1. A method for determining a total acid number and napthenic acid number of a sample selected from the group consisting of petroleum, petroleum cuts, and petroleum emulsions of water-in-oil type by mid-infrared attenuated total reflection Fourier transform infrared spectroscopy, the method comprising the following steps: a) measuring absorbance, at a temperature between 21 and 28° C., and at a wave number selected within 1002-1351 cm −1 and 1550-1822 cm −1 of a plurality of samples to determine total acid number as determined by ASTM D-664 and napthenic acid number of a known amount of naphthenic acids; b) obtaining at least one calibration curve, by multivariate regression model, correlating absorbance measured in step a) with values of total acid number and napthenic acid number of the plurality of samples at the wave number used in step a); c) determining total acid number and napthenic acid number of additional samples by measuring absorbance using attenuated total reflection Fourier transform infrared spectroscopy at the temperature and wave number in step a) and using the calibration curves obtained in step b) to determine corresponding values.