# Preparing a 700mL PBS Buffer Solution: Step-by-Step Guide

• WSR
In summary, to prepare 700 mL of PBS with a pH of 7.40 and concentrations of 25mM phosphate and 140 mM NaCl, you can use the Henderson-Hasselbalch equation and two equations in two unknowns to determine the concentrations of Na2HPO4 and NaH2PO4 needed. You can then dilute the original solutions to produce the desired volume of 700 mL. For the separate question, using the Henderson-Hasselbalch equation, the protonated form of x will be at a pH of 7.89.
WSR
Question

You are asked to prepare 700 mL of PBS (phosphate buffered saline). The PBS needs to be pH 7.40 with 25mM phosphate and 140 mM NaCl. You are given a bottle of NaCl (58.45 g/mol), 1 M Na2HPO4 stock solution and a 1.00 M NaH2PO4 stock solution. ( H3PO4 pk1=2.12, pk2=7.21, pk3=12.66) Describe how to prepare this buffer.

Equations

I'll use the Henderson-Haeelbalch Equation so pH= pka +log(base/acid)

The thing that really screws me up in this problem is how to incorporate the M concentrations. The pH has to be 7.40 so 7.40 = pka + log (base/acid). I don't know where to go with the concentrations.

Also, a separate question that I don't know if I did correctly.
At what pH will 62% of x be in the protonated form if pka = 8.10. So I did, pH= pka +log(base/acid) and got pH= 8.10 + log (.38/.62) with an answer of 7.89. Does that makes sense?

I'd appreciate any guidance.

Thanks

You know sum of concentrations of hydrogen and dihydrogen phosphates must be 25 mM. That, plus Henderson-Hasselbalch equation give you two equations in two unknowns - solve for both concentrations. Then calculate how much of the original solutions you need to dilute to produce 700 mL of the PBS.

Approach to the other question looks OK, I haven't checked numbers.

## What is a buffer in biochemistry?

A buffer in biochemistry is a solution that is able to resist changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

## Why are buffers important in biochemistry?

Buffers are important in biochemistry because they help maintain a stable pH, which is critical for the proper functioning of enzymes and other biological molecules. Without buffers, small changes in pH could significantly alter the structure and function of these molecules.

## How do you prepare a buffer solution?

To prepare a buffer solution, you need to mix a weak acid or base with its conjugate base or acid, respectively. The ratio of the two components should be such that their concentrations are in a 1:1 ratio. You can then adjust the pH of the solution by adding small amounts of a strong acid or base, if necessary.

## What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation is a mathematical relationship that describes the pH of a buffer solution. It is expressed as pH = pKa + log ([A-]/[HA]), where pKa is the acid dissociation constant of the weak acid, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.

## How do you choose the appropriate buffer for a specific experiment?

The appropriate buffer for a specific experiment depends on the desired pH range and the pKa of the weak acid or base. You should choose a buffer with a pKa that is as close as possible to the desired pH. Additionally, you should consider the compatibility of the buffer with the other components of your experiment, such as enzymes or proteins.

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