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Confused on how to calculate ammonium phosphate buffer molarity
quddit32:
Deeply appreciate your help in advance
I'm a bit confused about how to calculate the molarity of an ammonium phosphate buffer solution. Could you please provide some guidance?
1.If there is instruction that "prepare a 5.0 mM phosphate buffer solution by mixing 2.5 mM of ammonium phosphate monobasic and 2.5 mM of ammonium phosphate dibasic." However, since monobasic and dibasic contain different amounts of phosphate per molecule, I'm unsure if we should just add the moles of the two chemicals together to determine the final phosphate molarity. Could you clarify if statement 1 is correct?
2. If there is instruction that "prepare a 5.0 mM phosphate buffer solution by adding 5mM of ammonium phosphate monobasic and then adjust the pH to x.x with H3PO4 solution", can we still call this solution a 5.0 mM phosphate buffer? My concern is that we don't know how much H3PO4 solution is added. It doesn't matter for molarity calculation?
Borek:
--- Quote from: quddit32 on March 17, 2024, 10:37:35 PM ---since monobasic and dibasic contain different amounts of phosphate per molecule
--- End quote ---
They contain the same amount of phosphate per mole of substance, but different amounts of ammonia.
--- Quote ---If there is instruction that "prepare a 5.0 mM phosphate buffer solution by adding 5mM of ammonium phosphate monobasic and then adjust the pH to x.x with H3PO4 solution", can we still call this solution a 5.0 mM phosphate buffer?
--- End quote ---
Depends on the concentration of the phosphoric acid, if it is 0.005 M final concentration will be still 5 mM. If it is different final concentration of the buffer can be (almost) anything.
quddit32:
--- Quote --- Depends on the concentration of the phosphoric acid, if it is 0.005 M final concentration will be still 5 mM. If it is different final concentration of the buffer can be (almost) anything.
--- End quote ---
However, the instruction says that initial prep already added 5 mM of phosphate salt, and we add more phosphoric acid to adjust the pH - can we still say it is 5 mM phosphate buffer
Borek:
If you mix 10 mL of 5 mM phosphate with 10 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix 10 mL of 5 mM phosphate with 20 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix x mL of 5 mM phosphate with y mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
edit: you do understand "phosphate" in a the context of a buffer means ALL forms of HnPO4(3-n)-?
quddit32:
--- Quote from: Borek on March 18, 2024, 08:05:51 AM ---If you mix 10 mL of 5 mM phosphate with 10 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix 10 mL of 5 mM phosphate with 20 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix x mL of 5 mM phosphate with y mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
edit: you do understand "phosphate" in a the context of a buffer means ALL forms of HnPO4(3-n)-?
--- End quote ---
So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
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