SOME TECHNICAL METHODS OF TESTING

MISCELLANEOUS SUPPLIES,

Including Paints and Paint Materials, Inks, Lubricating Oils, Soaps, etc.

INTRODUCTION.

The Contracts Laboratory of the Bureau of Chemistry is called upon to examine a great variety of articles purchased by the Government. The character of the examinations made varies to such an extent that it is not possible to outline any general methods which are not subject to more or less variation according to the purpose for which the material is intended.

In this bulletin are assembled methods which have been found useful in a large number of cases. As a general rule these methods, while not original, have been compiled from a variety of sources and modifications introduced when necessary. No attempt has been made, therefore, except in a few cases, to refer to the original source. Acknowledgments, however, are due to the following authorities: Lunge, Lewkowitsch, Rideal, Fernbach, Hall, Parry, Hurst, Low, and many others. Nor has any attempt been made to give all of the methods for the examination of any of the materials mentioned, but only those which have been found most useful in this laboratory.

The methods are presented in this form because of their inaccessibility at present-scattered as they are throughout chemical literature-and because those selected from the mass of material on the subject have been tested in practice and found to be especially satisfactory.

SPECIFIC GRAVITY DETERMINATIONS.

1. Temperature Corrections.

It is almost the universal custom among oil chemists in this country to express the specific gravity of oils at 15.5° C. (60° F.) as compared with water at the same temperature. Since this temperature is generally far below that of the laboratory, the determination is frequently made at a higher temperature and the specific gravity calculated, using the formula G=G'+K (T—15.5° C.), in which G=specific gravity at 15.5° C.; G'=specific gravity at T, and K=mean correction for 1° C.

The correction K varies somewhat for different oils, but the value K=0.00064 may be taken as sufficiently accurate in most cases for both hydrocarbon and fatty oils. These corrections, however, only apply to specific gravities referred to water at 15.5° C. (60° F.). The common custom of determining specific gravity at higher temperatures compared with water at the same temperature

For the value of K for different fatty oils see Allen's Commercial Organic Analysis, third edition, 2 (1):33, and for lubricating oils see Archbutt and Deeley, Lubrication and Lubricants, page 183.

as the oil gives values to which this correction does not apply. If the correction is applied to such values, the results obtained for 15.5° C. will contain large errors. This is well illustrated by the following example:

The specific gravity of a sample of oil was carefully determined at two temperatures, with the following results:

If the temperature correction is applied to the second value, the result is as follows: 0.9298+9.5X0.00064=0.93588 (specific gravity at 15.5° C.), an error far too large to be admissible. Now, from the known density of water at vary25° C. ing temperatures," the specific gravity at 15.5° C. may be calculated from equa

tion 2, as follows:

0.9298X

0.997071
0.999050

=0.92795.

If to this value the correction is applied the result is 0.92795+9.5×0.00064= 0.93403, a value differing less than two in the fourth place from that actually determined, which is sufficiently accurate for most commercial work.

It is a simple matter to determine the specific gravity at any selected temperature as compared with water at 15.5° C., but to do so most easily it is necessary to properly calibrate the pyknometer or plummet. The coefficient of cubical expansion of glass varies somewhat, and, while this correction should be applied when necessary in calibrating, care should always be taken to apply the correction for only a very few degrees. The Bureau of Standards in calibrating volumetric apparatus assumes the cubical coefficient of expansion of glass per degree centigrade as equal to 0.000025, and if this value is always used, on determinations within a very few degrees of the desired temperature no appreciable error will be made. Since it is not safe to assume that the cubical coefficient of expansion of glass is the same for all temperatures, apparatus should be calibrated at a temperature very near that at which the determination is to be made.

2. Westphal Balance.

The Westphal balance, on account of its convenience, is frequently used in making specific-gravity determinations, but it is difficult to determine the errors for this instrument. The weights can be easily tested, but the divisions on the arm are not easily tested, and both may be somewhat incorrect. It is better not to use a Westphal balance, but a plummet may be used on an analytical balance, which, when properly calibrated, is a very useful piece of appàratus, though the temperature is rather hard to control.

3. Vacuum Weighing.

It is not necessary to reduce weighings to vacuum, since the corrections entering in both numerator and denominator would not make any appreciable difference in the result. It should be understood that both in calibrating the apparatus and in making the specific-gravity determination all weights are made in air.

Landholt, Bornstein, and Meyerhofer, Physikalische Chemische Tabellen, pages 37 to 39; Bureau of Standards, Circular No. 19, page 27.

4. Choice of Pyknometers.

For the most accurate work with very fluid oils the Sprengel tube is the most accurate form. This apparatus is difficult to clean, and for ordinary work a specific-gravity bottle, with a cap to take up expansion, is the most convenient.

Opening 0.1 mm.
in diameter

-0.5mm.
in diameter.

A thermometer as part of the apparatus adds to the cost and has little or no advantage; in fact, it has some distinct disadvantages. Aside from errors in the thermometer, the liquid, if placed in the pyknometer at a low temperature and then allowed to stand at room temperature until the thermometer registers 15.5° C., may not be of the same temperature throughout. If a pyknometer without a thermometer is used, it is necessary to bring it to its final temperature by keeping it in a bath for some time. By filling the pyknometer at about 14° C., inserting the stopper, and then placing it in a

-0.1mm diameter
of hole

--2 mm diameter of hole

FIG. 1.-Pyknometer for very fluid liquids (actual size).

