THERAPEUTIC DENTIFRICES 825 ........ •o .•...- Figure 6. Assembled dissolution unit constant-speed 1800-rpm motor.* The glass stirring rod is attached to the mo- tor with about 45 rain lcngth of 6-ram i.d., 16-ram o.d. pressure rubber tubing. It is convenient to make up dissolution units in sets of four which are mounted in a 37øC constant temperature bath (Fig. 7). Lactate Buffer To a 2-1. volumetric flask is transferred 211 g (2 moles) of 85.2% lactic acid, analytical reagent grade, and 400 ml of water is added. Next, 84 g (2.07 moles) of sodium hydroxide, analytical reagent grade, is dissolved in 600 ml of water and transferred completely to the lactic acid-containing volumetric flask. If the pH of the solution is not above 10.5, i g of sodium hydroxide is added and the pH is measured after standing 30 min. The solution is then diluted to 1 1. with water. Next, 211 g of 85.2% lactic acid reagent is transferred to a 2-1. volumetric flask and diluted to the mark with water to give 1M lactic acid. To a 3-1. beaker is transferred the 2 1. of 1M sodium lactate solution and about 425 ml of the 1M lactic acid solution is added to bring the pH of the mixture to 4.50. It is preferable to age this solution about a month or more to reduce the lactic acid polymer coutent. The 1M lactate buffer should be diluted *Dayton •oo H.P. shaded pole #4K903, Dayton Motor Co., Dayton, Ohio.

826 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 7. Constant-temperature bath with four dissolution units tenfold just prior to use. Prolonged storage of the 0.1M lactate buffer may result in mold growth. Conditioning o1' Mounts Each new tooth mount is conditioned in the dissolution apparatus with 40- ml po•ions of 0.1M plI 4.5 laetate buffer for 9, one-hour periods, with fresh buffer solution added after one hour of conditioning. hnmediately after each enamel solubility test, the individual mount should be conditioned for 9, one- hour periods. At the time when no further tests are contemplated on the same or successive days, the mounts are conditioned for 9, one-hour periods. Since there is the danger of effects h'om previous treatments, it is very important to condition the teeth properly before each test. Most mounts can be used for 10 individual tests before discarding. Enamel Solubility Test After proper pretest conditioning, the conditioned tooth •nount is etched in the dissolution apparatus with 40 ml, accurately measured, of 0.1M pH 4.5 lactate buffer for 15 rain at 37øC to determine the pretreatment solubility. A