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Author Topic: Which is more reactive with sodium hydroxide  (Read 1862 times)

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born2dive00

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Which is more reactive with sodium hydroxide
« on: December 29, 2018, 01:56:44 AM »

Ok i am trying to figure out what would form first i.e. which is more reactive

If I have a solution of say 500 ml of water, and add say 10 mg of 85% phosphoric acid, and 10 mg of citric acid, and then added say 10 mg of sodium hydroxide,

what would form first? sodium phosphate or sodium citrate ??

From what I have read and understand is that the enthalpy determines the reactivity of a substance, a lower number means it is less likely to react, and a higher number means it is more reactive.

Phosphoric acid has a enthalpy of −1288 kJ/mol and the citric acid has a enthalpy of −1548.8 kJ/mol[2]

so would I be correct in assuming that the citric acid would react first with the sodium hydroxide to form trisodium citrate before the phosphoric acid would react? or would the phosphoric acid react first?

Please let me know

Thank you for your help

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chenbeier

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Re: Which is more reactive with sodium hydroxide
« Reply #1 on: December 29, 2018, 02:45:46 AM »

No,
In the solution you have H+ Ions and Phosphate and citrate anions. The H+ reacts with OH- to water. Left are sodium ions which can form either (hydrogen)citrates and (hydrogen)phosphates.
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Babcock_Hall

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Re: Which is more reactive with sodium hydroxide
« Reply #2 on: December 29, 2018, 06:12:29 AM »

My first thought was to look at the pKa values.  The general problem is made somewhat more complex because both phosphoric acid and citric acid are triprotic, and therefore each has three pKa values.
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born2dive00

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Re: Which is more reactive with sodium hydroxide
« Reply #3 on: December 29, 2018, 11:26:11 AM »

Babcock,

The PKa value of phosphoric acid is (PKa 1) is 2.16 (PKa 2) is 7.21 and (PKa 3) is 12.32
The PK1 value of Citric acid is (PKa1) is 3.13 (PKa2) is 4.76, and the (PKa 3) is 6.40

Now I know that the desired pH of the solution is 3 so how would this determine which chemical compound is formed first?

I made a small mistake above the Enthalpy rule states
The more negative the enthalpy of formation, greater will be the stability of the compound formed. And, lower will be its tendency to react in a given environment.
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born2dive00

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Re: Which is more reactive with sodium hydroxide
« Reply #4 on: December 29, 2018, 11:29:52 AM »

Chenbeier
So how do i determine which compound is formed first? one chemical i.e. phosphoric acid or citric acid one or the other should form a compound first so how do i determine which one is formed first.

Thanks

No,
In the solution you have H+ Ions and Phosphate and citrate anions. The H+ reacts with OH- to water. Left are sodium ions which can form either (hydrogen)citrates and (hydrogen)phosphates.
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Babcock_Hall

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Re: Which is more reactive with sodium hydroxide
« Reply #5 on: December 30, 2018, 04:39:07 AM »

Do you want to know which reaction is fastest, or do you want to know what is the equilibrium mixture at pH 3?  Chenbeier provided an answer to the first question.
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born2dive00

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Re: Which is more reactive with sodium hydroxide
« Reply #6 on: December 30, 2018, 06:07:18 AM »

ultimatly i would like to know how much di or tri sodium phosphate is formed and tri or di sodium citrate is formed at a pH of 3 preferably as a percentage.

This is what I know I have 4.799 ml of phosphoric acid in 5 gallons of water.
added to this is 3343mg of sodium hydroxide

what is un known is how much citric acid is needed to bring the ph to 3 and if there is a preference of sodium hydroxide to v=favor a reaction with citric acid vs phosohoric acid.

there is also 1889.502453 mg of caffeine to consider as i believe citric acid reacts with caffeine to form caffeine citrate as the caffeine acts as a base as i understand it

I believe that the equlibirum of phosphoric acid with the sodium hydroxide is what I need. If i understand it correctly Phosphoric acid will react at a ph of 3 to form di and mono sodium phosphate. the excess sodium hydroxide will bind with the citric acid to form Tri sodium phosphate at a PH of 3, and the remaining citric acid will react with the caffeine to form caffeine citrate, But what I am un certain of is if the citric acid is increasing the ph of the mixture, or if it is the phosphoric acid.

Please give me a hand Babcock.
Thank you
Brad



Do you want to know which reaction is fastest, or do you want to know what is the equilibrium mixture at pH 3?  Chenbeier provided an answer to the first question.
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Babcock_Hall

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Re: Which is more reactive with sodium hydroxide
« Reply #7 on: December 30, 2018, 07:17:54 AM »

This is not exactly my area of expertise, but I think you need to apply the Henderson-Hasselbalch equation for a quantitative answer.  At a pH of 3, the concentration of trisodium phosphate will be very, very small, based on the value of pKa3.  The same is true of trisodium citrate. 

