In the one case three times the normal weight of milk is 61.53 and in the other twice the normal weight, 65.48 grams.
244. Error due to Volume of Precipitate.—Vieth states that the volume allowed for the precipitated solids in the original process, viz., two and four-tenths cubic centimeters, is not sufficiently large.[202] In such cases it is quite difficult to decide on any arbitrary correction based on the supposed quantities of fat and albuminoids present. A better method than to try to compensate for any arbitrary volume is to remove entirely the disturbing cause or eliminate it by indirect means. To wash the precipitate free of sugar without increasing the bulk of the filtrate unduly would be extremely difficult and tend, moreover, to bring some of the precipitated matters again into solution. It is better, therefore, to eliminate the error by double dilution and polarization ([86]). The principle of this method is based on the fact, that, within limits not sensibly affecting the gyrodynat by reason of different densities, the polarizations of two solutions of the same substance are inversely proportional to their volumes.
For convenience, it is recommended that the volumes of the samples in each instance be 100 and 200 cubic centimeters, respectively, in which case the true reading is obtained by the simple formula given in the latter part of [86].
In this laboratory the double dilution method of determining the volume of the precipitate is conducted as follows:[203]
In each of two flasks marked at 100 and 200 cubic centimeters, respectively, are placed 65.52 grams of milk, four cubic centimeters of mercuric nitrate added, the volume completed to the mark and the contents of the flask well shaken.
After filtering, the polarization is made in a 400 millimeter tube by means of the triple shadow polariscope described in [75]. From the reading thus obtained the volume of the precipitate and the degree of correction to be applied are calculated as in the subjoined example. The flasks should be filled at near the temperature at which the polarizations are made and the observation room must be kept at practically a constant temperature of 20° to avoid the complications which would be produced by changes in the gyrodynat of lactose and the value of the quartz plates and wedges of the apparatus by marked variations in temperature.
Example.—Weight of milk used in each case 65.52 grams.
| Polarimetric | reading | from | the | 100 | cubic | centimeter | flask, | 20°.84 |
| ” | ” | ” | ” | 200 | ” | ” | ” | 10°.15 |
- Then 10.15 × 2 = 20.30
- 20.84 - 20.30 = 0.54
- 0.54 × 2 = 1.08
- 20.84 - 1.08 = 19.76
- 19.76 ÷ 4 = 4.94,
which is the corrected reading showing the percentage of lactose in the sample used.