[mores1365]

how does citric acid (C6H8O7)and sodium phosphate (Na2HPO4) react with each other to act as a buffer system, suppose that we have a buffer solution consists of 0.4365 g Na2HPO4(41 mM ) and 1.252 g citric acid(86.9 mM ) to make 75 mL buffer with PH=3. if we add 10 mL KOH 1.5 M to the solution, how do the chemical and subsequently the PH change?

Thank you!

[AWK]

Hint
citric acid and disodium hydrogenphosphate do not form a buffer solution directly - buffer solution is formed after reaction between them.

Show some  your calculations

[mores1365]

Hi,

unfortunately, i am not from chemistry field. i am working on a project which i have a solution consists of citric acid mixed with na2hpo4 to form a 75 mL solution with initial PH=3 . now we want to add KOH 1.5 M to increase the PH to another point(like 6.5 or 9, etc) so we need to know how we can predict the PH change by adding defined volume of KOH.if you need other information i can tell you,

thanks for replying

[Borek]

This is not a trivial calculation to do. If I may suggest something - check out this program:

http://www.chembuddy.com/?left=Buffer-Maker&right=pH-calculator

[mores1365]

Thanks a lot

but i just gives me my initial condition, namely the PH when i mix citric acid and NA2HPO4. but i want to find a way to recognize PH after adding KOH 1.5 to the mixture which seems impossible for this software.

[AWK]

I do not know  how exactly Borek's program work, but I can see that program has option "add reagent" and addition of OH- (KOH) is possible.

[Borek]

I guess you downloaded the program and started the buffer calculator. There are two calculators in the program - one is a buffer calculator (which will calculate how to prepare pH 3.0 buffer from your reagents), but there is also a pH calculator (see the page I linked to) which allows calculation of pH of any mixture (so it can be used to check your case - after adding 10mL of KOH solution; actually I did it yesterday). Note you will need to trick the program into calculating correct concentrations of all substances present. I can show you how.

[mores1365]

Yes , i downloaded the software. my problem in using it is that, when i mix the initial components(citric acid and NA2HPO4) with correct molarities and mass i get a correct result(PH=2.99), but when i want to add another reagent like 10 ml KOH1.5M , the program changes the molarity after defining the volume from 1.5 M to another value.would you please show me how to use this program correctly?

[Borek]

This is a little bit tricky, because you have data of several types - amounts of some substances present in the final solution are given as masses, amount of KOH is given as volume and concentration. Program accepts input either as volumes and concentrations (it sums volumes then, calculating final concentrations before calculating pH), or as given concentrations (volume is ignored then)*, so we need to use a little trick.

Initially you have 75mL of solution containing 0.4365 g Na2HPO4 and 1.252 g citric acid. If we enter these things as 30 mL of solution containing 0.4365 g Na2HPO4 and 45 mL of solution containing 1.252 g citric acid we will get exactly 75 mL of the solution containing entered masses as the first stage.

Start the pH calculator, click database button, select disodium hydrogenphosphate, click use button, click on the C button to start concentration calculator, enter mass (0.4365) and volume (30 mL) - concentration (0.1025 M) is calculated automatically. This is not the final concentration after mixing yet, don't worry about it now. Click use button. Program calculates pH of the 0.1025 M solution of disodium hydrogenphosphate (should be 9.04).

Click Add reagent button. Click database button, select citric acid. Again start concentration calculator, enter mass of 1.252 g and volume of 45 mL, click use button. At the moment pH is calculated as 3.01, but it is not yet correct - as concentrations were copied just like they were calculated by the concentration calculator, and we know both citric acid and phosphoric acid were diluted after mixing. Find solution volume edit field - to the right there is a drop down list showing "ignore". Select "sum volumes" instead.  Solution volume is listed now as 75 mL (30+45) and all concentrations were recalculated accordingly. pH is calculated as 2.99 - and while it is not exactly 3.00 is it the best value you can get from the equilibrium calculations.

Click Add reagent button again. This time you can't use database, as KOH is not listed as a reagent (it can be easily added, but we won't waste time on it now). Just enter KOH as formula and potassium hydroxide as name. While KOH is not present in the reagent list, program knows its dissociation constant, so it is automatically properly recognized. Don't start concentration calculator, you don't need it this time, just enter 1.5 as a concentration and 10 as a volume. Done & ready, what you have is your 85 mL of the final solution with KOH added.

Note that this final solution has an ionic strength of 0.44 - that means final result is most likely slightly off. This is not a limitation of the program, but of the theory. There is not much than can be done about it.

*Actually it is also possible to enter the final volume as if the solution was diluted after mixing, but it won't help us here either.

[mores1365]

WOW, thank you sir,

so far, we can get PH value from program.
i need to know the calculation technique which program uses. i tried to understand the equation which program shows in following link:

http://www.chembuddy.com/?left=Buffer-Maker&right=buffer-calculation

but, would you elaborate the example of adding 10 ml KOH 1.5M to the existing solution by using the equation(i mean explaining step by step the calculations result in the PH value after adding  KOH). i actually want to see how PH can be calculated by the equation.i really appreciate your kind help

[Borek]

Sorry, but it is not that easy.

I mean - program solves the equation listed numerically, so it is not possible to show how it is done step by step, this is not some easy algebra. First step is to calculate all C_a values (usually from simple dilutions, or they are just given). All K_a values have to be known (they are present in the program database and can be added there if necessary). Then the equation is solved numerically using Newton–Raphson method. Following these calculations manually - while technically possible - will take hours at best.

[mores1365]

Thank you Sir for your explanation,

but, is it possible to find some one who can tell me the step by step equations? because i need to use the calculation algorithm in my program in Labview, so if i know the calculation process , i can do complete my Labview program.

[Borek]

Equations are explained on the page you have seen. To be honest, if you have problems following them, I doubt you will be able to implement the calculations.

[mores1365]

yes, you are true,honestly, i need one example to understand the equation, and actually it was the reason i asked you to explain the equation for me, because in some parts of equation, i dont know what should i substitute for one parameter and more important is to how use the equations when i add KOH to the mixture  would you please elaborate one example for using the equations?

[AWK]

All these calculations you can do step by step using calculator.
Start from all Ka values of citric and phosphoric acid and calculate stoichiometry of replacing reaction (stronger acid replaces the weaker - in each stoichiometry calculation step replace on hydrogen of citric acid or hydrogen citrate). Finally you can obtain two buffer solution with the same pH based on citrates and phosphates. Sometimes on of them can be neglected in calculation of pH. This need not more than 15 minuts and calculator. The same concerns addition of KOH.