July, 1928
INDUSTRIAL A N D ENGINEERING CHEMISTRY
thrive in lime solutions and high counts have been obtained of these, but the putrefactive organisms are almost entirely killed or inhibited. In general, it is quite unnecessary to add other preservatives during the liming process. SELECTION OF THE LIME
It is important to use care in the selection of the lime for hide swelling, for it has been found that dolomitic limes which contain Large amounts of magnesia are decidedly inferior t o
737
the high calcium limes. For some reason that is not entirely clear, the magnesia tends to offset the normal swelling induced by the lime. I n practice it is usually most satisfactory to procure a high-grade quicklime and slake it a t the plant just prior to use. The iron oxide content of the lime should likewise be low, as otherwise the color imparted by it to the finished glue or gelatin may be objectionable, and bleaching may then be necessary.
T h e Determination of Solids (or LA40isture) by Means of a Gauze Dish' By Armin Seidenberg
'
CHEMICAL LABORATORY, DEPARTMEKT OF HEALTH, N E W YORK,N. Y.
The determination of solids or dry substance (moisture by difference) of viscous organic liquids is one of the most frequent operations in chemical investigations. I n spite of its seeming simplicity, it has generally been found impossible to secure strictly quantitative results bg a n y of the methods so f a r proposed. Pumice stone, sand, and other finely divided materials usually used as distributing media, have a tendency, after being heated, slowly to adsorb gases and liquids. The slight traces of adsorbed moisture are held very tenaciously and tend greatly to increase decomposition of solids, so that it is dificult to differentiate between the loss due to evaporation of moisture and that due to decomposition of solids. For this reason it is not possible to secure a definite end-point followed by a correct constant weight that indicates the actual solids present. In order to secure more satisfactory results a dish of corrugated wire gauze has been proposed. T h i s permits the liquids to be dis-
tributed over a wide surface; it also permits uniform dehydration, is unaffected by chemical action, and does not change in weight during cooling or weighing. It has no tendency to retain moisture, and largely for this reason the decomposition of the solids distributed over it has been found to be much less than is the case with pumice or sand. I t is always possible with the gauze dish to secure a sharp end-point followed by a true constant weight that remains constant on prolonged heating and that indicates the correct amount of solids present. A large number of comparative determinations were made on trarious sugar and other solutions between the gauze dish and pumice stone methods, and very considerable differences were noted in the results by the two methods--equal in m a n y instances to 3 per cent and more of the solids present. B y using higher temperatures rapid results (in about h a y a n hour) can be secured on many materials with the gauze dish.
T
SUCROSESOLUTIONS RESULTSWITH PUMICE SToivE-Several hundred parallel determinations a t temperatures ranging from 55' to 175' C. were made under exactly the same conditions, by the gauze dish and pumice stone method^,^ on sucrose solutions of varying concentrations. The pumice stone used was subjected to careful preliminary purification; it was used only after standing under atmospheric conditions for several weeks and then dehydrated in each instance in the manner recorded in the table. The pumice, before being weighed, was always placed in a desiccator for exactly 10 min. It was found that pumice, as well as any other material consisting of finely divided particles or containing openings of capillary size, had a tendency to adsorb both gases and liquids, particularly after having been heated. Material in a colloidal state such as this is affected by the conditions of temperature and pressure so that its weight changes, not only during evaporation, but also during cooling and weighing. Considerable evidence will be presented in another paper4 indicating that, although decomposition of organic residues distributed over pumice does not take place in the presence of a large excess of water, it is greatly accelerated by the slight traces of adsorbed moisture held throughout the dehydration by the pumice. Radically different results were secured by the pumice stone method, dependent upon the manner in which the pumice had been dehydrated before being weighed. In those instances (KO. 3, Table I) where it was first heated over the Bunsen flame, results were secured which after a
HE most favorable condition for the thorough removal of the volatile portion of a liquid is a wide and uniform distribution of the liquid over a large surface which will itself remain entirely unaffected throughout the drying operation. In a previous paper2 the writer has described a device, a modification of which was found to meet these conditions t o a marked degree. This device, or gauze dish, is made from a fine mesh wire gauze with an area of 200 sq. cm., corrugated into 31 to 33 lateral ridges and compressed in this way into an area of 8.5 X 5.5 cm. The gauze dish referred to in this paper was made of platinum and rested upon a platinum-gold stand. I n order to protect hygroscopic material it was placed during weighing in a closed dish made of thin, light-weight lead sheets. The drops of any liquid when delivered from a slight height (about 2 cm.) upon a gauze in this form are broken up by the impact of the fall and held entirely as a fine film within the meshes. I n this way the grooves formed by the corrugations exert a capillary action and 5 cc. of any liquid can readily be distributed over the gauze dish without any going through the meshes. Owing to the l u g e surface and to the effect of the meshes and corrugations in maintaining a wide and even distribution of the liquid, dehydration is greatly facilitated. Evaporation of the liquid takes place from both surfaces. After all the water has been removed the solids will be found to be held uniformly within the meshes, forming an integral part of the gauze dish. All dehydrations were carried on in an electric oven, those in parallel being conducted in dishes placed side by side upon a small cardboard tray. 1
Received October 25, 1922.
