PROCESS FOR STABILIZING THIOACETIC ACID FIELD OF THE INVENTION The present invention relates to thiocarboxylic acids and more particularly to the stabilization of thioacetic acid. STATE OF THE ART This acid of formula CH3-C0SH, prepared by the reaction of hydrogen sulphide with acetic anhydride (Patent FR 1,417,154), is currently used as an intermediate for the synthesis of pharmaceutical products such as captopril (antihypertensive agent). It is known that thioacetic acid tends, after hydrolysis, to form various sulfur-containing or oxygen-containing compounds, which reduce the title of the commercial product to thioacetic acid. This instability of thioacetic acid is due in particular to the formation of hydrogen sulphide and acetic acid according to the reaction:
CH, - C SH + H20 CH3 - COOH + H2S II or
catalyzed by acid species (Tetrahedron, 1965, vol.21, pp. 835). Diacetyl sulfide (CH3C0-S-C0CH3) and diacetyl disulfide (CH3C0-S-C-C0CH3) are also formed. In order to stabilize the thioacetic acid, JP 1,345,605 has proposed the addition thereto of an inorganic acid containing phosphorus or a strong organic acid (pKa < 3,3). Apart from oxalic acid, the other mentioned compounds (phosphorous acid, citric acid, methanesulfonic acid) do not have a sufficient stabilizing action to achieve a good preservation of the thioacetic acid in storage. SUMMARY OF THE INVENTION It has now been found, surprisingly, that chloroacetic acids allow to stabilize the thioacetic acid in a satisfactory way at 50 ° C, doing so for a period of 1 to 2 months which is the time necessary to be able to transport this product to long distance. It has also been observed that in the series of chloroacetic acids comprising monochloroacetic acid (pKa = 2.9), dichloroacetic acid (pKa = 1.3) and trichloroacetic acid (pKa = 0.7), dichloroacetic acid has the stabilizing action more effective DESCRIPTION OF THE INVENTION Therefore, a first object of the invention consists of a process for stabilizing thioacetic acid, characterized in that a sufficient amount of a chloroacetic acid, preferably dichloroacetic acid, is added thereto. Another object of the invention also consists of a composition comprising essentially thioacetic acid and a small amount of chloroacetic acid, preferably dichloroacetic acid. The amount of stabilizing chloroacetic acid can range between 50 and 5000 ppm, preferably between 100 and 2000 ppm (0.01 to 0.2% by weight). In order to obtain the optimum stabilizing action, it is advantageous to add the stabilizing chloroacetic acid to a freshly distilled thioacetic acid containing the least possible amount of water (water <0.5%). In addition, and in order to avoid an atmospheric oxidizing action, it is convenient to isolate the stored product under nitrogen. The following examples illustrate the invention without limiting it. EXAMPLE 1 (Comparative) Freshly distilled thioacetic acid (purity = 99.27%) was introduced into several glass vials which were then hermetically sealed and kept in an oven at 50 ° C. The vials were periodically cooled and opened in order to hate the thioacetic acid titre by gas chromatography. The results indicated in the following table show that, after storage for 100 days, the purity of the product had dropped to 63%. EXAMPLE 2 (Comparative) The procedure of Example 1 was followed, but adding 3000 ppm of oxalic acid to thioacetic acid. After storage for 100 days, the purity loss is about 10%. EXAMPLES 3 AND 4 The process of Example 1 was performed but adding 3000 ppm of dichloroacetic acid (Example 3) or only 1000 ppm of this same acid (Example 4) to thioacetic acid. The results shown in the following table demonstrate that dichloroacetic acid is much more effective than oxalic acid since, after storage for 100 days, the tioacetic acid titre had only decreased by 2-3%, even at a dose of three. times so low (Example 4).