# How to make a sodium carbonate/sodium hydrogen carbonate buffer

• chem_is_lovex
In summary, to make a 100mL buffer solution with a pH of 10.8 using only sodium carbonate, sodium hydrogen carbonate, and water, you need to calculate the ratio of concentrations of carbonate and hydrogencarbonate, and then use that ratio to determine the concentrations of each compound. From there, you can calculate the amounts of sodium carbonate and sodium hydrogen carbonate needed to prepare the desired buffer solution.
chem_is_lovex

## Homework Statement

"Plan how to make 100mL of a buffer solution with a pH of 10.8 to be made using only sodium carbonate, sodium hydrogen carbonate and water"

## Homework Equations

pH= pKa+ log(CO32-)/(HCO3-)

## The Attempt at a Solution

I tried rearranging it, but I'm not sure what to do to find both of the concentrations...

thanks :)

Calculate ratio of concentrations of carbonate and hydrogencarbonate, see how you can obtain this ratio mixing given reagents.

Borek said:
Calculate ratio of concentrations of carbonate and hydrogencarbonate, see how you can obtain this ratio mixing given reagents.

ok, I did that and I got the ratio to be 3.55. I then tried to find the conc. of H+, therefore the conc. of carbonate ions... is this how you're supposed to do it?

it also says "you should specify the amounts of sodium carbonate and sodium hydrogen carbonate that you would use"

thanks

No, once you know ratio of carbonate and hydrogencarbonate it is just a trivial calculation of concentrations - ignore dissociation and equilibrium, just assume you are preparing some volume of the buffer, you want it to be - say - 0.01M in carbonate, use the ratio you already know to calculate concentration of hydrogencarbonate, then calculate how much of each solid you need.

ok :) thanks! it was a lot simpler than I imagined it to be...

## 1. How do I calculate the correct ratio of sodium carbonate and sodium hydrogen carbonate for a buffer?

The ratio of sodium carbonate and sodium hydrogen carbonate in a buffer can be calculated using the Henderson-Hasselbalch equation. The equation is pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base (sodium carbonate) and [HA] is the concentration of the weak acid (sodium hydrogen carbonate). The pKa values for these compounds can be found in a chemistry reference book or online. Once you have the pKa values, you can use them to calculate the ratio of sodium carbonate to sodium hydrogen carbonate needed for your desired pH.

## 2. What are the best methods for preparing a sodium carbonate/sodium hydrogen carbonate buffer?

There are a few different methods for preparing a sodium carbonate/sodium hydrogen carbonate buffer. One common method is to dissolve the desired amounts of sodium carbonate and sodium hydrogen carbonate in water and adjust the pH using a strong acid or base. Another method is to add a strong acid or base to an existing buffer solution until the desired pH is reached. It is important to use high purity chemicals and accurate measurements when preparing buffers.

## 3. Can I use other salts in addition to sodium carbonate and sodium hydrogen carbonate to prepare a buffer?

Yes, other salts can be added to the buffer solution to adjust the ionic strength or to provide additional buffering capacity. However, it is important to consider the potential interactions between the added salts and the sodium carbonate and sodium hydrogen carbonate. It is also important to ensure that the added salts will not significantly alter the desired pH of the buffer.

## 4. How do I store a sodium carbonate/sodium hydrogen carbonate buffer?

Buffers should be stored in airtight containers to prevent contamination and evaporation. It is best to store buffers at 4°C to slow down any chemical reactions that may occur. Before using a stored buffer, it is important to check the pH and adjust if necessary.

## 5. Can I reuse a sodium carbonate/sodium hydrogen carbonate buffer?

Buffers can be reused multiple times as long as they are stored properly and have not been contaminated. However, it is recommended to make a fresh buffer solution for each experiment to ensure accuracy and avoid potential errors. If reusing a buffer, it is important to check the pH and adjust if necessary before use.

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