Oct., 1922
T H E JOURNAL O F INDUSTRIAL A N D ENGINEERING CHEMIXTRY
941
The Determination of the pH Value of Commercial Glucose as a Substitute for the Candy Test‘ By Otto A. Sjostromz CORNPRODUCTS RBFININQCo
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In the following paper a simple method is given for the accurate determination of the actual aciditg of glucose by means of colorimetric comparison with standards of known p H . The diflculties and inconueniences connected with the use of the candg test as a measure of the inuerting power of commercial glucose are pointed out. The relation between the inuerting effect of glucose as found from the candy test and the p H value for a definite concentration has been determined and expressed in the form of a curue.
OMMERCIAL glucose is used mainly for two purposes;
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for the mixing and blending of table sirups and for the manufacture of candy. Corresponding to these uses the glucose is manufactured and put on the market in two brands, mixing glucose and confectioners’, or crystal, glucose. The mixing glucose is lighter, about 40’ BB.,the gravity varying according to the season of the year and to the climate of the territory in which the table sirup is to be consumed. The crystal glucose is furnished in brands of definite gravity, from 42” to 45’ Be. The essential difference between these two brands is that of gravity. I n one respect, however, the standards of quality are more definite for the crystal than for the mixing glucose and that is in regard to the acidity of the finished product. As is well known, dextrose is readily decomposed by the action of even very small amounts of alkali and a slight acidity is therefore essential to the keeping quality of glucose. This acidity is of no detriment in the mixing glucose and might even be considered beneficial, as it promotes the inversion of the cane products with which the glucose is blended. But in the case of crystal glucose an excess of acidity, even if slight, over a certain limit established on the basis of practical experience is detrimental to the quality of the product from the candy-maker’s point of view. When in the manufacture of candy glucose is boiled with cane sugar, inversion of the latter takes place to an extent determined by the acidity of the glucose. A small degree of inversion does no harm and is possibly desirable in some cases, but when the formation of invert sugar goes beyond a certain limit the quality of the candy suffers; this applies particularly to the different kinds of so-called “hard candy.” The candy becomes hygroscopic or, to speak with the candy-maker, “sticky” very easily. This is due to the presence of levulose, which is more hygroscopic than the other sugars. Of course, under certain conditions of temperature and humidity all candy will become “sticky.” There are no particular standards to meet in this respect but the candy-maker knows by experience how much the candy can be expected to stand under certain atmospheric conditions, and when the candy is “out of the ordinary” he lodges a complaint with the glucose manufacturer. The acidity of glucose is low, usually about 0.01 per cent expressed as HC1, using phenolphthalein as indicator. This acidity is due to acid phosphates and organic acids which are present in varying amounts. It is natural that there should 1
Presented before the Division of Sugar Chemistry a t the 63rd Meeting
of the American Chemical Society, Birmingham, Ala., April 3 to 7, 1922. 2
Research Chemist, Corn Products Refining Company.
., ARGO,ILLINOIS
be certain fluctuations in the acidity of the finished batches of glucose, due to varying conditions in the different stages of a rather complicated manufacturing process and sometimes to differences in the raw material of manufacture. The most convenient way of keeping track of the acidity would of course be to determine the same for each batch by titration, but while this is always done as a matter of record it has long been known that the titration is not a sufficient or correct index of the suitability of glucose for the candy manufacturer. The practice has therefore been to apply a test to the glucose which imitates the conditions under which the product is to be used. This “candy test” is carried out in the following manner: Glucose, cane sugar, and water in definite proportions are
heated in a copper pan at such a rate that the temperature of the mixture reaches 320’ F. in 25 min. The contents of the pan are then poured out on a marble slab and left to cool. All operations are carried out according to detailed and rigid instructions. A table showing the relation between temperature and time has to be followed to the second. After cooling, the candy is broken up, an average sample is taken, and from this sample 5 g. are weighed out and dissolved in a 100-cc. flask which is filled to the mark. In this solution the reducing sugar is determined in the ordinary way with Fehling’s solution by titration. The reducing sugar found represents the sum of the dextrose from the glucose used in the test and the invert sugar formed from the cane sugar during the heating. For the sake of simplicity the reducing sugar is all counted as dextrose. The dextrose from the glucose can be calculated from the dry substance of the latter, and a table has been worked out which gives this dextrose as per cent of the total dry substance of the candy for varying purities. In another table the results of the titration for ’ total dextrose are expressed as per cent of total dry substance. The difference between the latter value and the value found from the first table gives the amount of invert sugar in the candy. The permissible maximum of invert sugar has been fixed to 8.5 per cent, and any batch of glucose which gives a higher inversion is diverted to the table sirup department.
