Why syrup is always kept from pH 3 to 6, except for antacids, what is the purpose behind it, please explain in detail.
Generally, plain syrup (like Sugar syrup IP is 66.7% sugar solution in water) does not require pH adjustment. Only medicated syrups require pH adjustment in the range of 3 to 6. This is because general medicaments (drugs) are stable in acidic pH. Hence to ensure the stability of drugs in syrup up to the defined shelf life (expiry period) the pH of medicated syrups is adjusted.
For antacids like Aluminum hydroxide, Magnesium hydroxide the inherent pH of these drugs (salts) is alkaline and hence medicated syrups containing these antacid drugs have pH at the alkaline side (more than 8 to 10) to ensure the stability of formulations up to shelf life.
The pH of the syrup was adjusted based upon the stability of API in the formulation, for e.g if you formulate a syrup using CPM + PHE.HCL as API means, the pH of your formulation should be maintained between 4 to 4.5, otherwise both the API will react each other to form maillard product.... pH also plays major role stability as well as preservative efficacy in the formulation.
Yes, the same aspect I have explained in my previous communication. The pH of medicated syrup is adjusted to ensure the stability of the drug up to shelf life.