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larities in the glass, may be stoppered readily, require less oil and, being thinner, may be more easily seen through. The bulb is suspended in the top of the test tube, as shown in Fig. 2. The tube is held in the hand a t arm's length, in such a position t h a t some distinct horizontal line in the distance may be seen through the bulb. This may be, for instance, the horizon or t h e border line between a grass lawn and some distant buildings. T h e tube is tipped forward or backward slightly until it is vertical, as is shown by the fact t h a t the line as seen through the center of the bulb appears straight. Then by slowly raising and lowering the bulb about a n eighth of an inch i t may be readily observed whether the oil .in t h e bulb has a greater or lower refractive index t h a n the surrounding medium, according as i t shows the phenomenon represented in the upper or the lower line in Fig. 3. The refractive index of a n unknown oil in t h e bulb may be obtained by comparing i t with a series of known oils contained in test tubes. A series of such oils was prepared and arranged in order so t h a t with a bulb of a n unknown oil it was a simple matter t o obtain the refractive index. These, together with a solution of glycerol (1.4545) a n d a sample of toluene (1.496), constitute a series with which t h e refractive index of oils may be determined with a n accuracy greater than the normal variation between different specimens of the same oil. This method was proved t o be accurate t o 0.0005.
547
The exact values for these oils were obtained on the Abbe refractometer, and were as follows:
............................. .............................. ........... ......................... Corn ............................... Rape-seed .......................... Castor.. ........................... Linseed. ........................... Sperm Olive Olive-cottonseed mixture.. Cottonseed
1.4655 1.4703 1.4713 1.4735 1.4758 1.4778 1.4796 1.48aO
(Corrected to I S o C.)
The AbbC refractometer is said t o be accurate t o 0.0002, but does not always check up as closely as that. The lard oil and olive oil with which the author was working were very close t o each other in refractive index. The lard oil is usually listed below olive oil, but was shown t o be higher by the above-described method. Values obtained on the Abbe refractometer showed this t o be correct, the difference being 0.0005. There is one difficulty in the use of this method which should be mentioned. The glass of which the bulbs are made produces a slight effect similar t o t h a t of a concave lens in air. SO for a bulb filled with a n oil and immersed in a tube of the same oil there is a slight effect, as shown in t h e middle line of Fig. 3. With a little practice one can tell how much of the effectis due t o the oil and how much is due t o the glass in the bulb. This effect is reduced t o a minimum by the use of bulbs with extremely thin walls blown a s shown in Fig. 1, which is drawn a little smaller than natural size. The bulbs should have a diameter between three-eighths and one-half inch.
Microanalytical Methods in Oil Analysis' By Augustus H. Gill and Henry S. Simms MASSACHUSETTS INSTITUTE OD TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS
Although much work has been done on perfecting the methods for identifying oils, little attention has been paid t o reducing the quantity required for analysis. Occasionally, as in extracting oils from leather, the oil chemist is called upon t o identify a quantity of oil so small in amount as t o handicap him in obtaining accurate results. The purpose of this paper is t o show t h a t a n accurate proximate analysis may be made upon a n oil when only a few drops are available, and with a n accuracy comparable t o t h a t of the usual methods. For t h e present work attention has been focused on four oils, selected because of their widely differing properties. These were olive, lard, cottonseed, and raw linseed. It is safe t o assume t h a t these oils represent in their properties all classes of saponifiable oils. Any adaptation of the general tests which would apply t o them would apply equally well t o others. The tests t o which most attention was given were the iodine number, saponification value, and specific gravity. APPARATUS
The apparatus used in obtaining the iodine numbers and saponification values is shown in Figs. 1 t o 7. Fig. 1 represents t h e ordinary titration apparatus on a small scale. The bottle was a liter bottle and 1
Received January 21, 1921.