The candy test gives also important information about the quality of the glucose in other respects, when interpreted on the basis of practical experience. It is therefore indispensable to the glucose manufacturer. For general information about the quality of the glucose occasional tests are sufficient. But for information about acidity every batch has to be tested, if the record is to be kept complete, and with only this test available the control work becomes quite a problem for the laboratory, especially if the output of the plant is large. The test is cumbersome and time-consuming and requires the undivided attention of the operator, who consequently can attend to only one test a t the time. The least carelessness on the operator’s part results in incorrect data on the acidity effect, as the degree of inversion is very sensitive to any deviRtion from the time-temperature schedule and the analytical part has to be carried out accurately to give reliable results. HYDROGEN-ION MEASUREMENTS It occurred to the writer that sufficient information about the inverting power of glucose could be gained in a much more simple and rapid way by applying some of the prin-
942
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T H E JOURNAL OF INDUSTRTAL A N D ENGINEERING CHEMISTRY
ciples of physical chemistry to this problem. The varying quantities of organic and inorganic acids in the glucose determine the degree of inversion by virtue of $he concentration of the hydrogen ion t o which they give rise. The methods for the determination of this concentration have been worked out carefully and are finding widespread industrial application. The investigation was therefore a matter of determining the p H values of a number of glucose samples, the inversion figures of which were known from the candy tests, and expressing the connection between these values in the form of a curve. Both the colorimetric and the electrometric method have been used. The latter, of course, has its place in an investigation of this nature; but for routine work the colorimetric method is the only one to be considered. It is simple, rapid, quite accurate, if carried out with due attention to details, and-last but not least-almost foolproof. The p H values of glucose solutions, under the conditions of test which will be spoken of later, range between 3.5 and 5.5 pH. For standard solutions covering this range ’either the Sorensen citrate mixtures or the more recently suggested phthalate mixtures can be used. The writer from long familiarity prefers the former and cannot find that the latter offer any advantages, in spite of claims for greater convenience in preparation. Besides, citric acid is cheap and can easily be obtained in sufficiently pure quality. The composition of the citrate mixtures for different p H values is given in Clark’s well-known treatise.3 However, the intervals of p H in Clark’s table are quite irregular and considerable interpolation is necessary to find the relations for increments of 0.1 pH. The writer has therefore used the excellent diagram found in Michaelis’s book14from which a table can be made up covering the desired range. The indicator which covers most closely the range in this work is methyl red. For acidities in the vicinity of and above 4.0 pH bromophenol blue would be more convenient, as in this part of the range the color changes of the latter are much more marked than those of methyl red. Besides, bromophenol blue is very permanent, whereas the methyl-red colors fade rather rapidly and can be used only for immediate readings, within half an hour after addition. The great difficulty with bromophenol blue is, however, that the color which it imparts to glucose solutions is of a different quality from that of a standard solution of the same pH. The main difference seems to be that the red is more strongly absorbed in a glucose solution, so that the latter in daylight appears blue, whereas a standard solution is purplish blue or violet. Clark, in the chapter on “Optical Aspects,” discusses this phenomenon as observed in turbid ,solutions with bromophenol blue and suggests the use of a light screen of a certain quality in order approximately to equalize the colors. Accordingly, various screens were tried, but in the case of glucose the difference in color character is so great that it was not possible to devise an arrangement for the equalizing of the colors without a t the same time reducing the sensitiveness of the method too far for accurate color comparison. It might be mentioned that by working with electrometrical measurements as a guide the eye can be trained to these peculiarities, so that after some practice very close guesses of pH values can be made. Methyl red, however, gives practically the same hue of color with glucose solutions as with the standards and the color changes are sufficiently marked to allow determinations to 0.1 pH, although in the more acid part of the range the changes are of course in the color value rather than in the hue. For the color comparison a simple comparator on the Wal8 4
“The Determination of the Hydrogen Ions,” p. 82. Michaelis, “Die Wasserstoff Ionen Concentration,” p. 173.
