# Adjusting PBS with HCl: pH drift after dilution of phosphate buffer

It is some kind of everyday lab science and with a feel of shame I cannot answer this observation.

I noticed a pH drift (~0.5 units) of my 10X phosphate buffer (PBS), when I adjust the pH with $\ce{HCl}$ and dilute the 10X buffer afterwards. When I prepare another buffer by mixing $\ce{Na2HPO4}$ and $\ce{NaH2PO4}$ to achieve the desired pH value, there is virtually no drift after dilution. If using carbonate buffer, $\ce{CO2}$ is created if I set the pH with $\ce{HCl}$, but none with the also acidic $\ce{NaH2CO3}$ is used.

I just want to know how the phosphate buffer system is working and why there is a pH drift after dilution if I use $\ce{HCl}$ to set the pH. This might answer the general question, why adjusting the pH with $\ce{HCl}$ in PBS is not suggested.

I feel there could be something wrong with the counter ion maybe?!

• Do you know what kind of electrode you're working with? – John Snow Sep 13 '14 at 0:14
• It's not clear in your question whether you are trying to prepare 1X PBS from an existing 10X PBS or you are preparing 10X PBS from its constituting salts? Please precise! – Yomen Atassi Sep 15 '14 at 8:16
• "I noticed a pH drift (~0.5 units) of my 10X phosphate buffer (PBS), when I adjust the pH with HCl and dilute the 10X buffer afterwards" It means exactly what I wrote. I prepare 10X and dilute it to 1X. Then I notice a pH drift or not. Dependent on how I prepared the 10X PBS. – dgrat Sep 15 '14 at 11:22
• How much HCl are you using? Could the amount affect ionic strength of the solution? – Abel Friedman Oct 18 '14 at 0:53

The differences between PBS and the buffer obtained by mixing $\ce{Na2HPO4}$ and $\ce{NaH2PO4}$ are in ionic strength ( because of the presence of $\ce{KCl}$ d $\ce{NaCl}$ in PBS) and in the ratio of [acid]/[conjugated base]. We have to signal that the buffer capacity (buffer effectiveness) is maximum when $\ce{pH=pKa}$. The best way to dilute 10x PBS into 1PBS (let's assume we want to prepare 1L of 1xPBS from 10xPBs) is to begin by diluting 100 mL of 10xPBS solution with 800mL water, then adjusting the pH and finally completig the volume to 1000mL. In doing so, there will be no drift in pH.