the buret was a n ordinary buret-pipet of 10-cc. capacity. For this purpose one 30 cm. long was selected. A ball or bead valve was used. It was of course necessary t h a t the drops falling from the nozzle tip be as small as possible. T o this end the tip was so drawn
out t h a t the outlet was on the side of t h e t i p about half way down, t h e lower half being a fine glass rod down which the solution would run and fail off in fine drops. (The same effect may be produced with a finely pointed tube smeared with a layer of grease.) Apparatus of this description was used for the sodium thiosulfate in the iodine number determinations
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and for the standard hydrochloric acid in the saponification value determinations. Figs. 2 and 3 show the arrangement for filling the pipet used for the iodine solution, the potassium iodide solution, and the alcoholic potash solutiofi. The capacities of these pipets were 3 cc.’, 1 . 5 cc., and 2 . 5 cc., respectively. They were blown for this particular work with unusually fine stems. They were fitted into stoppers of the respective reagent bottles, together with a fine capillary tube, as shown. On blowing through the capillary tube the pipet was filled with the reagent. It was raised (Fig. 3), allowed t o drain t o the mark on the stem, and emptied in the usual manner. With a very fine stemmed pipet small quantities of liquids may be handled with negligible error. With the arrangement described above, such liquids as iodine solution may be handled without danger of breathing the fumes, and alcoholic potash solution may be handled without danger of serious contamination. It might be advisable in this latter case t o seal a bulb of lime in the mouth tube t o absorb the carbon dioxide in the breath. This was not done by the authors, and no trouble resulted from this, since blanks were run with each determination. Fig. 4 shows the syringe pipet devised by the authors t o measure the chloroform in the iodine number determinations. It was blown and ground t o fit the bottle, and was calibrated to deliver 1 cc. of chloroform. Fig. 5 shows the dropper used t o deliver two large drops of starch solution in the iodine number determinations. A similar dropper used for phenolphthalein would deliver one small drop. Twenty-five-cc. Erlenmeyer flasks with small funnels made from glass tubing, as shown in Fig. 6, were used in obtaining the saponification FIG.8-GRAVITOM~ETERvalues. Iodine numbers were determined in 25-cc. glassstoppered weighing bottles, as shown in Fig. 7. A stirrer was used t o stir the solution while titrating. This proved t o be very necessary in keeping the solution mixed during the titration. The apparatus used in determining the specific gravities will be discussed later (Fig. 8). The general size of the apparatus used may be seen by comparing the hand in Fig. 3 with the apparatus in Figs. 1 t o 7. QUANTITIES U S E D
The quantities of reagents were, in general, onetenth those usually required for the same tests. It would be possible, without doubt, t o obtain good results with still smaller quantities. This, however,
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seems unnecessary. More dilute reagents would be required, and this would involve many difficulties, aside from the trouble of preparing the reagents. T h e rate of addition of iodine t o the oil would have t o be studied with the new strength of iodine solution. Difficulty might be encountered in saponifying some oils in more dilute alcoholic potash solution. The reagents used by the authors were those customarily used for saponification and iodine number determinations (Hanus method), except t h a t the standard hydrochloric acid was 0 . 1 N rather than 0 . 5 N . Similarly, the thiosulfate solution might be diluted from 0 . 1 N t o 0 . 0 5 N , if desired. However, t h e results obtained by the authors could be reproduced more easily than could results obtained with more dilute solutions. Furthermore, i t is doubtful if it would be desirable t o obtain the saponification value with less t h a n a n ordinary drop of oil or t o find the iodine number on less t h a n 11 mg. (a small drop of oil). It was found t h a t the samples of oil could be reduced t o considerably below one-tenth the normal quantity, while still using the same quantities of reagents. I n the case of the iodine number, the quantity was reduced t o 11 mg., which is one-fourteenth t o onetwenty-seventh of the usual sample. Even better results were obtained with the saponification value, for here the weight was reduced t o less t h a n 25 mg. without preventing accurate results. This is one-fortieth t o one-eightieth of the usual amount. It is not t o be expected t h a t the oil analyst would be called upon t o make a n analysis with such small samples on any but rare occasions. On such occasions he would find i t a simple matter t o make such a n analysis if the size of his apparatus and the quantities of his reagents were all reduced by the same factorone-tenth. METHOD O F WEIGHING
The method of weighing used throughout this work was the single-swing method of Paul.’ The principle involved is t h a t of taking a single scale reading as the pointer makes its first swing. The pans are so adjusted t h a t when the pans are released the pointer will swing t o the right. The distance t o which i t swings is observed, and the pan release is thrown off. This can be done very quickly. The balance which was used worked so well by this method t h a t its accuracy could be relied upon without repeating the swing, though i t was always checked up by a second reading taken in the same way. For convenience, the scale was so numbered t h a t the mark three spaces t o the right of the center was zero. Scale divisions t o the right of this were positive, while those t o the left were negative. The weight on the pointer arm was so adjusted t h a t a swing of one scale division was equal t o a difference in weight of 1 mg. The balance was so adjusted t h a t when there were no weights or equal weights on both pans the pointer would swing t o the right as far a s the zero mark on releasing the pans. Thus, in adding the weights in making a weighing, the pans are released when each weight is added just 1
J . Am. Chcm Soc ,4f (l919), 1151.
