Steve You might want to switch to Fotospeed stop. It is acetic acid and is diluted at 1+29 so a one litre container makes 30 litres which makes it very cheap - even cheaper than some lower priced stop such as Champion which needs a greater ratio of stop to water. Agphotographic sells it. I have used Fotospeed stop for several years in both colour RA4 and B&W printing without problems.

Pure citric acid powder may be OK but I'd alway be dubious about what else goes into household vinegar. Great for chips but maybe not so good for RA4 paper