out is defined as the end of melting. The usual precuations regarding the heating and stirring of the bath are observed. The result thus obtained as the melting interval is adjusted for the calibration correction of the thermometer and the correction for the emergent stem. In this method the following liquids are suggested for use as heating baths: For temperatures up to 200° C, pure concentrated sulfuric acid; for temperatures up to about 350° C, a pure grade of cotton-seed oil (almost colorless). Other, though less desirable, substitutes for sulfuric acid at high temperatures are pure paraffin freshly distilled, and clean, white artificial (cotton-seed) lard. A very desirable bath for high temperature work is prepared by cautiously boiling together, for 5 or 10 minutes under a hood, a mixture of 70 parts of purified concentrated sulfuric acid and 30 parts of potassium sulfate, stirring constantly until the sulfate is completely dissolved.

In place of the simple tube described above, many workers prefer to use the Thiele melting-point tube (No. 2) or the Thiele-Dennis

J V

tube (No. 3). These tubes are so designed that heating causes a circulation of the liquid in the tube, making stirring unnecessary. A further modification (figure 128) consists of a tube similar to the above with a side tube sealed in at an angle of about 30° to the axis of the tube, which permits the capillary containing the sample to be inserted or removed without removing the thermometer. The use of the side arm also has the advantage that the capillary containing the sample can be placed in contact with the thermometer bulb and the usual unsatisfactory methods of fastening the capillary to the thermometer are eliminated. The various modifications of the Thiele tube and a discussion of their advantages are given in reference [2].

3. ADDITIONAL METHODS

Another method of determining melting points employs the Maquenne Block. This consists of a block of brass mounted in a frame above a long gas burner. In the top surface of the block are a number

of small cavities. A hole is bored lengthwise of the block, just below its upper surface, to permit the insertion of a thermometer.

To make a melting-point determination, place a small amount of the sample in one of the cavities and cover with a small glass plate. Insert the thermometer so that the bulb is just under the cavity containing the sample. Heat the block slowly until the substance begins to melt, then adjust the thermometer so that the mercury column just projects beyond the end of the block and note the temperature. As the block has approximately a uniform temperature, the stem correction of the emergent mercury column is eliminated.

Dennis and Shelton [3] have devised an apparatus for the accurate and rapid determination of melting points. It consists of an electric heater controlled by a rheostat and mounted on the end of a square pure copper bar. The temperature of this bar can be varied from room temperature to 300° C by means of the rheostat. A voltmeter shows the potential across the heater. The heat applied at one end of the bar causes a temperature gradation along the length of the bar, with a temperature variation of from 10° to 30° C. The temperature of the bar at any point is determined by turning a knob which lowers a constantan element onto the copper bar. This contact forms a thermocouple, the potential of which is read on a potentiometer graduated directly in degrees centigrade. To determine the melting point of a substance, drop a few finely ground particles along the surface of the bar. If the bar has been heated to the proper temperature, a distinct line of demarcation will be noted between the melted and unmelted particles. Move the knob along its slider and lower the constantan element onto the bar exactly at this line. Read the temperature directly by means of the potentiometer. The apparatus is claimed to give values for melting points to an accuracy of about 0.25° C.

4. REFERENCES

[1] U. S. Pharmacopoeia XI, p. 455-457 (Mack Printing Co., Easton, Pa., 1936). [2] A. A. Morton, Laboratory Technique in Organic Chemistry, p. 27–30 (McGraw-Hill Publishing Co., Inc., New York, N. Y., 1938).

[3] L. M. Dennis and R. S. Shelton, J. Am. Chem. Soc. 52 3128 (1930).

PART 4. GENERAL INFORMATION

XXXIV. STANDARD SAMPLES

1. SUCROSE 44

This Bureau is prepared to issue standard sucrose samples (see p. 392) prepared by recrystallization from aqueous solution or by precipitation with alcohol. For each sample, the process of purification has been continued until analysis shows a satisfactory product. A certificate of analysis accompanies the sample. The sugar usually contains but little moisture, and in a moderately dry atmosphere shows very little hygroscopicity. It is advisable to store the sample in a cool, dry place. If placed in a desiccator, the drying agent used should be pure. Vapors from impure sulfuric acid or phosphorus pentoxide frequently damage the sugar.