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549
long enough t o see which way t h e pointer starts t o swing, and, when weights have been added t o t h e nearest 10 mg., a reading is taken of t h e distance which itbe pointer swings. If, for instance, it swings t o t h e $ 2 mark, one knows t h a t the object weighs 2 . 0 mg. more t h a n the weights on t h e pan. Accordingly, t h e rider is placed on the 2 . 0 mg. mark and another “reading is taken, which should be exactly zero. This method gave entire satisfaction, being not only quick b u t accurate.
The gentle heat of a water b a t h may be used t o aid the drying if this seems advisable. T h e authors used a water bath consisting of two tin pans of 2- or 3-qt. capacity, 26 cm. in diameter b y about 8 cm. deep. One of these was filled a third full of water and placed on a tripod over a burner. The other pan floated on t h e water in the first. Flasks containing solutions t o be evaporated could be placed in the upper pan a n d receive a constant and even heat.
D A T A F O R OILS B Y U S U A L LARGER-SCALE M E T H O D S
The apparatus has already been described. I n general, the method is the same as t h a t commonly used for the Hanus method. The quantities of t h e reagents were reduced t o one-tenth the usual amounts. The samples were reduced t o 11 mg. A few grams of oil were poured into a small beaker. A short stirring rod was placed in t h e beaker and the weight obtained t o the fourth decimal place. A drop of oil was allowed t o run from t h e rod into t h e weighing bottle, and t h e beaker was weighed again. T h e second sample was removed and the beaker was weighed t h e third time. T h e difference between two weighings gives the weight of t h a t sample. Six samples and two blanks were usually run a t a time. T h e oil was dissolved in 1 cc. of chloroform delivered from the syringe pipet. After all (or nearly all) of the samples had been weighed out, 3 cc. of iodine solution were delivered from the pipet (Figs. 2 and 3) into the first sample. Five minutes later iodine was added t o the second sample, a n d in like manner all the samples were treated a t 5-min. intervals. After each sample had been acted upon by t h e iodine solution for exactly 15 rnin., 1 . 5 cc. of potassium iodide solution were added from a pipet, and immediately titrated with 0 . 1 N Na2S20s solution. The above method allows 5 min. for each titration. It is essential t h a t t h e time of reaction be exactly 15 min., as the iodine number will be high or low, according a s the action is allowed t o continue more or less t h a n this time. During the titration t h e solution cannot be mixed b y putting the cover on and shaking, a s this is sure t o cause a loss. T h e mixing can be done very satisfactorily b y t h e stirrer shown in Fig. 7. With this stirrer t h e standard solution may be r u n i n fairly rapidly right u p t o t h e poiht where t h e starch is added without danger of running past t h e end-point. THE BEAD vALvE-Another precaution has t o do with the use of the bead valve of t h e buret. It is customary when using one of these to’pinch the bead between t h e fleshy surfaces of the t h u m b a n d forefinger, thus producing a wrinkle in the tubing through which the solution may pass. This invariably produces a n error which, while it may be negligible in ordinary work, is a serious matter when a n error of 0.01 cc. is not allowable. The surface of t h e t h u m b presses not only on t h e tubing around t h e bead b u t also on t h e tubing above and below t h e bead. On removing t h e pressure of t h e t h u m b t h e tubing resumes its normal shape a n d a bubble of air is drawn u p into t h e tip. This would be permissible if the bubble
Before going into the microanalysis of t h e oils which had been selected it was necessary t o determine first t h e properties of these oils b y the usual methods a n d with t h e usual amounts. This was done with care, a sufficient number of determinations being run in each case t o warrant reliance on t h e results obtained. OIL
Specific Gravity 15O C. Lard.. 0.932 Olive.. 0.918 0.922 Cottonseed., 0.934 Linseed..
.......... ......... .... .......
