Check oil content in TRI. For this purpose, it is charged 200 – 250 ml „tri” it cools it to room temperature, pours into a measuring cylinder and introduces a hydrometer, slowly pulling it out of the fingers, so that it does not touch the bottom or the wall of the vessel. After waiting 1 – 2 min read the hydrometer, and the percentage of impurities is determined from the diagram. The acidification of the "tri” is marked by shaking 100 ml of a sample of a bath with water and the use of bromophenol blue as indicator. If the water layer turns yellow, this bath is unusable. For the blue tint, alkalinity is checked by titration 0,1 HCl and converted to NaOH. Quantity consumed 0,1 n HCl should not exceed 0,2 ml. It needs to be highlighted that, that using trichlorethylene and similar solvents it is not possible to simultaneously remove contaminants such as inorganic salts from the metal surface, e.g.. hardening, strongly embedded mechanical impurities, such as chromium oxides included in the polishing belts and other impurities insoluble in chlorinated solvents.