March, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the preceding paragraph. I n this manner the results given in Table I were obtained. The sable indicates that the solubility of the 2,6 salt rapidly diminishes as the quantity of the 2,7 salt increases. All results are given on the basis of the anhydrous salts. The total solubility increases regularly, as shown in Fig. 1. PROCEDURE-In order to determine the proportion in which the isomers are present in any mixture of the two acids, the following procedure is suggested: Dilutc the mixed acids with water and heat to boiling. Add slowly Ittad carbonate until the acid solution is neutralized and a slight excess of lead carbonate is present. (In neutralizing the acid mixture it is necessary to use a grade of lead carbonate equal in purity to Kahlbaum's precipitated lead carbonate. The commercial C. P. lead carbonate, owing to its preparation by precipitation with sodium bicarbonate from lead acetate or lead nitrate atohtion, contains alkalies. The presence of even small amounts of alkalies vitiates the results, because of the formation of clisulfonates of the alkali present.) Allow the precipitate to settle. Decant off the hot solution and wash the precipitate several times with boiling water. Evaporate the filtrate and washinm to dryness and dry the residue at 100' C. for one hour. Determine the water of hydration in one portion of the dried residue (2 to 3 grams) by heating it at 190' to 200' C. over night. This is necessary since the two salts contain different quantities of water of hydration.
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Weigh out another portion of the dried residue which is exactly equivalent to 5.2 grams of anhydrous lead salts. (The anhydrous salts are not used here because they absorb moisture so rapidly that it is almost impossible to make accurate weighings.) Place this portion in a flask fitted with a ground stopper. Add 100 cc. of water and place the flask in a tumbling apparatus contained in water bath maintained at 25" C. and tumble for 24 hours. Remove the flask and filter off a portion of the solution, discarding the first runnings, and determine the lead as chromate in a weighed quantity of the solution. With the solubility thus obtained, the curve (Fig. l), and the quantity of the anhydrous mixture taken, the composition of the mixture can be readily ascertained.
To show that the foregoing proposed method is accurate, the following experiments were performed: Two mixtures of lead 2,7 and 2,6 salts were prepared, one containing 47.3 per cent lead 2,7 salt, the other 18.4 per cent lead 2,7 salt. (These percentages are for the anhydrous salts.) The mixtures were separately subjected to the same operations that have been described. The per cents of lead 2,7 salt found were 46.3 and 17.5, respectively. Thus it will be seen that the error of the method is about 1per cent, which, for the majority of practical purposes is not objectionable. The method cannot be employed if 1,6-naphthalenedisulfonic acid is present, owing t o the high solubility of its lead salt.
Melting and Freezing Points of Pure Substances and of Eutectic Mixtures*'* Apparatus for Their Determination By Edward W. Washburn NATIONAL RESEARCH COUNCIL,WASHINGTON, D. C.
H E apparatus is shown in the figure. If constructed of Pyrex glass i t can be used for any temperature up to 500" C!., and a multiple thermocouple or a resistance thermometer can be substituted for the mercury thermometer. 1nasmuc.h as the walls of the Dewar tube are a t substantially the same temperature as the thermometer bulb, no stem correction :s necessary, except possibly a very small one in the case of subslances melting at very high or very low temperatures. For work a t such temperatures a thread thermometer may be fasteneci to the Anschutz.
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thermometer to some temperature above the melting point of the substance. Melt a sample of the substance in a small casserole, remove the Dewar tube and thermometer from the heating chamber, pour into the Dewar tube a sufficient amount of the liquid substance, insert the thermometer and the stopper, A , and take readings as under (1). In case of substances that tend to super-cool without crystallization, the stopper may be momentarily removed as soon as the liquid is cooled below its freezing point and a few crystals added in order to seed it. Procedure 2 may also be used for substances melting below room temperature, provided the Dewar tube is kept surrounded by a transparent bath at a temperature below the melting point of the substance during the readings. For melting points lying in the neighborhood of room temperatures the vacuum jacketing is unnecessary and a small test tube may be substituted for the Dewar tube. Constancy of melting or freezing point is not in itself a complete criterion of purity, except when checked with different samples of the substance which have been subjected to repeated purifications, preferably by different procedures. The following are some results obtained with this apparatus:
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FOR USE DIRECTIONS (1) -For substances melting below room temperature. Place the Dewar tube, B, and its loose-fitting asbestos stopper, A , in a compartment surrounded by a cooling bath somewhat below the melting point of the substance under investigation. Place the calibrated Anschutz thermometer, T, in the same cooling bath. With the aid of a small casserole or other convenient vessel, cool a sample of the substance under investigation until it crystallizes, stirring it so as to obtain the frozen material in the form of loose crystals When the Dewar tube has attained the temperature of the bath, remove it from the bath and with a glass rod or small spatula quickly transfer to the Dewar tube a small amount of the crystals. Dry the thermometer quickly and place it in position. Then fill in a sufficient amount of the powder so as to insure the complete covering of the bulb of the thermometer after the powder is melted, Insert the stopper, A , and tap and read the thermometer at regular intervals until all the powder is melted, using the thermometer as a stirrer during the process. Plot the time-temperature curve. The horizontal portion of the curve (after applying the necessary thermometer corrections) corresponds to the melting point of the substance. If no horizontal portion is obtained, the substance is impure and further purification should be carried out. (2) For substances melting above room temperature. With the aid of a suitable heating chamber bring the Dewar tube and the
SUBSTANCE, MANNITE c. Commercial product , . 165.10 First crystallization.. . . 165.84 Second crystallization.. , . 165.05 Third crystallization, . 165.05 Fourth crystallization.. . . . . . . . . . , 165.05 Constant to 0.01' C. during the crystallization. 10 to 15 minutes
Received January 23, 1924. description of this apparatus has been previously published: Washburn and Read, Proc. Not. Acad. Sci., 1, 193 (1915). The present paper contains a more detailed description of the apparatus and method, prepared in response to requests received by the National Research Council.
The procedure for determining a eutectic temperature or other invariant point is obvious. Evidently the apparatus may be also used for determining a phase equilibrium diagram by the method of thermal analysis.
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* A brief
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