Measurement of acid strength of excipients by photoacoustic spectroscopy.

Acid-base properties of excipient surfaces have been studied by photoacoustic spectroscopy (PAS) in the visible absorption range using seven acid-base indicators. The acid strength (Ho) of each excipient was determined from the PA spectral patterns of the indicators. The Ho values of silica-alumina, silica-magnesia and crystalline cellulose were determined to be between 3.3 and -3 .0, between 4.8 and 2.0, and between 4.8 and 3.3, respectively. In the cases of some excipients, the acid strength measured by PAS was weaker than the value which was measured from the color changes of the indicators on the surface of the excipients in benzene solution. These differences are ascribed to water adsorption on the surface during PAS sample preparation. In the case of crystalline cellulose, the hydrogen bonding network was so tight in benzene solution that the indicators could not enter the network.