(b) TOLERANCES

The tolerances for flasks of various sizes are shown in table 9.

(1) PRECISION STAMPS.-Flasks tested by the National Bureau of Standards and found to comply with the foregoing specifications will be given the official precision stamp of the Bureau. The stamp consists of the letters "NBS" and the year in which the test is made, surrounded by a circle. Thus for the year 1941 the stamp will be

NBS
1941

The stamp will be placed on the neck of the flask, above the graduation mark.

(2) SUGAR-TESTING FLASK (BATES).-The type of flask used in the sugar laboratories of this Bureau and in the United States Customs Service is shown in figure 30, No. 1. It is especially designed for sugar polarizations and is described in the Customs Regulations, 1931, chapter XI, Sampling, Weighing, and Testing of Sugars, Sirups, and Molasses, as follows: "The flasks shall have a height of 130 millimeters, the neck shall be 70 millimeters in length and have an internal diameter of not less than 11.5 millimeters and not more than 12.5 millimeters. The upper end of the neck shall be flared, and the graduation marks shall be not less than 30 millimeters from the upper end and 15 millimeters from the lower end of the neck." All flasks shall be standardized to contain 100 ml at 20° C. These specifications permit a maximum internal diameter of the neck of 12.5 mm, which exceeds the maximum allowed in the National Bureau of Standards specifications for 100-ml precision flasks by 0.5 mm. However, since the sugar flask is used almost exclusively for polariscopic work, it is tested as precision volumetric apparatus, and if the tests show it to be in accordance in all respects with the specifications given above, it will be given the precision stamp of this Bureau, described under (b) (1) above.

In the Bates type of flask the neck, while smaller than that of the ordinary 100-ml sugar flask, is made slightly larger than the Bureau's requirements for a flask of this size in order that the neck shall not become clogged when the sugar is being introduced into the flask. The upper end is flared to facilitate pouring. The height of only 130 mm minimizes the dilution of the solution by moisture on the upper part of the neck and gives a length which readily permits the flask to be closed with the thumb while the forefinger rests on the bottom, thereby facilitating a thorough mixing by shaking, with no loss by spilling.

(3) SPECIAL. A number of flasks designed for special purposes are shown in figure 30.

Flask 1 is the Bates-type sugar flask, specifications of which are given above. This flask is used extensively in the United States Customs Service and elsewhere in routine sugar analysis.

Flask 2 is the Kohlrausch flask with a funnel neck to facilitate the transfer of the solid sugar without loss.

Flask 3 is typical of the double-graduated flasks and is used in inversions and clarification methods where one volume is to be diluted to a different volume. The usual sizes are 50-55, 100-110, 200-220, and 500-550 ml.

Flask 4 was designed at the National Bureau of Standards for precision work. The inside diameter of the neck is 5 to 6 mm. The enlarged portion of the neck permits complete mixing of the contents of the flask without the solution coming in contact with the grinding or the thumb until the mixing is complete. Before making to volume, the bulb is dried inside by a current of filtered air. The glass tube is ground to fit the grinding of the flask and also the glass stopper of the tubulated water-jacketed polariscope tube, which permits the transfer from flask to polariscope tube without exposure to the air and the consequent evaporation of the solution. In addition to the 100-ml mark on the neck of the flask, there are supplementary graduations above and below the mark. With this flask it is possible to

make the calibration and reproduce the volume of solution with an accuracy of 0.002 ml.

Flasks 5 and 6 are special flasks used chiefly in the determination of the density of molasses and sirups.

(4) CORRECTION TABLES.-Tables 106 and 107, p. 612, are given for the convenience of those who wish to verify the graduation of volumetric apparatus. More complete data for this purpose will be found in National Bureau of Standards Circular C19.

4. THERMOMETERS*

(a) GENERAL

Thermometers [1] should be of good design and workmanship and should be made of suitable materials, with special attention to the thermometric properties of the glasses used. Detailed specifications, covering the necessary items for the various types of thermometers suitable for use in polarimetric measurements, have been prepared and are available upon request. A sample form of the specifications is printed in this circular, page 382.

Mercury-in-glass thermometers are calibrated to agree as closely as possible with the International Temperature Scale adopted in 1927 and now in general use. On this scale, which conforms with the Centigrade Thermodynamic Scale as closely as possible with presentday knowledge, temperatures in the interval 190° to 660° C are defined in terms of the resistance of a standard form of platinum resistance thermometer calibrated at basic fixed points. The International Temperature Scale is defined in NBS Research Paper RP22 [2].

It is highly desirable that a fixed point appear on the scale of a mercury-in-glass thermometer. The ice point (0° C or 32° F) or steam point (100° C or 212° F) is convenient. The volume of the bulb of a mercury-in-glass thermometer is known to change with time and use by amounts which must be taken into consideration if the best results are to be obtained. By checking the fixed-point reading from time to time, these changes may be determined and allowed for. For example, if the ice-point reading is found to be higher or lower than the previous reading, all other readings on the thermometer will be higher or lower by the same amount.

Thermometers provided with graduated metal backs should, in addition, have graduations engraved on the glass stem. The thermometer should be securely and firmly fastened to the back. If the thermometer is of the inclosed-scale (Einschluss) type, such fiducial mark should be placed on the outer glass tube.

Thermometers should comply in all respects with the applicable specification.

(b) ICE BATHS [3]

Because of the changes in bulb volume, the ice bath, which provides a convenient means of determining the amount of such changes, is very important. The most convenient form consists of a widemouthed thermos bottle or Dewar flask, filled with a mixture of shaved ice and distilled water, or water obtained by the melting of the ice. Other containers may be used, but are likely to be less convenient because of the more rapid melting of the ice. Clear ice is considered sufficiently pure for the purpose and is readily obtainable. The ice may be shaved by means of a small plane, resembling a carpenter's plane, or other appropriate mechanism. There should be enough water in the mixture to make it soft or slushy, but not enough to cause the ice to float. Excess water which accumulates may be conveniently removed by means of a glass siphon, ending in a rubber tube with a pinch cock. Precautions should be taken to wash the ice and to avoid contaminating it in handling.

