TITER.
At the present time the prices of glycerol and caustic potash are abnormally high, but the committee has considered that the methods adopted are for normal times and normal prices. For routine work during the period of high prices the following method may be used for preparing the fatty acids and is recommended by the committee:
Fifty grams of fat are saponified with 60 cc. of a solution of 2 parts of methyl alcohol to 1 of 50 per cent NaOH. The soap is dried, pulverized and dissolved in 1000 cc. of water in a porcelain dish and then decomposed with 25 cc. of 75 per cent sulphuric acid. The fatty acids are boiled until clear oil is formed and then collected and settled in a 150-cc. beaker and filtered into a 50-cc. beaker. They are then heated to 130° C. as rapidly as possible with stirring, and transferred, after they have cooled somewhat, to the usual 1-in. by 4-in. titer tube.
The method of taking the titer, including handling the thermometer, to be followed is the same as that described in the standard method. Even at present high prices many laboratories are using the glycerol-caustic potash method for preparing the fatty acids, figuring that the saving of time more than compensates for the extra cost of the reagents. Caustic soda cannot be substituted for caustic potash in the glycerol method.
UNSAPONIFIABLE MATTER.
The committee has considered unsaponifiable matter to include those substances frequently found dissolved in fats and oils which are not saponified by the caustic alkalies and which at the same time are soluble in the ordinary fat solvents. The term includes such substances as the higher alcohols, such as cholesterol which is found in animal fats, phytosterol found in some vegetable fats, paraffin and petroleum oils, etc. Unsaponifiable matter should not be confused in the lay mind with insoluble impurities or soluble mineral matter.
The method adopted by the committee has been selected only after the most careful consideration of other methods, such as the dry extraction method and the wet method making use of the separatory funnel. At first consideration the dry extraction process would seem to offer the best basis for an unsaponifiable matter method, but in practice it has been found absolutely impossible for different analysts to obtain agreeing results when using any of the dry extraction methods proposed. Therefore, this method had to be abandoned after numerous trials, although several members of the committee strongly favored it in the beginning.
Iodine Number—The iodine number adopted by the committee is that determined by the well-known Wijs method. This method was adopted after careful comparison with the Hanus and Hübl methods. The Hübl method was eliminated from consideration almost at the beginning of the committee's work for the reason that the time required for complete absorption of the iodine is unnecessarily long and, in fact, even after absorption has gone on over night, it is apparently not complete. In the case of the Hanus and Wijs methods complete absorption takes place in from 15 minutes to an hour, depending on conditions. Formerly, many chemists thought the Hanus solution rather easier to prepare than the Wijs solution, but the experience of the committee was that the Wijs solution was no more difficult to prepare than the Hanus. Furthermore, absorption of iodine from the Wijs solution appeared to take place with greater promptness and certainty than from the Hanus and was complete in a shorter time. Results by the Wijs method were also in better agreement in the case of oils showing high iodine absorption than with the Hanus solution and showed a slightly higher iodine absorption for the same length of time. However, the difference was not great. The committee investigated the question of substitution since it has been suggested that in case of the Wijs solution substitution of iodine in the organic molecule might occur, and found no evidence of this in the time required for the determination, namely, 1/2 hr., or even for a somewhat longer period. One member of the committee felt that it was not desirable to introduce the Wijs method into these standard methods since the Hanus method was already standardized by the Association of Official Agricultural Chemists, but the committee felt that it must follow the principle established at the commencement of its work, namely, that of adopting the method which appeared to be the best from all standpoints, taking into consideration accuracy, convenience, simplicity, time, expense, etc., without allowing precedent to have the deciding vote.
Iodine Number, Tung Oil—The committee has made an extensive study of the application of the Wijs method to the determination of iodine value in the case of tung oil with the result that it recommends the method for this oil but has thought it desirable to limit the conditions under which the determination is conducted rather narrowly, although reasonably good results are obtained by the committee method without making use of the special limitations.
The co-operative work of the committee and the special investigations conducted by individual members bring out the following points:
Influence of Temperature—From 16° C. to 30° C. there is a moderate increase in the absorption, but above 30° the increase is rather rapid so that it was thought best to limit the temperature in the case of tung oil to 20° to 25° C.
Influence of Time—The absorption increases with the time but apparently complete absorption, so far as unsaturated bonds are concerned, occurs well within one hour's time. Consequently, one hour was set as the practical limit.
Influence of Excess—The excess of iodine solution also tends to increase the iodine number, hence the Committee thought it necessary to limit the excess rather rigidly to 55 ± 3 per cent, although with greater latitude results were reasonably good.
Influence of Age of Solution—Old solutions tend to give low results although up to 2 mo. no great differences were observed. Nevertheless, it was thought best to limit the age of the solution to 30 days—long enough for all practical purposes.
Amount of Sample—As a practical amount of sample to be weighed out the Committee decided on 0.15 g. with a tolerance of 0.05 g. in either direction according to preference. In other words, the amount of sample to be taken for the determination to be from 0.1 to 0.2 g. in the discretion of the analyst.