Saponification Value 195.1 193.9 195.0 191.4
Iodine Number 60.9 84.2 110.3 173.8
EBFECT O F E X T R A C T I O N O N O I L S
It was highly important t o know with certainty whether or not any change is brought about in the properties of a n oil when i t is extracted with a n organic solvent, such a s gasoline or ether, and subsequently freed from t h a t solvent by evaporation. If the solvent were able t o affect the iodine number, for instance, of a n oil, t h e practical application of microchemical methods for identifying oils would be seriously handicapped. If t h e analyst has a small sample of leather from which he can extract only a few drops of oil, he can identify t h a t oil b y microchemical methods only if he is certain t h a t there is no change i n its properties as a result of the extraction. This was tested with t h e four oils by the following method: A few drops of the oils were poured into a n Erlenmeyer flask and dissolved in ether or gasoline. T h e solvent was then evaporated off in a current of air in the case of the nondrying oils, and of carbon dioxide in the case of the drying oils. When t h e oil had been dried t o a constant weight, samples were removed b y means of a stirring rod, t h e weight of the samples being determined b y difference. Both t h e iodine number a n d saponification value were determined on each oil. The results of the iodine number determinations are given below. OIL
From Gasoline
.......... 84.0 60.8 Olive.. ......... Cottonseed. ..... 110.6 Linseed.. ....... 173.0 Lard..
From Ether 60.7 83.2 111.0 172.0
Original Value 60.9 84.2 110.3 173.8
T h e saponification values agreed with the true values within t h e limits of error of t h e determination. From t h e above results it may be seen t h a t there need be no doubt in the mind of the analyst as t o the possibility of error a s a result of chemical action during the extraction, provided, of course, t h a t t h e precaution is taken in the case of drying oils t o evaporate off the solvent in a n inert atmosphere, such as carbon dioxide or illuminating gas.
DETERMINATION O F IODINE NUMBER
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were the same size in all cases, but its size depends upon the manner in which the thumb is removed, and i t may be large or not form a t all, according t o the amount of pressure on the tubing below the bead after the flow had ceased. Instead of pressing with the fleshy surfaces of the thumb and forefinger, one should press on one side with the end of the thumbnail and on the other side with the end of the forefinger near the nail. I n this way no air is drawn u p and the flow may be so easily regulated t h a t no drops of solution are allowed t o remain on the tip. READING THE BURET-In analytical laboratories various devices are used t o aid in reading burets. Among these are the use of paper (usually black paper) wrapped around t h e buret and fastened with a pin; the telescope which is fastened t o the buret and slides up and down on it; and the use of burets having a blue line flashed on the side opposite the scale. The experience of t h e writers is t h a t these methods are all cumbersome, inconvenient, slow, and not so accurate as might sometimes be desired. Accordingly a different method, which has none of these faults, was adopted. A pocket mirror was placed against the back of the buret a t the level of the meniscus and swung around so t h a t the observer was looking directly into it. By raising or lowering his head until the scale graduation nearest the meniscus was in line with its image in the mirror he was sure t h a t there could be no error through parallax. Without moving his head, the observer then swung the mirror around (keeping i t in contact with the buret all the while) until the image of the light from the window could be seen in such a position as t o make the meniscus very clearly defined. The observer must be facing in a direction neither toward nor directly away from the window. This can be done very quickly, and one can read with remarkable accuracy, without taking any more time t h a n would be required t o get a n approximate reading without any appliance. The tip of t h e buret was of the type described above. Even in this way i t was difficult t o get drops smaller than one-sixtieth cc. Since the required accuracy did not permit an error of 0 . 0 1 cc., this caused another difficulty which was overcome by transferring, near the end-point, half a drop a t a time from the tip t o t h e solution by means of the stirring rod. More accurate results were obtained, however, by t h e use of a tip having a longer glass thread on the end which dipped into the solution. I n this way drops were not formed a t all, but the standard solution could run out in quantities much smaller than a drop. There was the serious difficulty, however, t h a t it was hard t o tell how fast the solution was flowing. The results obtained show t h a t the method is practicable. Time did not permit the continuation of the work after this point. The values for linseed oil were obtained in the first attempt t o use the microchemical apparatus. T h e samples in Determinations 3 and 4 were only 0 . 9 and 11.2 mg., respectively. While they both gave
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rather high values, i t is worthy of notice t h a t t h e error was no greater. TABLE I-IODINE NUMBERS DETERMINED BY MICROCHEMICAL METHODS No. 1 2 3 4 5
Weight of Sample Gram
Linseed Oil 0.0167 0.0196 0.0109 0.0112 0.0155
Lard Oil 0.0140 0.0112
.............................