Vel. 14, No. 10
pole principle, such as the one recommended by Clark, answers a11 purposes. The comparisons are facilitated by the fact that glucose solutions are clear and not a t all or very slightly colored. The work should be done in daylight and it is very essential to accuracy that the light be uniform. An excellent illumination for observations is the reflect,ed light from a large plate of white enamel glass, of the kind commonly used for sanitary table tops, which is laid on a table close to a window. I n this light all irregularities due to clouds in the sky are eliminated. In this connection some points concerning the physiological side of color comparison should be mentioned which apply more or less to all colorimetric work. All individual color comparison is eminently a matter of rapid and concentrated action, especially when judging different values of the same color. It is in the first moment of vision that the eye perceives most sharply the ’difference between values. Prolonged observation only lowers the power of discrimination. The best way to observe is therefore to “flash” the colors. The comparator block is held in a tilted position with the field dark, and by a uniform angular movement of the hand the color disks are made to appear and disappear in the same manner as if a screen were raised and lowered over the observation holes. The operation is repeated as many times as necessary. By working in this way the eye also gets a short rest between each observation, and color values can be read with great accuracy and with very little tiring effect on the eye. Color fatigue from prolonged observation manifests itself in a peculiar way. At first there is a decrease in the power of discrimination; then a rather sudden change takes place into a gross irregularity of perception and the observations become entirely unreliable. Fortunately the eye recovers very quickly from a fatigue of this kind but in the meantime mistakes might easily have been made. I t is therefore advisable to guard against errors from this source by making the observations in the manner suggested, by “flashing,” especially in the range close to 4.0pH. COLORIMETRIC TESTOF GLUCOSE The colorimetric test of finished glucose batches is made as follows: The samples are first diluted to 22’ BB. This arbitrary concentration has been chosen as a compromise, in consideration of the facts that on one hand the p H value is less influenced by other factors and consequently more representative the less the sample is diluted, and that on the other hand dilution is necessary in order to handle the samples in test tubes and get a uniform mixture with the indicator. A practical point of advantage in regard to dilution is that the gravity does not have to be exactly the same for every sample. The effect on the pH value of a variation of a few tenths on either side of 22’ BB. is so small that it can be neglected. The comparisons are made on 10-cc. samples of glucose solutions and standards, to each of which exactly 0.2 cc. of a 0.02 per cent alcoholic solution of methyl red has been added. This amount of indicator was determined by experiment. When rather exact information on pH values is needed, as in this case, it is essential to establish as closely as possible that strength of color which will give the maximum of color differences for variations of pH. It was found that with the above amount a maximum of sensitiveness was attained. The strength of color for a given proportion between indicator and volume of solution depends of course upon the lumen of the test tubes used. By carrying out the color comparisons in the manner described the pH values can be determined to 0.1 pH in the upper, more acid part of the series, and to 0.05 p H in the lower part. For ordinary routine work the standards are made up in increments of 0.2 pH and the pH values estimated to 0.1 pH.
