I N D U S T R I A L ALVDENGINEERING CHEMISTRY
A u ~ u s t 1923 .
837
The Use of Dyes as Temperature Indicators' By Paul A. Kober EDISONLAMPWORKS, GFNERALELECTRIC Co., HARRISON, N. J.
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N M A N Y industries
A brief oufline is given of a reliable and eficienf method f o r interval of disappearance, baking O r heating constiall Other factors being conchecking temperatures in heating processes. The disappearance of stant, I n the accompanytutes an important Part or the change in color indicates that the part to which the color has ing figure curves are given of manufacturing Processes. been applied has actually received a definite amount of heat. These are necessary to bring of indicators shoWing the The method prouides a simple and inexpensiue means f o r temperaabout Some desired Physical ture checks on an entire fact0r.y production. Its apIjlication in few relation between temperaor chemical change, and in ture and time of disappearis most cases it is essential ance. These values were that the amount of heat obtained by inserting glass applied comes within certain fixed limits, since quality rods, on which a thin coating of the dye was applied, into an and production are dependent upon it. The amount of heat oven held at constant temperature, and noting the time when applied is usually determined by the time of heating and the color had entirely disappeared. The c6rves show only the temperature, the latter being indicated by the more common approximate relative values of the different dyes. The time methods of pyrometry. I n many cases, however, the of disappearance for the higher temperatures may vary in methods used do not indicate the actual temperature condi- any particular test according to the mass and heat capacity tions sought, and where a large production is involved are of the substance on which the color is applied. It will be often costly both as to loss of time and material. noted from accompanying curves that all the dyes function A method recently developed for this purpose, based on the about the same, the curves being parabolic. The indicadecomposition of an organic dye, has proved to be a very re- tors represented in Fig. 1, with their relative time of disapliable and efficient heat indicator. The absence or presence pearance when subjected to constant oven temperatures, of the color on or in the heated product to which it is applied are given in the following table: determined accurately whether or not the product has reApproximate Time of Temperature of Dye Disappearance Designated by ceived a definite amount of heat. Some of the essential a t 350' C. a t 375" C. Melting Point Salt considerations in the use of dyes as heat indicators and a few Min. Min. c. of its applications will form the basis of this paper. TEMPERATURE INDICATOR REQUIREMENTS OF DYES In ordei that a dye may act as a heat indicator, it is nec- Erythrosin essary that when heated it break down readily into gases ilside from showing the relations of the various dyes by and leave no appreciable residue behind. Most dyes on curves, they may also be designated by some specific temperaheating do not possess this characteristic. Usuallj. they ture. This may be done by comparilZgFa dye with a melting decompose to some extent into other solid substances and then point salt which just melts when a certain dye just disappears disappear only after a high temperature has been applied, under the same conditions. The dye may then be designated thus carbonizing the dye with subsequent oxidation. There by the melting point of the salt. The temperatures given in are, however, a considerable number of dyes which act as last column of the table are so designated. very good indicators when subjected to heat. These break down easily with the complete disappearance of the color. Dyes which change on heating through many shades will obviously not act as good indicators. Colors such as blue, green, and red, when applied to various substances, act as the best indicators, while colors like yellow and brown show insufficient color changes. Dyes that break down readily on heating are not confined to any particular chemical group. Most of those found, however, are identified with the triphenylmethane group of coloring matters. The disappearance of a dye, or its complete decomposition into gases, is a function of the time of heating as well as of the temperature. Unlike a melting point salt, which indicates a definite temperature regardless of the time element, a dye will completely disappear a t various time intervals and temperatures. dside from these two factors the disappearance of a 20 24 dye is also a function of the amount of dye applied to the subErne /n Mmufes stance. The heat which causes a dye to break down is conFIG. 1, veyed to the dye by both convection and conduction of the APPLICATION OF DYESAS INDICATORS material to which it is applied. For all practical purposes it I n the application of dyes as heat indicators three methods may be QaEelyassumed that when the dye disappears the part to which it is applied has absorbed a definite amount of heat. may be employed depending on the conditions of heating: It will be apparent that a dye cannot indicate any particular ( a ) Applying a thin film or coating by means of a standard temperature, but acts only as an indicator of the amount of solution to the part t o be heated. ( b ) Mixing a standard solution of a suitable dye to compounds heat applied to the part measured. or mixtures t o be baked. Dyes which act as indicators differ in temperature and time (c) Applying the proper dye on a test plate, which, with the prodO
1 Received
February 16, 1923.
uct to be measured, is subjected to the same heating conditions.
838
I N D UXTRIAL A N D ENGINEERING CHEiWIXTRY
To obtain consistent results it is necessary to make up standard solutions whose purity and tinctorial power are known. Absolute alcohol (either ethyl or methyl) has been found best as a solvent. The presence of water will cause some dyes to hydrolyze and others to combine with one another, thus destroying their temperature-indicating properties. All dye solutions should be filtered before using to eliminate solid matters and gums. The concentration of the dye solution will depend on whether it is to be added to a mixture, as in a cement, or applied to some surface. I n the former case the concentration will determine to some extent the time of disappearance, while in the latter the varying concentration which is likely to be applied to any surface will not appreciably vary the time of disappearance. It is, however, desirable for surfaceuse to make the concentration as low as possible, yet high enough to give sufficient tinctorial power. Where an excess amount of dye is applied to any surface there is R tendency for a slight smudge to remain.