*THISJOURNAL, 7 (19151, 769.
3
4
Assoc. Official Agr. Chem., Methods, 1920, p. 101. T o be published in J . Assoc. Oficial A g r . Chem., August 15, 1923.
INDUSTRIAL AND ENGINEERING CHEMISTRY
738
DISTRIBUTING MEDIUM
METHOD OF DEHYDRATING
Time, hrs.. . . 'C. . . . . . . . . . . . . . . . Temperature hrs, 5S".c: . . . . . . . . .
TABLEI-SERIES B, SUCROSE SOLUTION Solids, calculated from specific gravity = 15.93 per cent
-
7-
6 58 16,542 16.303
20 50 16.430 16.229
Time, hrs.. ..................... Temperature, C . . . . . . . . . . . . . . . . Pressure, in.. .................... Pumice 4 hrs., 60' C.
6 58 10 16.943
4 55 25 16.438
Time hrs.. . . . . . . . . . . . . . . . . . . Temderature, 99' C. 1.5 hrs., 100' C. Pumice Heated t o redness Gauze dish Heated to redness
16 075 16.339 16.043
Time, hrs.. ..................... Temperature, 100' C. Gauze dish Heated to redness ,
16,143
Time. hrs.. . . . . . . . . . . . . . . . Temperature 108' C Pumice 46 hrs., l i 7 ' C. Gauze dish Heated to redness
16.067 16.081
Time, hrs.. . . . . . . . . . . . . . . . . . Temperature, 112' C. Gauze dish Heated t o redness
16.175
).
Pumice Gauze dish
Heated t o redness
Time hrs.. . . . . . . . . . . . . . . . . . . . . . Temierature, 117' C. Pumice 4 hrs., 117' C. Gauze dish Heated t o redness
.................
Vol. 15, No. 7
3
6
2.5
5
2 15.803 16.176 4
2 15.844 16.306 16.043 3 15.995 1.5 15.994 16.045
2 15.430 16.063 2
16.036
Time, hrs ........................ Temperature, 148' C. Gauze dish Heated to redness
15.927
Time, hrs.. . . . . . . . . . . . . . . . . Temperature, 155' C, Pumice 15 hrs., 100' C. Gauze dish Heated t o redness
26.631 16.951
14.802 15.906
Time, min ....................... Temperature, 160° C . . . . . . . . . . . . . Gauze dish Heated to redness
15 160 16.110
15 175 15.908
Time, min.. . . .. .C . . " . . " ....... Temperature, ................ Gauze dish Heated t o redness
30 140-155 15.945
30 155 15.913
Time, min ....................... Temperature, C . . . . . . . . . . . . . . . . Gauze dish Heated t o redness
20 155 16.142
30 , 155 15.938
0.5
60 16.151 15.976 Number l b
16.259 Number 2 2.5 16,657 16.290 16.009 Number 3 3
15.967 2 15.849 0.5
2 55 16,214 15.984
15 60 16.139 15.957
15 4 15.390 16.254 15.936 3
15,926
15.839 15.976 Number 5
15,640 15.891
3
---
PER CENT SOLIDS-
15.926 Number 4 2
4 15.961
Time hrs.. Temderature, 125' C. Gauze dish Heated t o redness
1
Nlcmber l a 45
3
3
3 15.242 16.234 15.919
15 15.926
3 15.409 15,863 15
32 60 16.139
....
.... .... .... 2
15 60 16.109
.... .... .... .... ....
.... ....
