On the spontaneous crystallisation of monochloracetic acid and its mixtures with naphthalene

The object for which the present investigation was undertaken was to study the spontaneous crystallisation of mixtures of two substances which form mixed crystals and possess a minimum, or eutectic, freezing point. According to Cady, naphthalene and monochloracetic acid are such a pair of substances, and possess a minimum melting point of 53°·5 for the eutectic mixture of 29·4 per cent, naphthalene, 70·6 per cent, monochloracetic acid. We have attempted to verify Cady’s freezing and melting point curves, but, although our experiments confirm the former, we were quite unable to verify the latter. Pickering mentions four modifications of monochloracetic acid, and traces the freezing point curves for three of them. Our study of mixtures of monochloracetic acid and naphthalene has not given us the information we expected concerning the composition of the “mixed crystals” which separate spontaneously from a solution, but it has led to interesting results concerning the spontaneous crystallisation of the different modifications of a substance dissolved in another substance which is not polymorphous.The Different Modifications of Monochloracetic Acid and their Mutual Transformations . Microscopic examination of crystals of monochloracetic acid obtained from fusion or solution show clearly that three different modificationsα, β , andγ of the acid exist. These modifications have melting points 61˚˙5, 55°, and 50°; they are formed on the microscope slide under different circumstances, each modification yielding rhombs quite distinct from those of the two other modifications. If fused monochloracetic acid be cooled suddenly it crystallises as theγ -modification in rhombs having a plane angle of about 59°. If these rhombs be touched they at once transform into theβ -modification and give rhombs having a plane angle of about 72°; or occasionally theγ -rhombs are transformed at once into the stable a-modification of the acid, which exists as broad needles having an acute angle of 43˚. The transformation fromγ toα takes place much more rapidly than the transformation fromγ toβ . Similarly, if rhombs ofβ are formed on a microscope slide they may be at once transformed into theα -modification by inoculating with a fragment ofα . Each transformation is accompanied by a rise of temperature. A remarkable feature of the change is that the more stable modification crystallises with sharp edges in the solid mass of the less stable substance, as though it were growing in a liquid.