PH: both HPMC and HEC are stable to common acids and alkalis. Alkali can speed up its dissolution and slightly increase its viscosity. HPMC solution is stable in the range of ph = 2-12. caustic soda and lime water can not affect HPMC properties. Hydroxypropyl methylcellulose is stable to common salts. But when the concentration of salt solution is high, the viscosity of HPMC solution tends to increase.