.

Cit3-  + H2O. <=> HCit2-  + OH-
HCit2- + H2O <=> H2Cit- + OH-
H2Cit2- + H2O <=> H3Cit  + OH-

The negative charges compensated by Na+.

Calculations can be done with Henderson Hasselbalch equation.

Offline born2dive00

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So if I understand everyone correctly, at a 100% saturated solution of water and Trisodiumcitrate  at a pH of 8.1 all of the sodium will be disassociated and we will have the following
Cit3-  + H2O. <=> HCit2-  + OH-
HCit2- + H2O <=> H2Cit- + OH-
H2Cit2- + H2O <=> H3Cit  + OH-

The negative charges compensated by Na+.

which means scenario 2 is accurate?
with a pH/pKa of 6.401 there is (Zero amount of) NaC6HO7 and (100%) C6H5O7 and all the sodium from the trisodium citrate will be disassociated.
at pH /pKa 4.76, there is (Zero amount of) Na2C6HO7 and (100%) NaC6H5O7
and when Ph/ pKa: is at 3.138 it will have (0 amount of) Na3C6H5O7, and (100%) Na2C6H6O7, meaning all the the loose sodium that is floating around in the water will rebind back to the citrate

Now as the ph decreases by the addition of phosphoric acid, to the following pKa values 6.41, 4.76, 3.138 means that the sodium will start to rebind with the citrate? Is this correct? or will the loose sodium bind to the phosphoric acid first to form trisodium phosphate?

Offline chenbeier

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In the solution the ions citrate and phosphate will exist seperat to the sodium.  It will only form sodiumcitrates and sodiumphosphates if the water will be evaporated.

Offline born2dive00

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Thank you ChenBeier for this help you are painting a much clearer picture of what is going on.

So let me get this strait. so regardless of the pH of the water due to the phosphoric acid, the sodium from the tri sodium citrate will disassociate 100% when it is in a solution of water. is this correct

And as you stated the sodium will not react with the citric acid/citrate nor will it react with the phosphoric acid UNLESS the water is evaporated. correct

So what/ and when do I concern myself with the disassociation of the sodium at the at the pKa values?

In the solution the ions citrate and phosphate will exist seperat to the sodium.  It will only form sodiumcitrates and sodiumphosphates if the water will be evaporated.

Offline born2dive00

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But when I took the Trisodium citrate and added water and made it a 100% saturation, it was slightly base with a ph of 8.1 so doesnt that mean that there would be some phosphoric acid reacting with the TSC or is there something else causing it to be slightly base? un reacted sodium hydroxide?

In the solution the ions citrate and phosphate will exist seperat to the sodium.  It will only form sodiumcitrates and sodiumphosphates if the water will be evaporated.

Offline Borek

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You were already told: citrate anion reacts with water producing OH-, that's why pH goes slightly up.
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