Page images
PDF
EPUB

DR. O. G. FREYERMUTH

racy for all practical purposes, as water increases its volume from 4° to 100° C. only to the extent of 0.012, or about 4

The principle of the loaded cylinder has been utilized in the construction of the Mohr specific gravity balance, of which the Westphal modification is a most desirable improvement (see Fig. 31). The specific gravity of a liquid can be quickly taken at any temperature between 7° and 30° C., since the loaded cylinder has been replaced by a short glass thermometer, which is suspended from the

[merged small][graphic][subsumed][subsumed][merged small]

end of the beam by a thin platinum wire; the adjustment having been made at 15 C., a slight variation will be observed for any higher or lower temperature. The small thermometer has a range of twenty-three degrees on the Centigrade scale, and, when suspended in air from the longer arm of the beam, establishes perfect equilibrium; when completely immersed in distilled water at 15° C. it displaces its own volume of the water and is buoyed up by a force equal to the weight of the water displaced-equilibrium of the beam being re-established by attaching the necessary counterpoise, which

is called 1.000: at 7.5° C. the necessary weight was found to be 1.001, while at 27° C. it was 0.998. As seen in the illustration, the longer arm of the beam is accurately divided into ten even spaces, and the weights, or riders, used to counterbalance the thermometer when immersed in any liquid, are made of brass and aluminum ; they are so constructed that each smaller rider is of exactly the value of the next larger, the largest rider and the counterpoise used to balance the thermometer in water, however, being of the same weight or value. Without the necessity for calculation, if the temperature of the liquid be at 15° C., the specific gravity of the liquid can be at once read off, after the equilibrium of the beam has been established; for instance, in testing alcohol at 15° C., the counterpoise necessary to balance the beam in water will be found too heavy if attached at the same point in alcohol, hence it is removed, and the largest rider is placed in the first, or, if necessary, in the second notch on the beam, where it may appear a little too light, and then the smaller riders are added as may be necessary to balance the beam perfectly. The value of each of the two larger riders, when suspended from the end of the beam, is considered as 1.000, while the three smaller riders are valued at 0.100, 0.010, and 0.001 respectively; when removed to the top of the beam the value of each rider is reduced by for every notch. If one of the large riders be placed at the notch marked 8, a second rider at 2, and a smaller rider at 1, the specific gravity of the alcohol must be read as 0.821. In the case of chloroform and all other liquids specifically heavier than water the large counterpoise is suspended from the end of the beam, and the other riders are placed in the notches as may be necessary; thus chloroform may require all four riders on the beam, the largest at 4, the second at 8, and the smaller two at 9, which would be read as 1.489 specific gravity. Whenever two riders of different weight are required in the same notch on the beam, the lighter of the two is suspended from the hook of the heavier, as shown in Fig. 32; thus the specific gravity of liquids can be read with accuracy to four decimal places. The Mohr or Westphal balance cannot be used, however, if only very small quantities of liquid are available, as sufficient liquid is required to immerse the glass thermometer completely.

Specific gravity beads, also known as Lovi's beads, are small, sealed, pear-shaped glass bulbs of various specific weights, which have been carefully ascertained and are marked on them; these beads will float indifferently in any liquid having the same specific gravity, and may be used in reducing liquids to a fixed specific gravity by dilution or evaporation. If a bead marked 0.93 be placed in a jar of alcohol it will sink-unless the liquid happens to be official diluted alcohol-but will slowly rise upon the addition of water, until a sufficient quantity has been added to increase the specific gravity of the mixture to that indicated on the bead, when it will float about midway in the liquid. Results obtained

with specific gravity beads are never so accurate as with other methods.

Hydrometers, or areometers, are instruments intended to indicate either the density or specific gravity of liquids, and in some cases also the percentage by volume or weight of certain liquids. They

[merged small][merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small]

consist of a glass tube having a bulb blown at one end, a little above which the tube is usually expanded cylindrically for a short distance, and then terminates in a long stem in which is securely fastened a graduated paper scale (see Fig. 33). The bulb is filled with mercury or small shot, so as to enable the instrument to assume a vertical position when floated in any liquid. Hydrometers, like all floating bodies, displace their own weight of a liquid and sink in it to a depth proportional to the volume of liquid displaced, which volume is equal in weight to the weight of the instrument; thus, by

FIG 33.

comparison of volumes displaced, the densities and specific gravities of various liquids can be ascertained. While the great majority of hydrometers are so constructed that with constant weight they will sink to varying depths in different liquids, some are made to sink to a uniform depth in all liquids by the addition or subtraction of weights, and the density, or specific gravity, is calculated from such change of weight; this latter class can also be conveniently used for taking the specific gravity of solids.