AVERAGE.. Value previously obtained..
170 175 179
I!%
........................... ............ ..............
AVERAGE.. Average of first three values.. Value previously obtained..
1 2
Iodine Number
177.4 174.7 173.8 61 63 62 .61
.................
Cottonseed Oil
1 2 3 4 5 6
..............................
AVERAGE Value previously obtained..
...............
107 110.3
The weight of the second sample of lard oil was only 1 1 . 2 mg. Of the six values for cottonseed oil, it is noteworthy t h a t the one coming closest t o the true value is t h a t obtained from the smallest sample, 1 2 . 5 mg. DETERMINATION O F SAPONIFICATION VALUE
Samples of t h e oil were weighed out b y t h e methods already discussed. The samples were of one or two drops. After i t had been found t h a t good results could be obtained with a single drop, no larger samples were used. The samples were dropped into a 25-cc. Erlenmeyer flask, and 2 . 5 cc. of alcoholic potash were added from a fine-stemmed pipet (Figs. 2 a n d 3). The flasks were covered by means of small funnels made from glass tubing. The saponifications were carried out in the arrangement of two sauce pans, previously described. This gave a low even heat. The ebullition of the water produces a movement of the upper pan which serves t o keep the flasks agitated, thus aiding the reaction and preventing bumping. With some oils a greater heat may be required. After the reaction was complete the excess of potash was titrated against standard 0 . 1 N hydrochloric acid. It was impossible t o obtain sufficient accuracy in reading the buret with the 0 . 5 N acid ordinarily used. However, two out of six values for lard oil with 0 . 5 N acid were 196, t h e true value being 191.5. The other values were all abnormal. TABLE 11-SAPONIFICATION VALUES DETERMINED BY MICROCHEMICAL MRTHODS Weight of Sample Saponification No. Gram Value Cottonseed Oil1
0.0750 0.0833 0.0628 0.0244 0.0511
.............................. ..............
AVSRAGE Value previously obtained.. Olive Oil
1 2 3 4
0.0486 0.0470 0.0244 0.0255
............................ ................
194 195 193 195 195 194.4 195 193.7 196 194 192 193.5 193 9
AVERAGE.. Value previously obtained. 1 The values for cottonseed oil represent the results of five simultaneous determinations.
.
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The above values speak for themselves. Good results were obtained with a s little a s 15 mg. These were usually mixed in with poor values, however. Probably the greatest error occurred in the titrations, since t h e end-points were not sharp. Phenolphthalein was used as t h e indicator. This normally gives a sharp, distinct end-point; but with dilute solutions, when a n accuracy of 0 . 0 1 cc. is required, the end-point is less satisfactory. ATTEMPTS TO PERFORM MORE T H A N O N E TEST ON T H E SAME DROP OF OIL
Considerable time was spent in a n attempt t o make two or more determinations on t h e same sample of oil. It was thought, for example, t h a t i t might be possible t o get the refractive index on a drop or two of oil, absorb the oil on filter paper, obtain the saponification value, and then get t h e melting point of the f a t t y acids and their iodine value. These attempts were entirely unsuccessful. I n recovering t h e oil after having obtained t h e refractive index b y the Abbt refractometer, the method first employed was t o absorb the oil on filter paper, extract with ether, evaporate off the latter, weigh, and run a saponification number on the oil. The values obtained were low in all cases. This was not due t o anything absorbed by t h e ether from the filter paper, as blanks run with t h e filter paper showed no error. T h e error may have been due t o the condensation of moisture during t h e evaporation of the ether. Although t h e oil was dried carefully, the water may not have been totally removed. I n t h e second method t h e filter paper was weighed before a n d after absorbing the oil, and the saponification was made in the presence of the filter paper. The results were all high, Subsequent investigation showed t h a t the filter paper itself was hydrolyzed and used u p some of t h e potash. Hardened filter paper was also acted upon by t h e alcoholic potash. Asbestos was unsatisfactory as a n absorbent because i t absorbed the caustic t o such a n extent t h a t a distinct end-point was