The uses of standard sucrose samples may be stated briefly as follows: (1) As a primary saccharimetric standard; (2) as a source of pure invert sugar for the standardization of analytical determinations of reducing sugar; (3) as a standard for the calibration of viscometers; and (4) as a material for standardization of bomb calorimeters. Sucrose has the advantage of being nonvolatile and nonhygroscopic. It is rather difficult to ignite and sometimes does not burn completely. It has a heat of combustion of about 3,950 calories, or only about half that of coal. The more exact value for each sample will be given in the certificate. For details of the standardization of bomb calorimeters, see NBS Circular C11.

2. DEXTROSE 45

The standard dextrose samples are prepared by purification of the purest glucose of commerce in the manner described on page 390. A certificate of analysis will show the degree of purification of the sample. The standard dextrose sample is intended to assist in research work of a general nature and, in particular, to serve as a standard reducing sugar for analytical work.

XXXV. TESTS

1. SPECIAL

The special requirements of scientific investigators, manufacturers of apparatus, and others, for higher precision than is considered in the following schedules, will be met as far as the regular work of the Bureau will permit. The application for a special test should state fully the purpose for which the apparatus has been used or is to be used in the future, the need for the test, and the precision desired. The test should be arranged for by correspondence before shipment of the apparatus. The special fee charged will depend chiefly upon the time consumed and the amount of alteration required in the regular Bureau testing set-ups. An estimate will be given when possible.

44. 48 See list on p. 559.

551

2. GENERAL INSTRUCTIONS TO APPLICANTS FOR TESTS

(a) APPLICATION FOR TEST

The request for test should be made in writing, addressed to the National Bureau of Standards, Washington, D. Č., and should enumerate the articles submitted for test, giving the identification marks of each-for example, maker's name and number-and should state the nature of the test desired.

(b) NATURE OF TEST

The classification of tests in this Circular should be followed, and the schedule numbers should be used to indicate the test desired. When the desired test is not included under the regular schedules, the applicant must comply with the requirements for special tests. When apparatus is sent simply for test, without definite instructions, the Bureau will, if practicable, decide upon the nature of the test.

(c) IDENTIFICATION MARKS

All packages should bear the shipper's name and address and, when convenient, a list of the contents.

Each separate piece of apparatus or sample of material should be provided with an identification mark, which in many cases may be the maker's name and number. The identification mark should be given in the application for the test.

(d) SHIPPING DIRECTIONS

Apparatus or test specimens should be securely packed in cases or packages which will not be broken in transportation. The shipment in both directions is at the applicant's risk. To facilitate packing and shipping, the tops of the cases should have the return or forwarding address on the underside and should be put on with screws. Transportation charges are payable by the party desiring the test and must be prepaid. Unless otherwise arranged, articles will be returned or forwarded by express collect.

(e) RETURN OF APPARATUS

Regular tests will be made in the order in which the applications are received, except as this practice may be varied by grouping similar tests together. It is suggested, therefore, that the applicant, if possible, make request for a test from 2 weeks to 2 months preceding the shipment of the apparatus. This facilitates the work of the Bureau as well as the prompt return of the apparatus.

(f) ADDRESS

Apparatus submitted for test, as well as all correspondence, should be addressed to the National Bureau of Standards, Washington, D. C. Apparatus delivered in person or by messenger should be accompanied by a written request for the test.

(g) REMITTANCES

Fees should be sent with the request for test, in accordance with the following schedules, or promptly upon receipt of bill. Certificates are not given nor is apparatus returned until the fees due thereon have been received. Remittances may be made by money order or check drawn to the order of the National Bureau of Standards.