The recommended concentration of acidulated phosphate (APF) ﬂuoride gel is:
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
2%
8%
1.23%
10%