Boiling rhubarb leaves for aphid spray

I have read about this too. You must wear gloves when you make the spray, and do it in a pot that won’t be used again in the kitchen due to the toxicity of the oxalic acid in the leaves. In the pot, boil 1.5kg chopped rhubarb leaves with 30g soft soap in 3.5 litres water. Boil this for about half an hour and strain and cool well before you spray onto the plants. I understand that the mix doesn’t last long so you need to use it within a day or two of making it.