Part 48

Weight of sample used 26.048 grams. First polarization 88°.5 Polarization after inversion 10°.5 Temperature 20°.0 Percentage of sucrose 58.4 Rotation due to dextrose 30°.1

Percentage of dextrose:

66.5 : 53 = x : 30.1; whence x = 37.8.

The sample examined therefore contains 58.4 per cent of sucrose and 37.8 per cent of dextrose.

It is evident that the method just described is also applicable when maltose, dextrin, or any other sugar or polarizing body, not sensibly affected by the process of inversion to which the sucrose is subjected, is substituted for dextrose. When, however, more than two optically active bodies are present the purely polariscopic process is not applicable. In such cases the chemical or the combined chemical and optical methods described further on can be employed.

=243. Lactose in Milk.=—By reason of its definite gyrodynat lactose in milk is quickly and accurately determined by optical methods, when proper clarifying reagents are used to free the fluid of fat and nitrogenous substances. Soluble albuminoids have definite levogyratory powers and, if not entirely removed, serve to diminish the rotation due to the lactose.

Milk casein precipitated by magnesium sulfate has the following gyrodynatic numbers assigned to it:

Dissolved in water [a]{D} = -80° ” ” very dilute solution [a]{D} = -87°. ” ” dilute sodium hydroxid solution [a]{D} = -76°. ” ” strong potassium hydroxid solution [a]{D} = -91°.

The hydrates of albumen have rotation powers which vary from [a]{D} = -71°.40 to [a]{D} = -79°-05. From the chaotic state of knowledge concerning the specific rotating power of the various albumens, it is impossible to assign any number which will bear the test of criticism. For the present, however, this number may be fixed at [a]_{D} = -70° for the albumens which remain in solution in the liquids polarized for milk sugar.

Many reagents have been prepared for the removal of the disturbing bodies from milk in order to make its polarization possible. Among the precipitants which have been used in this laboratory may be mentioned:

(1) Saturated solution basic lead acetate, specific gravity 1.97:

(2) Nitric acid solution of mercuric nitrate diluted with an equal volume of water: (=88.=)

(3) Acetic acid, specific gravity 1.040, containing twenty-nine per cent acetic acid:

(4) Nitric acid, specific gravity 1.197, containing thirty per cent nitric acid:

(5) Sulfuric acid, specific gravity 1.255, containing thirty-one per cent sulfuric acid:

(6) Saturated solution of sodium chlorid:

(7) Saturated solution of magnesium sulfate:

(8) Solution of mercuric iodid in acetic acid, formula; potassium iodid, 33.2 grams; mercuric chlorid, 13.5 grams; strong acetic acid, 20.0 cubic centimeters; water 640 cubic centimeters.

Alcohol, ether, and many solutions of mineral salts, hydrochloric and other acids are also used as precipitants for albumen, but none of them presents any advantages.

Experience has shown that the best results in polariscopic work are secured by the use of either the mercuric iodid or the acid mercuric nitrate for clarifying the milk. The latter reagent should be used in quantities of about three cubic centimeters for each 100 of milk. It is evident when it is desired to determine the residual nitrogen in solution, the former reagent must be employed. The quantity of albuminoid matter left in solution after clarification with mercurial salts is so minute as to exert no sensible effect on the rotation of the plane of polarized light produced by the lactose.

For purposes of calculation the gyrodynat of lactose in the ordinary conditions of temperature and concentration may be represented by [a]_{D} = 52°.5 (=107=).

Polarization.—The proper weight of milk is placed in a sugar flask, diluted with water, clarified with the mercuric salt, the volume completed to the mark, and the contents shaken and poured on a filter. The filtrate is polarized in tubes of convenient length. The observed rotation may be expressed either in degrees of angular measurement or of the sugar scale. The weight of milk used may be two or three times that of the normal weight calculated for the instrument employed. Instead of weighing the milk a corresponding volume determined by its specific gravity may be delivered from a burette-pipette (p. 231). For the laurent polariscope three times, and for the half-shadow instruments for lamplight, twice the normal weight of milk should be used. For approximately sixty cubic centimeters of milk the flask should be marked at 105 cubic centimeters in compensation for the volume of precipitated solids or the reading obtained from a 100 cubic centimeter flask, decreased by one-twentieth.

For the laurent instrument the normal weight of lactose is determined by the following proportions:

Gyrodynat of sucrose, 66.5: lactose: 52.5 = x: 16.19.

Whence x = 20.51, that is, the number of grams of pure lactose in 100 cubic centimeters required to read 100 divisions of the sugar scale of the instrument.

For the ventzke scale the normal quantity of lactose required to read 100 divisions is found from the following equation:

66.4: 52.5 = x: 26.048

Whence x = 32.74.