impossible. Inasmuch as there seemed t o be no material which would act satisfactorily as a n absorbing agent, this part of the work was abandoned. It was next attempted t o isolate the fatty acids produced during the saponification and t o use these t o obtain t h e iodine number and melting point. After t h e saponification values had been determined the solution was acidified with a n excess of hydrochloric acid. It was allowed t o stand in contact with ether until the water layer became clear. The ether was separated, a n d the aqueous layer was washed twice with ether. The combined ether extracts were evaporated in a 25-cc. weighing bottle t o constant weight, and t h e iodine number was determined. Although extraction with chloroform was tried, as well as other modifications, t h e results were in all cases low. An attempt which was made t o remove the last traces of water from the f a t t y acids from a n olive oil saponification by means of a current of air resulted in their oxidation. It is probable t h a t if the f a t t y acids were dried carefully in a n inert atmosphere, after alcohol
55 1
had been added t o lower the vapor pressure of the water, satisfactory results could be obtained. SPECIFIC GRAVITY
Specific gravity may be obtained by a variety of methods. The Westphal balance is the most common, but requires a large amount of oil. Small pycnometers may be used, but require much care in handling, especially when dealing with small quantities of oils. T h e gravity may be determined by weighing a definite volume measured from a pipet, but i t is hard t o deliver a definite volume from a pipet, on account of t h e viscosity of t h e oil. It was accordingly deemed advisable t o devise a different method for obtaining specific gravities when only small quantities of the oil are obtainable. It was thought possible t o obtain the gravity by smearing a weighed quantity of oil on the bottom of a very delicate hydrometer and noting the difference in the height t o which t h e hydrometer would rise. Hydrometers were made for the purpose, but the method proved t o be impracticable, even when the bottom of t h e bulb was made concave t o prevent drops of oil from rising t o the surface of the water. The sensitivity of a hydrometer depends on t h e size of the stem and t h e difference between the densities of the medium and air. The density of air being negligible in comparison with t h a t of liquids, we may say t h a t t h e sensitiveness of a hydrometer is inversely proportional t o the diameter of the stem squared. Hence t o make a hydrometer sensitive i t is necessary t o make the stem as fine as is practicable. The hydrometer' shown in Fig. 8 was devised t o obtain the specific gravities of small quantities of oils. After experimentation with different sized stems and bulbs, t h e hydrometer shown was found t o give t h e best results. There are two bulbs: the lower one, t h e usual hydrometer bulb, while the upper one (not connected with the lower) is filled with the oil or other substance whose gravity is desired. This is filled by dipping the little side tube into some of the substance placed on a flat watch glass and sucking through the stem, which is open a t the top. The bulb is filled t o a little above the mark on the stem with the liquid, which is then drawn down t o the mark by means of a piece of filter paper applied t o t h e end of the short tube. The stopper of soft rubber with a small hole in i t is then placed on t h e end of the stem. This prevents the entrance of water into the bulb by capillary attraction when the bulb is immersed in water. T h e bulb having been filled with oil, the hydrometer is immersed in water in a 2-liter graduate. The height of t h e hydrometer in the water is observed by reading on the scale of the graduate the height of the bottom of the hydrometer. I n this way it is not necessary t o calibrate t h e stem of the hydrometer, and a smaller stem may be used. T h e bulb is emptied by blowing through t h e stem. It is rinsed out with alcohol and ether. 1 With regard to this reservoir hydrometer it should be said that the same principle has been used before by Eichhorn [Z.anal. Chem., SO (1891), 2161, but the instrument here described has advantages over other instruments in the ease of filling and emptying the bulb and the method of reading the height of the liquid.
