A dentist asks for a tooth gel with NaF but with a pH = 4

ANSWER (20130403)

Give a correct answer to your question "why....." I do not shake out my sleeve because I am not a specialist in products for the treatment of the teeth. I have entered the following keywords in Google  

"Fluoride tooth gel acidic pH" and there appear various references where it seems the most interesting:   Carifree. com/patient/learn/ Protective-factors/ fluoride. HTML  

The pH has apparently an influence on the reinforcement of the tooth enamel.  

I suggest to use the following composition: NaF 0.9 g; NaH2PO4 2 aq; 1 g; Citric acid 0.7 g; HPMC 4000 3.75 g; Sorbic acid 150 mg and water up to 100 ml  

Often those gel becomes red-pink colored with the help of dye. But is there still some red dye available in the pharmacy?  

The gel is used 1 times a week.