DOUBLE INDICATORS I n order to indicate in heating processes whether the heat applied has passed a maximum as well as a minimum temperature limit, two suitable indicators would ordinarily be necessary, To avoid the’application of two different dyes, an indicator was developed indicating both the minimum and maximum temperature limits. It was found that two dyesfor example, a red and blue forming a violet-can be dissolved in absolute alcohol without combining chemically. When such a solution with the right proportions of the components is applied to the part to be heated, the lower temperature dye will disappear a t its own particular temperature and time interval, thus changing sharply the violet mixture to either a red or blue, depending on which is the lower component dye, I n other words, the higher component dye is left on the material. If the time interval and temperature are then
A Modified Test for Phthalates With Particular Reference to the Detection of Diethylph t hala t e’ By R. E.Andrew CONNECTICUT AGRICULTURAL EXPBRIMENT STATION, N E W HAVEN,CONN.
HE use of diethylphthalate as a denaturant for industrial alcohol to be used in the manufacture of toilet preparations has brought about the necessity of reliable tests for its detection, either in the manufactured preparations or in products which may have been illegally derived therefrom. Several tests have been described2 in which heating with resorcin is recommended, the final results being the development of green or yellow-green fluorescence if the test is positive. I n the author’s experience the criticism of all these tests is that satisfactory blanks cannot be obtained; the reagents alone may develop fluorescence of such a color and intensity as to make conclusions with regard to an unknown sample doubtful. I n a series of trials in which reduced periods of heating the acid-resorcin mixture were tried, it appeared that the difficulty arose a t this stage in the process. It
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1 Received February 27, 1923. Presented before t h e Division of Agricultural and Food Chemistry a t t h e 65th Meeting of the American Chemical Society, New Haven, Conn , April 2 to 7, 1923. 2 Mulliken, “Detection of Pure Organic Compounds,” Vol. I, p. 107; Allen, “Commercial Organic Analyses,” Vol. 111, p 546; Lyons, J . Am. Phcrm. Assoc., 11 (1922), 262, Handy and Hoyt, I b s d , 11 (1922), 928.
Vol. 15, KO. 8
increased to a point corresponding to the higher component dye, it wilI also disappear. This “double indicator” in a single solution, then, will enable one to tell by inspection whether the heat applied was below, within the limits set, or above the maximum limit. The following dyes when mixed in right proportions will act as good double indicators : (a) Chemco blue-Chemco red, ( b ) C. P. 196-malachite green, (c) malachite greenrubine red, (d) erythrosin-neptune blue. APPLICATIONS (1) In many heating processes where a definite amount of heat must be applied during a certain time interval, the use of a dye as a heat indicator will be found efficient. To determine whether the heat applied was within the proper limits, the disappearance or change of color is noted. ( 2 ) A very important use, and one to which a dye is admirably adapted, is that of determining the heat distribution of any material to which it is applied. The heat distribution is noted by the distribution of the remaining color or the order in which the color disappears. (3) For glass annealing a suitable dye is applied to the part to be measured, or on a test plate which is subjected to the same heat conditions. The dye chosen is one that will just disappear dr change to another color. (4) I n baking cements or mixtures which require a definite amount of heat, the proper indicator may be mixed in the compound, the amount used being determined by tests. The disappearance of the color in the compound will indicate that sufficient heat has been supplied. ( 5 ) Still another use of dyes is an indicator of overheating which any electrical instrument or part may receive. An inspection of the part coated with the proper dye will immediately indicate whether it has been overheated by the presence or absence of the color.
was found that no heat other than that developed on neutralizing the alkaline mixture with sulfuric acid was necessary or advisable. The modified test is as follows: METHOD
To 10 cc. of the solution to be tested in a small porcelain capsule add 5 drops of a 10 per cent sodium hydroxide solution. Evaporate over a steam bath to dryness and then add 0.5 cc. of a 5 per cent resorcin solution and again evaporate to dryness. Remove from the steam bath and at once add 6 drops of concentrated sulfuric acid and mix thoroughly by rotating the capsule. When cool add 10 cc. of water and transfer to a test tube, rinsing the capsule with 10 cc. of water. Add 5 cc. of a 10 per cent sodium hydroxide solution. A green fluorescence will at once appear if the test is positive, the color of the fluorescence varying from green to yellow-green, according to the amount of phthalate present. Alcohol of known purity gave a negative test-i. e., no fluorescence. Ten cubic centimeters of pure alcohol to which had been added 0.1 cc. of a 1: 500 solution of diethylphthalate (0.0002 gram) gave an unmistakable fluorescence. Positive tests were also obtained when small amounts-i. e., 0.0010 to 0.0020 gram-of this denaturant were added to gin and whiskey, the procedure as outlined being applied directly to these substances without previous distillation. I n the presence of considerable amounts of extractive material it was found best to (1) extract the diethylphthalate with petroleum ether, evaporate the solvent a t room temperature, take u p the residue with alcohol and apply the test, ( 2 ) distil rapidly and apply the test to the distillate.
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