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T h e hydrometer must first be calibrated by filling t h e bulb with water. After the reading has been taken, i t is filled with a n oil of known specific gravity a n d another reading taken. The gravity is calculated as follows:
-1 --G
1- GS
R-W - Rs-W where G = specific gravity sought Gs = specific gravity of the standard oil R = reading with unknown oil Rs = reading with standard oil W = reading with water The gravity sought is then:
After the hydrometer has been standardized t h e ratio
(’ - Gs) is known. (Rs- W)
Substituting for this the letter F, we have: G = 1-(R-W)F where W is known when the jar is filled t o a certain height. All t h a t is then necessary is t o take a reading with the oil and t o substitute this in the last formula. T h e factor F for t h e hydrometer used b y t h e writers 1 was 2570’ The factor depends not only on t h e size of the stem, but also on t h e capacity of the reservoir bulb. This bulb, in the writers’ instrument, has a capacity of 1 . 2 cc. Smaller bulbs were tried b u t found impracticable, owing t o t h e extreme fineness of the stem necessary t o produce the desired accuracy and sensitivity. T h e diameter of t h e stem used was 1 mm. It is true t h a t this does not offer a method for obtaining specific gravities when the total amount of oil available is less t h a n 1 cc. It offers, however, certain advantages over t h e use of t h e pycnometer or t h e weighing of a definite volume measured from a pipet: 1-The method is much simpler, since no weighings are required. 2-The chances for error are much smaller, since there is always a danger of a large error in weighing a glass vessel, owing to the adsorption of an appreciable layer of moisture, the amount depending upon the humidity of the atmosphere. When a pycnometer is used, three weighings must be made, unless the instrument has been calibrated and the weight etched on it. With the hydrometer described, one reading is sufficient after it has once been standardized. The only source of error lies in reading the scale, providing there is not much variation in temperature and care is exercised in filling the bulb exactly t o the mark on the stem. A glance at the instrument after taking the reading would show whether the meniscus of the oil in the stem had crept up, because of a leak in the stopper. The error in reading the scale would be negligible if a graduate were used having graduations passing all the way around the cylinder. The errors involved in weighing a measured amount of oil are large, both in measuring the volumes and in weighing the oil. 3-The time required is less. It is much easier to take a reading of the height of the hydrometer in the jar than to weigh a pycnometer. As in the pycnometer, t h e oil may be totally recovered a n d used for other determinations. T o distinguish this instrument from a n ordinary hydrometer, it might be called a “gravitometer.”
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It was used t o determine t h e specific gravities of the four oils. T h e bulb was filled roughly t o about the right height, and the readings were taken rather hastily. The variations from the readings obtained on t h e Westphal balance were as follows: OIL
Lard
.......................... ......................... ....................... ..................
Olive Linseed Cottonseed.. AVERAGE..
..................
ERROR 0.000
0.003 0.004 0.001
0.002
These results are remarkable when i t is considered t h a t there was no mark on the stem of the instrument a t the time t h a t the readings were taken. It was found necessary t o etch a mark on the stem and t o be very careful t o fill exactly t o the mark. This having been done, t h e error was reduced t o less t h a n 0.001. Had t h e marks on the graduate passed all around the cylinder, i t is probable t h a t t h e results would check t o 0.0001. This is a greater accuracy t h a n is required for oils, because various specimens of t h e same oil vary among themselves from 0.003 t o 0.01 or more. However, i t is worth knowing t h a t this accuracy may be obtained when i t is desired. An instrument t o be used exclusively for oils might have a smaller bulb and still be sufficiently accurate. Another possible method for determining specific gravities is as follows: If a drop of oil is suspended in a mixture of alcohol a n d water of such a concentration t h a t t h e drop neither rises nor sinks, t h e oil a n d the mixture have t h e same density. If the density of t h e alcohol-water mixture is determined, t h a t of t h e oil will be known. This might be done by running alcohol into water until a drop of oil floating on t h e surface of the water would remain suspended in the solution without tending t o rise or sink, and determining the specific gravity of the solution with a pycnometer or Westphal balance. When this method was tried in a qualitative way t h e following objection was discovered: If, after the proper mixture had been obtained t o suspend a drop of oil, a fresh drop of t h e same oil is dropped in, i t sinks t o the bottom and remains there for a time (possibly half a minute, depending on t h e size of t h e drop) before i t can be made t o remain suspended in the solution. Evidently t h e oil absorbs a certain amount of alcohol from the mixture and then, becoming lighter, will remain suspended. Hence i t may be seen t h a t t h e values obtained would all be low. The magnitude of this error was not determined quantitatively. It is furthermore difficult t o mix t h e alcohol solution so thoroughly t h a t it will have t h e same density a t the top as a t t h e bottom. SUMMARY
1-Very close analytical results on the saponification a n d iodine values of oils are obtainable with 15 and 11 mg., respectively, or about one one-hundredth and one-thirtieth t h e usual quantities. 2-Good results can be obtained for specific gravity with 1-g. samples. 3-The apparatus is t h a t ordinarily found in t h e laboratory or easily made by a good manipulator.
