One of these plates is placed in the instrument, and the field observed; it will be seen that the uniform appearance of the field is changed. The milled head is turned to the right until the exact point of neutrality is re-established, just as described above in setting the zero. The scale is read, the observation repeated, the reading taken again, and so on until five or six readings have been made. The average is taken, readings being rejected which show a divergence of more than 0.3, and the result corrected for the deviation of the zero point, if any was found, the deviation being added if it was to the left, and subtracted if to the right. If the adjustment of the instrument be correct, the result should be the value of the control plate used, as ascertained from the table, for the temperature of 20°. Each of the three plates is read in the instrument in this way. A variation of 0.3 from the established values may be allowed for errors of observation, temperature, etc., but in the hands of a careful observer a deviation greater than this with one of the three plates, after a careful setting of the zero, shows that the instrument is not accurately adjusted.
The complete verification of the accurate adjustment of the polariscope by means of three control plates, as given above, should be employed whenever it is set up for the first time by the officer using it, whenever it has sustained any serious shock or injury, and whenever it has been transported from one place to another. It should also be done at least once a week while the instrument is in active use.
After the complete verification has been performed as described, further checking of the instrument is done by means of one control plate alone, the one approximating 90°, and the setting of the zero point is dispensed with, the indication of the scale for sugar solutions being corrected by the amount of deviation shown in the reading of the 90° control plate from its established value as ascertained from the table, at the temperature of the room.
For example: A sugar solution polarizes 80.5; the control plate just before had given a polarization of 91.4, the temperature of the room during both observations being 25° C. According to the table the value of the control plate at 25° C. is 91.7; the reading is, therefore, 0.3 too low, and 0.3 is added to the reading of the sugar solution, making the corrected result 80.8. The temperature of the room should be ascertained from a standardized thermometer placed close to the instrument and in such a position as to be subject to the same conditions.
PREPARATION OF THE SUGAR SOLUTION FOR POLARIZATION.
If the sample is not entirely uniform it must be thoroughly mixed before weighing out, after all the lumps are broken up, best with a mortar and pestle. Then 26.048 grammes are weighed out on the balance in the tared German silver dish furnished for this purpose. Care must be taken that the operations of mixing and weighing out are not unduly prolonged, otherwise the sample may easily suffer considerable loss of moisture, especially in a warm room. The portion of sugar weighed out is washed by means of a jet from a wash bottle into a 100 c.c. flask, the dish being well rinsed three or four times and the rinsings added to the contents of the flask. The water used must be either distilled water or clear water which has been found to have no optical activity. After the dish has been thoroughly rinsed, enough water is added to bring the contents of the flask to about 80 c.c. and it is gently rotated until all the sugar has dissolved. The flask should be held by the neck with the thumb and finger, and the bulb not handled during this operation. Care must be taken that no particle of the sugar or solution is lost. To determine if all the sugar is dissolved, the flask is held above the level of the eye, in which position any undissolved crystals can be easily seen at the bottom. The character of the solution is now observed. If it be colorless or of a very light straw color, and not opalescent, so that it will give a clear transparent liquid on filtration through paper, the volume is made up directly with water to the 100 c.c. mark on the flask. Most sugar solutions, however, will require the addition of a clarifying or decolorizing agent in order to render them sufficiently clear and colorless to polarize. In such case, before making up to the mark, a saturated solution of subacetate of lead is added.
The quantity of this agent required will vary according to the quality of the sugar; for sugar which has been grained in the strike pan and washed in the centrifugals, from 3 to 15 drops will be required; for sugar grained in the strike pan but not well washed in the centrifugals, that is, sugar intended for refining purposes, from 15 to 30 drops will be required; for sugar not grained in the strike pan, that is, "wagon" or "string sugar," "second sugar," etc., from 1 to 3 c.c. will be required. After adding the solution of subacetate of lead the flask must be gently shaken, so as to mix it with the sugar solution. If the proper amount has been added, the precipitate will usually subside rapidly, but if not, the operator may judge of the completeness of the precipitation by holding the flask above the level of the eye and allowing an additional drop of subacetate of lead to flow down the side of the flask into the solution; if this drop leaves a clear track along the glass through the solution it indicates that the precipitation is complete; if, on the other hand, all traces of the drop are lost on entering the solution, it indicates that an additional small quantity of the subacetate of lead is required. The operator must learn by experience the point where the addition should cease; a decided excess of subacetate of lead solution should never be used.
The use of subacetate of lead should, in all cases, be followed by the addition of "alumina cream" (aluminic hydrate suspended in water)[2] in about double the volume of the subacetate solution used, for the purpose of completing the clarification, precipitating excess of lead, and facilitating filtration. In many cases of high grade sugars, especially beet sugars, the use of alumina alone will be sufficient for clarification without the previous addition of subacetate of lead.
The solution is now made up to the mark by the addition of distilled water in the following manner. The flask, grasped by the neck between the thumb and finger, is held before the operator in an upright position, so that the mark is at the level of the eye, and distilled water is added drop by drop from a siphon bottle or wash bottle, until the lowest point of the curve or meniscus formed by the surface of the liquid just touches the mark. If bubbles hinder the operation, they may be broken up by adding a single drop of ether, or a spray from an ether atomizer, before making up to the mark. The mouth of the flask is now tightly closed with the thumb, and the contents of the flask are thoroughly mixed by turning and shaking. The entire solution is now poured upon the filter, using for this purpose a funnel large enough to contain all the 100 c.c. at once, and a watch glass is placed over the funnel during filtration to prevent a concentration of the solution by evaporation.
The funnel and vessel used to receive the filtrate must be perfectly dry. The first portion of the filtrate, about 20 to 30 c.c., should be rejected entirely, as its concentration may be affected by a previous hygroscopic moisture content of the filter paper. It may also be necessary to return subsequent portions to the filter until the liquid passes through perfectly clear.