Citric acid question

[Mrs Ava]

Sorry if this has been asked before.  I am trying to use up the glut of last years frozen fruit before this years begins and have jammed and chutneyed so I am now making fruit cordial.  I have been trying to find how much citric acid is needed to preserve the cordials, I don't have fridge space for masses of bottles.  My first small attempt it rhubarb cordial and I only have 500ml of liquid before adding the sugar.  Any help would be gratefully received. 

[Mrs Ava]

[BarriedaleNick]

In my elderflower cordial I add 55g of citric acid to 3 pints of water - not sure if that works across the board though...