FIG. 2.-Pyknometer for viscous liquids (one-half actual size).

bath which is kept at 15.5° C. until no more liquid rises out of the capillary tube, one may be sure that the whole of the liquid in the pyknometer is at 15.5° C. The form of pyknometer shown in figure 1 is excellent for very fluid oils, but it is not suitable for those that are very viscous. For the latter use the form shown in figure 2.

20470-Bull. 109-10- -2

5. Calibration of Pyknometers.

Clean and dry the pyknometer and weigh carefully after it has stood in the balance case for about thirty minutes. Fill with freshly boiled distilled water at about 14° C., insert the stopper, and place in a bath kept at exactly 15.5° C. When the temperature of the water in the pyknometer has risen to 15.5° C., which will be shown by the water ceasing to rise out of the capillary tube of the stopper, wipe off the water on the top of the stopper, place the cap on the stopper, remove from the bath, wipe dry, and let stand until it has come to room temperature, then weigh. This will give the water value of the pyknometer at 15.5° C. Calibrate the same pyknometer at 50° C., proceeding in the same way, except that the water used in filling the apparatus is about 47° C. and is brought up to 50° C. in a bath kept at exactly that temperature. Multiply the 0.99905 weight of water at 50° C. by =1.011125. This product is the water value 0.98807

of the pyknometer at 50° C. In determining the specific gravity of any oil proceed as in calibrating, only substitute the oil for the freshly boiled water, and divide the weight of oil by the water value of the pyknometer. Results are

15.5° C. 50° C.
15.5° C. 15.5° C.

or

If it is desired to calibrate

expressed in specific gravity at the instrument at any other temperature, proceed in a similar manner and multiply the weight of water found by the density of water at 15.5° C. (0.99905) and divide by the density at the temperature used. (Use the tables of Landholt, Bornstein, and Meyerhofer, previously mentioned, page 8.)

A plummet may be calibrated in a similar manner by calculating the water equivalent at 15.5° C. of the volume displaced at the different temperatures.

6. Hydrometers.

Hydrometers are frequently used by oil chemists, and when properly made are very good instruments. They should be calibrated at 15.5° C. (60° F.) and used at that temperature when exact results are desired. If a hydrometer is T used at any other temperature (T), the results should be recorded at 15.5°C. by applying the formula: G=G'+K (T—15.5° C.).

and also reduced to

15.5° C.
15.5° C.

Unless the determination is made at the temperature at which the hydrometer is calibrated the result should not be relied on beyond the third place. If more exact results are desired, do not use a hydrometer.

7. Arbitrary Scales.

The expression of specific gravity in terms of arbitrary scales, such as Baumé, is very objectionable, since there are a number of Baumé tables in print which differ widely. It is sometimes necessary, however, to report specific gravity in degrees Baumé. When this is the case determine the specific gravity in the 15.5° C. usual manner, reduce, if necessary, to and find the degrees Baumé by 15.5° C. reference to an American standard table. Such a table has been recalculated and published by the Bureau of Standards, Circular 19, and is calculated by means of the following formulæ :

PAINT MATERIALS.

LINSEED OIL.

1. Preparation of Sample.

All tests are to be made on oil which has been filtered through paper at a temperature of between 15° and 30° C. immediately before weighing, with the exception of tests No. 6, Turbidity; No. 7, Foots; No. 9, Moisture and Volatile Matter, and No. 10, Ash. The sample should be thoroughly agitated before the removal of a portion for filtration or analysis.

2. Specific Gravity.

Determine with a pyknometer, plummet, or hydrometer at 15.5° C.

3. Viscosity.

Use the Engler-Ubbelohde method, making the determination at 20° C.

4. Flash Point, Open Cup.

Set a nickel crucible 60 mm in diameter at the top, 40 mm in diameter at the bottom, and 60 mm in height in a hole in the middle of a sheet of asbestos board 200 mm square. The bottom of the crucible should project about 25 mm through the asbestos. Support the asbestos on a tripod and suspend a thermometer reading to 400° C. in degrees in the center of the crucible, so that the lower end of the thermometer is 10 mm from the bottom of the crucible. Then pour in the oil until its level is 15 mm below the top of the crucible. Place a Bunsen burner below the crucible and regulate the size of flame so that the thermometer rises 9° a minute. As a test flame use an ordinary blowpipe attached to a gas tube. The flame should be about 6 mm long. Begin testing when the temperature of the oil reaches 220° C., and test for every rise of 3°. In applying the test move the flame slowly across the entire width of the crucible immediately in front of the thermometer and 10 mm above the surface of the oil. The flash point is the lowest temperature at which the vapors above the oil flash and then go out.

5. Fire Point.

After noting the temperature at which the oil flashes continue the heating until the vapors catch fire and burn over the surface of the oil. The temperature at which this takes place is the tire point. In determining the flash point note the behavior of the eil. It should not foam or crack on heating. Foaming and cracking are frequently caused by the presence of water.

6. Turbidity.

Note whether the oil is perfectly clear or not.

7. Foots.

Let a liter of the oil stand in a clear glass bottle for eight days, and then note the amount of sediment formed. The highest grades of oil show no turbidity or foots by this test. The claim is made that sometimes what would be called foots by the above method is due to the freezing out of fats of rather high melt