[Unclenick]

Citric acid at 5% by weight with water (7 oz dissolved in a gallon of water is close enough) is an old Frankford Arsenal formula. You can use it in the ultrasonic until it stops working. I usually add just a drop or two of dishwashing liquid as a wetting agent.

Citric acid will be sold in your neck of the woods by wine making supply outfits and can probably be ordered through a pharmacy (apothecary, chemist's), but those are likely the most expensive options. Some canning supply outfits sell it, here, so I assume they will where you are. It's also available in Kosher grocery stores as Sour Salt. I am able to get 10 lbs of food grade citric acid for about $30 here, or just under 2£/lb., to give you an idea of a reasonable price, but I have to get the 10 lb quantity to get the price that low. It's enough for about 23 gallons of solution.