5. WEIGHTS*

(a) SUGAR

It is generally advantageous to have special 26- and 13-g weights for weighing out sugar samples for direct polarization. For this work, the Bureau recommends that the ordinary screw-knob type of weight be avoided in favor of a strictly one-piece weight in which the knob forms an integral part of the weight. Gold- or platinum-plated Tobin bronze weights have been found satisfactory. Recently,

*See test fee schedule 226, p. 557.

stainless-steel weights have been introduced, and the indications are that they will prove satisfactory if made from steel of the proper composition. The working standards should conform to the Bureau's requirements for class B and the reference standards (i. e., those used only for checking the working standards) should conform to the requirements of class A. The maximum error allowed in both classes is 2 mg on the 26-g weight and 1.5 mg on the 13-g weight.

(b) ANALYTICAL

In most analytical procedures in the sugar laboratory, a good grade of analytical weights should be accurate enough, but it is not safe to rely on them unless they are tested. For work requiring considerable accuracy in the weighing, the weights should conform to the Bureau's requirements for class S. The maximum errors allowable range from 0.5 mg on the 100-g weight to 0.1 mg on the 1-g weight and 0.02 mg on the 10-mg weight. For less accurate work, weights conforming to the requirements of class S2 having allowable errors of five times those of class S, may be used.

(c) REFERENCE STANDARDS

Weights are liable to change. They cannot be used without a certain amount of wear, which will ultimately make an appreciable change in their values. Ordinary analytical weights sometimes suffer serious change from the oxidation of adjusting material placed in the cavity under the knob. Weights must therefore be retested from time to time according to the nature of the weights and of the work for which they are used. Reference standards are therefore needed, since it is seldom advisable to send the weights to this Bureau as often as would be needed. There is no gain in the purchase of complete sets for this purpose when this is done at the expense of quality, as must usually be the case. A set of working standards can be tested readily by intercomparison of the weights among themselves, if one or two reference standards are available on which to base the calibration. The Bureau will furnish information in regard to series that are much better than the ones generally used and yet involve no great amount of additional labor. Probably the best denominations for reference standards would be one 100- or 50-g weight, one 1-g weight, one 10-g weight, one 26-g weight, and one 13-g weight. The 1-g weight should be made of platinum, as it is the starting point for the determination of the milligram weights.

For the best reference standards, the Bureau recommends one-piece weights. Gold- or platinum-plated Tobin bronze weights are the most satisfactory ones now available for this purpose for weights above 1 g. Weights having a bard metal driven plug would rank Standards for the analytical sets should conform to the requirements for class S and be tested under that class. Reference standards for the "sugar weights" should come under class A, as stated above.

(d) CERTIFICATION

Sugar weights and analytical weights are among the weights tested by the Bureau, but the rougher weights are not regularly accepted for test.

The same kind of weights, but less accurately adjusted, may be obtained as class A and class B weights. For extreme accuracy (seldom, if ever, needed in weighings for polarimetry), when careful corrections must be made for the buoyant force of the air, only one-piece weights can be relied upon, and the volume of each weight must be determined. This requires that the weights come under class M.

Information as to the precision to which corrections will be certified for weights of classes A, M, or S, and lists of tolerances for any class, will be supplied on application to this Bureau. Full details as to specifications, tolerances, and precision of corrections, together with other information as to standard weights and some of the methods of testing them, are given in National Bureau of Standards Circular C3.

To assist in the identification of the weights, the test number assigned to the weights by the National Bureau of Standards will be stamped on the bottom of the box provided for keeping them, thus NBS Test No. 4978.

The shipping case or the inner wrappings will always be sealed when tested weights are shipped from the Bureau.

(e) SUBMISSION OF WEIGHTS FOR TEST

A written request for the testing should be sent when the weights are shipped. This should always indicate the class of weights submitted, and if two different tests are available in that class, the character of test desired. Sufficient information should also be given to enable the Bureau to identify the package.

If weights have already been used as standards in exacting work, and it is important to know what their corrections were at the close of such work, this fact should be stated; otherwise, weights are carefully cleaned before being tested.

Weights should be packed tightly. Sets in covered cases generally need extra packing inside the case. The very small sheet-metal fractional weights are especially likely to work out of place and be damaged.

Address packages and correspondence, "National Bureau of Standards, Washington, D. C."

6. BALANCES

The methods of analysis employed in the sugar laboratory require the use of a high-grade analytical balance for such operations as the determination of ash, moisture, and specific rotation, as well as a special sugar balance for the rapid weighing of sugars, molasses, and sirup for the usual saccharimetric determinations.

In selecting an analytical balance, it is well to keep in mind several essential points. The sensibility of the balance is influenced by (1) distance between the center of gravity and the point of support, (2) coincidence of the planes of the three surfaces on which the three knife-edges bear, (3) length of the arms of the beam, and (4) reduction of friction to a minimum by finely ground and polished knife-edges and planes. In addition, the sensibility is affected by the weight of the beam; in general, a balance with a light beam is more sensitive than one with a heavy beam.

The Bates sugar balance, especially designed at the National Bureau of Standards for saccharimetric work, has a number of improved features. The customary bows supporting the pans have been replaced by single-arm hangers at the back, an arrangement which gives free access to the pans and reduces to a minimum the danger of spilling sugar on the pans. The weighing scoop is adjusted to balance exactly either of the pans, thereby avoiding the use of a counterpoise weight.