There was a very recent post on this topic 
https://chemistscorner.com/cosmeticsciencetalk/discussion/7475/how-to-use-buffers-in-cosmetics#latest

Basically if you know what pH you want your formula to be you can select a buffer that has a range encompassing your selected pH. Citric acid should be fine for 5.5, and with a little base (sodium hydroxide would work well) you can create the sodium citrate in situ.  

So make your toner and adjust pH at the end using a combination of your acid and base. You would then check the sample over a period of time to see if the buffer remains effective