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What is The recommended concentration of acidulated phosphate (APF) ﬂuoride gel is: and explain in detail?
An aqueous solution of acidulated phosphate fluoride is prepared by dissolving 20 grams of sodium fluoride in 1 liter of 0.1 M phosphoric acid and to this is added 50% hydrofluoric acid to adjust the pH at 3.0 and fluoride ion concentration at 1.23%.
It is also called as Brudevold's solution.

Essentials of preventive and community dentistry
Soben Peter
5th edition
page no. 258