30

Hydrome

ter plain.

Specific gravity hydrometers are made with the unit mark 1.000 at a point to which the instrument sinks in distilled water at normal temperature (usually 15.6° C. or 60° F.), and then have the scale carried above and below this point, each mark on the scale indicating either 0.001, or 0.005, or 0.010, according to the intended delicacy of the instrument. As specific gravities of liquids range from 0.700 to above 2.00, the tube of a hydrometer carrying such a scale would have to be inconveniently long to permit of a fair reading of it; hence specific gravity hydrometers usually come in sets of four, ranging from 0.600 to 1.000, from 1.000 to 1.400, from 1.400 to 1.800, and from 1.800 to 2.200. When intended for testing the specific gravity of special liquids the scale is usually much shorter, and thus permits of more accurate gradu

ation.

spe

By far the larger number of hydrometers are intended for determining the density of liquids irrespective of cific gravity; they are extensively employed for technical purposes and are based on arbitrary scales, no two of which agree, but which can be converted into specific gravity by certain rules. To this class belong Baume's, Twaddell's, Cartier's, Zanetti's, Sikes', Beck's, Jones' and other hydrometers. Since Baumé's hydrometers are largely used by manufacturing chemists in this country, and the degrees Baumé are often stated on labels, the instrument is of special interest to pharmacists.

Baumé had two hydrometers, one for liquids heavier than water and the other for liquids lighter than water; the former was called Pèse-Acide, or Pèse-Sirop, and the latter Pèse-Esprit. For liquids heavier than water the zero was placed at the point to which the instrument sank in distilled water at 15.6° C., and the point to which it sank in a solution of 15 parts of dry table salt and 85 parts of distilled water, also at 15.6° C., was marked 15; the distance between these two points was then divided into 15 equal parts, called degrees, and the scale extended as far as the length of the tube would permit. The zero for liquids lighter than water was found by immersing the instrument in a solution of 10 parts of dry table

salt and ninety parts of distilled water at 15.6° C. in such a way that the long stem would be almost entirely out of the liquid; the point to which the instrument sank in distilled water, also at 15.6° C., was marked at 10, the space between the two points being divided into 10 equal parts and the scale extended as in the other case. The slightest error in obtaining the first interval is increased upon extension of the scale; hence it is almost impossible to find two instruments adjusted by the old method to correspond exactly. A more accurate and equally practicable method is to obtain the exact specific gravity of two liquids compared with distilled water at a fixed temperature, place these at the extremes of the scale, and then divide the intervening space into the requisite number of degrees. The liquids chosen in this country, for liquids heavier than water, are concentrated sulphuric acid having the specific gravity 1.8354 at 15.6° C., and distilled water; and for liquids lighter than water, highly rectified ether having the specific gravity 0.725 at 15.6° C., and distilled water; the space between the points to which the hydrometer sinks in the water and the acid is divided into 66 parts, or degrees, and the space between the points to which it sinks in the ether and the water into 53 parts. For all liquids heavier than water the scale is read from above downward, while for liquids lighter than water it is read from below upward. (See 30 Figs. 34 and 35.)

As it is frequently desirable to know the specific gravity for any given degree on the Baumé scale, and vice versa, the following rules have been formulated.

For liquids heavier than water: Subtract the degree Baumé from 145 and divide the remainder into 145 to find the specific gravity.

Divide 145 by the specific gravity and subtract the quotient from 145 to find the degree Baumé.

For liquids lighter than water: Add the degree Baumé to 130 and divide. the sum into 140 to find the specific gravity.

Divide 140 by the specific gravity and from the quotient subtract 130 to find the degree Baumé.

FIG. 34.

10.

20

20

00

FIG. 35.

70

70

60

60

50

50

[ocr errors]

40

40

30

50

20

60.

10

[blocks in formation]

The moduli or constants employed in these rules express the proportion which the weight of water displaced by the hydrometer when

« PreviousContinue »