But let me indicate one way to think about this problem qualitatively.  If I mix (for example) citric acid and trisodium phosphate, there will be proton exchange such that the strongest acid(s) and strongest base(s) are consumed.   
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Borek

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Re: Which is more reactive with sodium hydroxide
« Reply #8 on: December 30, 2018, 08:02:48 AM »

Best approach is to write equations describing all equilibria, all mass balances and the charge balance, then solve the system of equations.

http://www.chembuddy.com/?left=pH-calculation&right=general-pH-calculation

(alternatively - Buffer Maker will solve the system for you, although to make the solution of pH 3.0 you will need a bit of trial and error - prepare mix H3PO4/H3Citrate in the proportion they are present in your solution, then play with amount of NaOH added; shouldn't take more than a few minutes)
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born2dive00

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Re: Which is more reactive with sodium hydroxide
« Reply #9 on: December 30, 2018, 01:33:24 PM »

hello Babcock, So if i understand this correctlyat a ph of 3 citric and phosphoric acid are dissassociated from the tri configuration, to a Di configeration, and as the citric acid on the second and third  dissassociation are greater than the second and third dissassociation of phosphoric acid, the strong base of the Sodium hydroxide, will prefer to "bond" or be associated with the stronger acid of citric acid, Is this correct?

Thank you with your help with this
Brad

This is not exactly my area of expertise, but I think you need to apply the Henderson-Hasselbalch equation for a quantitative answer.  At a pH of 3, the concentration of trisodium phosphate will be very, very small, based on the value of pKa3.  The same is true of trisodium citrate. 

But let me indicate one way to think about this problem qualitatively.  If I mix (for example) citric acid and trisodium phosphate, there will be proton exchange such that the strongest acid(s) and strongest base(s) are consumed.
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born2dive00

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Re: Which is more reactive with sodium hydroxide
« Reply #10 on: December 30, 2018, 01:48:49 PM »

The problem Borek is in egypt where i am currently living i can not get food grade sodium hydroxide, i am currently visiting the UK, but the same issue arises, food grade sodium hydroxide is not readily avaiable here, besides even if it was i could not bring any on an commercial plane due to its corrosive nature. nor do i want to be screwing arround with sodium hydroxide with this, i much rather use di sodium phosphate in granular form and tri sodium citrate to get rround the need of reacting sodium hydroxide, and these are allowed on the plane in cargo by the carriers.

hence the reason i am trying to determine the amounts of sodium phosphate with the amount of sodium citrate. from what i understand is the tri sodium citrate will dissassociate 1 sodium at a ph of 3 to form di sodium citrate, and that extra sodium that is dissociated will move to bond with the phosphoric acid to form di sodium phosphate until such time that all the sodium is consumed.

Do i understand this correctly?

Best approach is to write equations describing all equilibria, all mass balances and the charge balance, then solve the system of equations.

http://www.chembuddy.com/?left=pH-calculation&right=general-pH-calculation

(alternatively - Buffer Maker will solve the system for you, although to make the solution of pH 3.0 you will need a bit of trial and error - prepare mix H3PO4/H3Citrate in the proportion they are present in your solution, then play with amount of NaOH added; shouldn't take more than a few minutes)
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Borek

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Re: Which is more reactive with sodium hydroxide
« Reply #11 on: December 30, 2018, 10:05:18 PM »

While I suggested to use NaOH in calculations doesn't mean you need to use NaOH when preparing the mixture. Think about it this way: it doesn't matter whether you add 1 mole of disodium phosphate or 1 mol of phosphoric acid and 2 moles of NaOH - the resulting solution is identical. There is an infinite number of combinations of citric acid/citrate salts/phosphoric acid/phosphate salts/NaOH that yield pH 3.0 solution. They all share some dependency of the amounts three main components - main components being citric acid, phosphoric acid and NaOH. As amount of NaOH needed is a function of the citric acid/phosphoric acid ratio, I suggested starting calculations with their mix and calculating amount of NaOH needed. Once you know these you can easily calculate what salts will bring the right result.

i much rather use di sodium phosphate in granular form and tri sodium citrate

Solutions of both these salts have a pH above 7, no way to mix them and get pH around 3. You may have better luck mixing phosphate with just citric acid.

The real question is what you are really trying to do. Do you need just a right pH, or some specific concentrations of citrate/phosphate, or some specific ratio of them, or just a a concentration of P in the solution? You have mentioned caffeine, so my guess is that you are trying to recreate some basic characteristic of a soft drink?
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Enthalpy

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Re: Which is more reactive with sodium hydroxide
« Reply #12 on: December 31, 2018, 12:21:20 AM »

[...] From what I have read and understand is that the enthalpy determines the reactivity of a substance [...]
This is wrong. Forget it.
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born2dive00

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Re: Which is more reactive with sodium hydroxide
« Reply #13 on: December 31, 2018, 01:01:14 AM »

hello Borek

i am trying to get a specific pH with 5 of 6 knowns
the knowns are 5 gallons of water.
the pH of 3 of the final product,
the amount of phosphoric acid  4.799 ml THIS AMOUNT CAN NOT BE EXCEEDED
the amount of sodium hydroxide 3343mg THIS AMOUNT CAN NOT BE EXCEEDED
and the amount of caffeine 1889.502453 mg THIS AMOUNT CAN NOT BE EXCEEDED

what i do not know is how much citric acid is needed to bring the pH to 3.
What i am not sure or is whether or not the citric acid is augmenting the pH or if it would be the Phosphoric acid that is changing the pH in this

