The following documents, mentioned in the course of the description hereafter, illustrate the state of the art:    Designation ASTM D664-95: American Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration, Annual Book of ASTM Standards, vol. 05.01, p. 250-256,    Babaian-Kibala, E., Petersen, P. R., Humphries, M. J., 1998. Corrosion by naphthenic acids in crude oils. Pre-prints of the A.C.S., Division of Petroleum Chemistry vol. 3, 106-110,    Meredith, W., Kelland, S.-J., Jones, D. M., 2000. Influence of biodegradation on crude oil acidity and carboxylic acid composition. Org. Geochem. 31, 1059-1073,    Robbins, W. K., 1998, Challenges in the characterization of naphthenic acids in petroleum, 215th ACS Nat. Mtg. Dallas, preprints 43 (1), 137-140,    Luo, Liwen, Xia, Daohong, 2003. Total carboxylic acids contents in petroleum reservoir rock determined by chemical titration. Prepr.—Am. Chem. Soc., Div. Pet. Chem. (Preprints—American Chemical Society, Division of Petroleum Chemistry) 48, 261-263,    Roussis s g, Lawlor l j, 2002. Direct determination of acid distributions in crudes and crude fractions. Patent assignee: Exxonmobil res & eng co world 02/48698a1, p Jun. 20, 2002, f Nov. 6, 2001, pr us Dec. 14, 2000 (appl 60/255659) and us Sep. 21, 2001 (appl 957941) (g01n-024/00).
The acidity of an oil is generally defined in the petroleum industry from the number of milligrams of alcoholic potash (KOH) necessary to neutralize one gram of oil. This number is referred to as TAN (Total Acid Number). This TAN is obtained by potentiometric titration in a non-aqueous medium. This titration is carried out according to the standard ASTM method D664-95 described in the following document:    American Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration, Annual Book of ASTM Standards, vol. 05.01, p. 250-256.
This potentiometric method has the drawback of requiring a great volume of crude and consequently a large amount of samples (Table 1). In fact, this method is based on the measurement of an electric potential difference (expressed in mV) between a crude oil diluted in a solvent (toluene/propanol-2) before and after neutralization referenced in relation to a freshly prepared anhydrous acid buffer solution. The measurement depends on the sensitivity of the electrodes intended to measure very slight variations (millivolt scale). This is the reason why this measurement requires large amounts of oil; the lower the acidity, the larger the amounts (Table 1). Furthermore, it is essential to first eliminate any solid particle from the oil sample prior to applying this standard method. Calculation is carried out according to the following formula:
  TAN  =                              m          KOH                ⁡                  (          mg          )                                      m          e                ⁡                  (          g          )                      =                  (                  A          -          B                )            ·      M      ·              56.1                              m            e                    ⁡                      (            g            )                              with:                me(g): mass of the oil sample in g,        mKOH(mg): mass of alcoholic potash (KOH) required to neutralize the mass of oil sample me(g),        B: volume in ml of KOH required for titration of the solvent in the absence of oil (blank test),        M: concentration of the KOH solution in mole/l,        A: volume in ml of the KOH solution (0.1 mole/l) required for titration of the oil diluted in a solvent, until a turning point measurement as close as possible to the one obtained for the buffer solution is reached.NB: molar mass of KOH=15.9994+1.00797+39.103=56.1.        
TABLE 1Amount of oil required to determine the aciditythereof according to standard D664-95TANMass of sample required (g)Weighing accuracy (g)0.05-1.0 20.0 ± 2.0 0.11.0-1.5 5.0 ± 0.50.025-201.0 ± 0.10.00520-1000.25 ± 0.020.001100-250  0.1 ± 0.010.0005
An oil is considered to be acid if its TAN is above 0.5 mg KOH per gram of oil. From natural observations, the TAN scale of crudes ranges from 0.1 to 8 mg KOH/g oil (Babaian-Kibala et al., 1998; Robbins et al., 1998; Meredith et al., 2000). Although a TAN value of 0.5 is low, and the acid compounds remain minor constituents in crude oils, their role can be significant in terms of profitability during development of an oil field (reservoir quality uncertainty, low economic value of the crude, . . . ). These acids are, among other things, responsible for emulsion, foaming and soap deposition (during production), corrosion (during production, transportation and/or refining) and environmental (waste water treatment, site pollution) problems. This global acidity measurement is to date the only one that is used and recognized by oilmen.
Various known methods concerning measurement of the acidity of crudes and/or characterization of the acids in production oils are for example described in the following publications:                Liwen, 2003, describes a back titration method (with excess potash) on crushed cores tested according to different parameters: grain size, extraction time, solvent volume (acetic acid/alcohol), . . . . The acidity measurements (in mmoles/g core) are higher than those obtained with the standard method and optimum conditions are described,        Roussis describes another method intended for direct determination of the acid distribution in crudes or crude fractions by mass spectrometry in negative chemical ionization mode (Cl−). The chloride anions generated by CI react with the acid compounds of the oil samples. The chlorinated adduct ions are detected selectively and the acid species are quantified from the identified peaks.        
Furthermore, the various methods currently proposed have integrated no relation with the TAN value measured according to the ASTM standard D664 and are therefore not used by the petroleum industry. Besides, these analyses are performed at a late production stage, from stock-tank or test oils.
The method according to the invention leads to the evaluation of the acidity translated in terms of isotopic enrichment of oil samples, even in small amounts. The method is furthermore applicable at any stage of the petroleum industry (Exploration/Production/Refining/Environment).