Modified Maquenne Melting-point Block F. W. SCHNEIDER and E. W . BLANK Colgate-Palmolive-Peet Company, Jersey City, New Jersey
sity for capillary tubes (the sample may be placed in direct contact with the block) and permits working a t temperatures up to 500°C. or higher, depending upon the range of the thermometer employed. Jacquemain (4) recommends plating the block & h chromium to minimize radiation. Dennis and Shelton (5) have described an electrically heated modification of the Maquenne block which is commercially available (Burgess Parr Co., Moline, Illinois). The Fisher Scientific Company also sells an electrically heated Maquenne type block (No. 12-142 Melting Point Apparatus). Both of the latter instruments are limited to temperatures of 300°C. or less. DESCRIPTION OF BLOCK
SECTION THROUOH A-A
MELTING POINT BLOCK MODIFIED MAQUENNE TYPE
T .
HE determination of melting points in a liquid bath is a dangerous operation, especially a t high temperatures. Use of the Maquenne block (1, 2, 3) automatically eliminates most of the hazards involved in the operation and at the same time facilitates obtaining the melting point of substances that sublime or decompose on heating. It, likewise, avoids the neces-
This paper describes the construction of an efficient and novel modificationof the Maquenne block designed for rapid and uniform heating with a minimum of radiation. Figure 1 illustrates the block and gives the constructional dimensions. The vertical holes piercing the block not only assist in heating but also facilitate cooling. When a series of determinations must be made, the temperature of the block may he quickly lowered by directing a stream of air through it. The dimensions of the block may be varied slightly if desired. Brass, copper, or aluminum is suitable material for construction. If brass or copper is employed, oxidation of the block may be prevented by plating the working surface with chromium or gold. The finished block may be wrapped with asbestos tape and inserted in a metal shell. This shell may be made from any suitable material. In the case of the block here described, a section cut from an aluminum essential oil container was used. The bottom half was cut from the aluminum container and the center of the bottom of the resulting cup was removed before slipping over the asbestos-wound block. The metal shell gives a finished appearance to the instrument although it is
quite as efficient in operation when merely wrapped with the asbestos. For low temperatures the block is heated by means of a microburner flame directed a t the apex of the cone on the under surface of the block. A Bunsen burner is used for workat high temperatures. The copper tube which supports the thermometer in a horizontal position is provided with several longitudinal slits which are pressed inward and provide sufficient traction to support the thermometer. Thermometer lag can be minimized by packing copper filings around the bulb. PROCEDURE Insert a thermometer of suitable range in the side of the block. The top of the block should be clean. If it is not, i t should be polished with fine emery paper or metal polish. Powder a small quantity of the compound whose melting point is to be determined. Heat ' the block over a low flame so that its temperature rises very slowly. Place a small quantity of the compound in the center of the depression on the top surface of the block. If the compound does not melt instantly, remove i t from the surface of the block by means of a small wad of filter paper. After a suitable increase in temperature, say 10°C., repeat the operation. Continue in this manner until a temperature is reached a t which the compound melts. As the temperature of the block approaches the melting point of the compound, the thermometric intervals a t which the compound is placed on the block should be decreased. I t is also possible to use the block by placing the sample between two cover glasses inserted in the depression of the block. In this case the sample is in contact with the block throughout the whole determination. A cifcular shelf of large diameter (3 cm.) is provided so that the cover glasses may be covered in turn with a small watch glass. A low power lens may be used to observe the sample when working with very small quantities of material. The sample may be illuminated by an oblique beam of light from a microscope lamp or other source of illumination, to facilitate observation of the melting point.
with the melting points obtained by the usual capillary method. In determining the melting points by use of the block the samples were placed between cover glasses. Chamot and Mason (6) give a table of substances suitable for calibration purposes together with their melting points. Both sets of melting points were corrected in the usual manner. The results obtained are shown in Table 1. Both sets of melting point valnb may be plotted against the corresponding temperatures to provide a calibration curve for the instrument which may be used in comparing values obtained by the two d i e r e n t methods. Very close correlation was observed, the melting points obtained by use of the block being slightly less than those obtained by the capillary method. TABLE 1. M ~ T I NPOINTS C BY MAQOBNNB BLOC=AND CAPILLABY TUBE. MB~ODS
urea Anthraoilie Aeid Ammonium Thiocyanate Salicylic Aeid BNcine Silver Nitrate Pheoolohthaleio ~li=& Sodium Acetate. An
mate
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401
398
LITERATURE CITED
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cation'of 0rhanic Comoounds." 2nd E
CALIBRATION O F BLOCK
The melting points of 'a large number of substances were determined by use of the block and compared
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