Yes Borek you are correct this is for a soft drink, these values are directly from the nutrition label once converted from the eemental values to the compound values, by brother in law who is the regional manager for ConAgrafoods has confirmed that all the chemicals are used in soft drinks and the amounts of the simple compounds, i.e. phosphoric acid, sodium hydroxide... are accurate. what he can not confirm is the amounts of the complex compounds that are formed once added to the soft drink, i.e. phosphoric acid + sodium hydroxide which forms mono, di, or tri sodium phosphate depending on the pH. (which has replaced the emulsifier of gum arabic in dark softdrinks.

if i understand you correctly in what you say, in this formula it does not matter if the sodium compounds are of citrate or of phosphate, they will adjust by them selves. so for example if i use di sodium all of the sodium is used but i have excess of phosphorus if i use mono sodium phosphate i use up all the phosphosphorus but have excessive amount of sodium.

now if i add citric acid, the excess sodium /hydroxide will react with the citric acid to form sodium citrate. so i am trying to figure out how much sodium phosphate is formed and how much sodium citrate is formed, as well as if the caffeine citrate is formed and if so how much citric acid is needed to form the caffiene citrate, and or how much more is needed to change the pH to 3

i am trying to figure out exactly how much di sodium phosphate i need to add,
how much phosphoric acid i need,
how much sodium citrate i need
how much caffeine citrate is needed
how much citric acid i need


While I suggested to use NaOH in calculations doesn't mean you need to use NaOH when preparing the mixture. Think about it this way: it doesn't matter whether you add 1 mole of disodium phosphate or 1 mol of phosphoric acid and 2 moles of NaOH - the resulting solution is identical. There is an infinite number of combinations of citric acid/citrate salts/phosphoric acid/phosphate salts/NaOH that yield pH 3.0 solution. They all share some dependency of the amounts three main components - main components being citric acid, phosphoric acid and NaOH. As amount of NaOH needed is a function of the citric acid/phosphoric acid ratio, I suggested starting calculations with their mix and calculating amount of NaOH needed. Once you know these you can easily calculate what salts will bring the right result.

i much rather use di sodium phosphate in granular form and tri sodium citrate

Solutions of both these salts have a pH above 7, no way to mix them and get pH around 3. You may have better luck mixing phosphate with just citric acid.

The real question is what you are really trying to do. Do you need just a right pH, or some specific concentrations of citrate/phosphate, or some specific ratio of them, or just a a concentration of P in the solution? You have mentioned caffeine, so my guess is that you are trying to recreate some basic characteristic of a soft drink?
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Borek

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Re: Which is more reactive with sodium hydroxide
« Reply #14 on: December 31, 2018, 02:35:54 AM »

the amount of phosphoric acid  4.799 ml

Pure? Solution? What concentration?

Quote
What i am not sure or is whether or not the citric acid is augmenting the pH or if it would be the Phosphoric acid that is changing the pH in this

In a way - both. As I signaled earlier as long as you put the same amount of basic components the equilibrium composition of the solution will be the same. For example you can mix:
  • 1 mole of citric acid, 1 mole of phosphoric acid and 3 moles of NaOH
  • 1 mole of trisodium citrate and 1 mole of phosphoric acid
  • 1 mole of citric acid and 1 mole of trisodium phosphate
  • 1 mole of disodium citrate and 1 mole of monosodium phosphate
and many other combinations - and the final solution will be identical. You can't assign the final pH to action of any single acid, it is result of their interactions.

Quote
what he can not confirm is the amounts of the complex compounds that are formed once added to the soft drink, i.e. phosphoric acid + sodium hydroxide which forms mono, di, or tri sodium phosphate depending on the pH.

Equilibrium values can be calculated in a way I explained earlier.

Quote
if i understand you correctly in what you say, in this formula it does not matter if the sodium compounds are of citrate or of phosphate, they will adjust by them selves. so for example if i use di sodium all of the sodium is used but i have excess of phosphorus if i use mono sodium phosphate i use up all the phosphosphorus but have excessive amount of sodium.

now if i add citric acid, the excess sodium /hydroxide will react with the citric acid to form sodium citrate

No, it doesn't work this way. Acids are not neutralized to the end before others start to react, they react at the same time to the extent determined by their pKa values.

Quote
i am trying to figure out exactly how much di sodium phosphate i need to add,
how much phosphoric acid i need,
how much sodium citrate i need
how much caffeine citrate is needed
how much citric acid i need

You wrote you can't buy nor transport NaOH, but you list phosphoric acid - which is a highly corrosive liquid. I doubt you will be able to buy (and fly) concentrated food grade phosphoric acid, but the citric acid - which is a crystalline solid - shouldn't be difficult to find.
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