The Committee's study of the Hübl method which has been adopted by the Society for Testing Materials in the case of tung oil indicates that this method when applied to tung oil is subject to the same influences as the Wijs method and it has the additional very serious disadvantage of requiring a long period of time for absorption which cannot be considered reasonable for a modern analytical method. When using the Hübl solution, the absorption is not complete in the case of tung oil at 3, 7, 18 or even 24 hrs.
The Hanus method in the case of tung oil gives very high and erratic results, as high as 180 to 240 in ordinary cases for an oil whose true iodine number is about 165.
MELTING POINT.
A melting point is the temperature at which a solid substance assumes the liquid condition. If the solid is a pure substance in the crystalline condition the melting point is sharp and well defined for any given pressure. With increased pressure the melting point is lowered or raised, depending on whether the substance contracts or expands in melting. The lowering or raising of the melting point with pressure is very slight and ordinarily is not taken into consideration. Melting-point determinations are commonly carried out under ordinary atmospheric pressures without correction. The general effect of soluble impurities is to lower the melting point, and this holds true whether the impurity has a higher or lower melting point than the pure substance (solvent). Thus if a small amount of stearic acid be added to liquid palmitic acid and the solution frozen, the melting point of this solid will be lower than that of palmitic acid. Likewise the melting point of stearic acid is lowered by the addition of a small amount of palmitic acid. A eutectic mixture results when two components solidify simultaneously at a definite temperature. Such a mixture has a constant melting point and because of this and also because both solid and liquid phases have the same composition, eutectic mixtures were formerly looked upon as compounds. The phenomenon of double melting points has been observed in the case of a number of glycerides. Such a glyceride when placed in the usual capillary tube and subjected to increasing temperature quickly resolidifies only to melt again and remain melted at a still higher temperature. This phenomenon has not yet been sufficiently investigated to afford a satisfactory explanation.
Non-crystalline substances such as glass, sealing wax and various other waxes and wax mixtures, and most colloidal substances do not exhibit a sharp melting point, but under the application of heat first soften very gradually and at a considerably higher temperature melt sufficiently to flow. This phenomenon of melting through a long range of temperature may be due to the amorphous nature of the substance or to the fact that it consists of a very large number of components of many different melting points.
The fats and oils of natural origin, that is, the animal and vegetable fats and oils, consist of mixtures of glycerides and, generally speaking, of a considerable number of such components. These components are crystalline and when separated in the pure state have definite melting points, although some exhibit the phenomenon of double melting point. For the most part the naturally occurring glycerides are mixed glycerides. In the natural fats and oils there are present also certain higher alcohols, of which cholesterol is characteristic of the animal fats and oils and phytosterol of many of the vegetable fats and oils. In addition to the crystalline glycerides and the higher alcohols present in neutral fats, there are in fats of lower grade, fatty acids, which are crystalline, and also various non-crystalline impurities of an unsaponifiable nature, and the presence of these impurities tends to lower the melting point. They also tend to induce undercooling and when the liquid fat or oil is being chilled for purposes of solidification or in determination of titer.
The presence of water, especially when this is thoroughly mixed or emulsified with a fat or oil, also influences the melting point to a marked extent, causing the mixture to melt through a longer range of temperatures than would be the case if the water were absent. This is particularly true of emulsified fats and oils, such as butter and oleomargarine, both of which contain, besides water, the solids naturally present in milk or cream and including casein, milk sugar, and salts. The melting-point method recommended by the Committee is not applicable to such emulsions or other watery mixtures and the Committee has found it impossible to devise an accurate method for making softening-point or melting-point determinations on products of this nature. Not only the amount of water present but also the fineness of its particles, that is, its state of subdivision and distribution, in a fat or oil influences the softening point or melting point and causes it to vary widely in different samples.
As a consequence of the foregoing facts, natural fats and oils do not exhibit a definite melting point, composed as they are of mixtures of various crystalline glycerides, higher alcohols, fatty acids, and non-crystalline substances. Therefore, the term melting point when applied to them requires further definition. They exhibit first a lower melting point (the melting point of the lowest melting component) or what might be called the softening point and following this the fat softens through a shorter or longer range of temperature to the final melting point at which temperature the fat is entirely liquid. This is the melting point determined by the Committee's melting-point method. The range between the softening point and the final melting point varies greatly with the different fats and oils depending on their chemical components, the water associated with them, emulsification, etc. In the case of coconut oil the range between softening point and final melting point is rather short; in the case of butter, long. Various methods have been devised to determine the so-called melting point of fats and oils. Most of these methods, however, determine, not the melting point, but the softening point or the flow point of the fat and the great difficulty has been in the past to devise a method which would determine even this point with reasonable accuracy and so that results could be easily duplicated. It has been the aim of the Committee to devise a simple method for the determination of the melting point of fats and oils, but it should be understood that the term melting point in the scientific sense is not applicable to natural fats and oils.