.... ....
....
.... .... ....
15
75
15.186 16,234 15.927
14.629 16.266 15.906
12.596
.... ....
....
..... ... ....
....
..*.
2 15.260 15.843
3
15
....
14.577 15.855
.... ....
45
....
15.880
....
15.951 hrumber 6 2
15.938
15,151 15.981 Number 7 2
14.919
15
50
15.918 Number 8 2
15,908
15.894
15.908
15.874
15.826
.... ....
.... ....
.... ....
....
.... ....
.... ....
....
....
.... ....
....
.... ....
....
.... ....
.... ....
.... .... ....
....
.... ....
....
.... .... ....
....
....
.... .... ....
.... .... ....
.... .... ....
..... ... ....
15.802 Number 9 0 5 14.009 15.859 Number IO 15 175 15.840 Number 11
.... .... ....
hrumber 12 30 155 15.902
certain period remained approximately constant. However, these results were usually too high by amounts t h a t in two series of determinations (Series A and B)5 were 1.2to 3 per cent of the actual dry residue. I n other series of determinations even greater differences were observed, and it was generally impossible to secure satisfactory checks unless all the pumice used had previously been heated in one batch. I n other words, a fictitious constant weight was attained which was usually incorrect, and which could only be correct as the result of the balancing effect of two counteracting sets of errors-one due to the loss in weight produced by the disintegration of the organic material, the other due to the gain in weight resulting from the adsorptive power of the pumice. On the other hand, pumice dehydrated by being heated a t the temperature a t which evaporation is subsequently carried on (Nos. 1, 2, and 3, Table I) yields with sucrose solutions results that show a continuous loss in weight on heating. Similar results were also obtained with sand (25 to 30 g.) dehydrated a t looo C . This loss in weight is so decided and so continuous t h a t i t is not possible under these conditions to attain a sharp end-point t h a t will clearly differentiate the completion of the evaporation of the liquid from 6 Results secured with Series A are similar in character t o those secured with Series B and are therefore not tabulated.
2
....
2
36 12.605
....
30 175 15.670
.... .... .... .. .. .. ..
....
15.894 3 14,674 15.924 2
.... ....
.... .... .... ....
15.911 3 14.436 15.918
2
2 14.297 15.912
....
.... ....
....
45 13.522 15.884
. . I .
the decomposition of the solid. This is true no matter what temperature is used for the evaporation. A lower temperature will retard the rate of decomposition of the solid matter, but it will generally reduce to about the same degree the rate at which the last traces of tenaciously held moisture are volatilized. I n this way a uniform rate of loss occurs in the dry residue a t a percentage above the correct percentage and continues t o one indefinitely below it. While some authorities recommend t h a t heating be continued until a uniform rate of loss is noted, it will be seen that a uniform rate of loss does not necessarily indicate the correct percentage of solid matter. FACTORS I N DETERMINING CONSTANT WEIGwr-It is therefore possible to secure accurate and reliable results only if decomposition is reduced to a point where its effect is negligible for an extended period of time-at least 2 and preferably 3 t o 4 hrs.-and where in any case the rate of loss due to this cause is decidedly less than t h a t due to evaporation. I n this respect it will be found t h a t the gauze dish adds greatly t o the reliability of the results, since i t practically adsorbs no moisture and since largely for this reason the dry residue on it is not readily decomposed and there is therefore always a distinct difference in the rate of loss due to decomposition and that due to evaporation.
July, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
RESULTSWITH THE GAUZEDISH-Although the rate of evaporation varies with the temperature, the constant weights attained on sucrose solutions with the gauze dish, a t temperatures ranging from 55' to 125" C., were in close agreement with each other and with the correct percentage indicated by the specific gravity. Furthermore, the same results were obtained with vacuum as at atmospheric pressure. I n Series A, where six determinations were carried on at temperatures ranging from 57" to 117" C., the average of all constant weights secured by the gauze dish represented 23.417 per cent,6 the percentage calculated from the specific gravity being 23.40. The average variation from this average was 0.005 per cent, the maximum 0.013 per cent. I n Series B, Table I, the average constant weight on the series of determinations made on the gauze dish at or below 1 2 5 O C. represents 15.930 per cent, the percentage calculated from the specific gravity being 15.93 per cent. The average variation from this average is 0.018 per cent, the maximum 0.041 per cent. In a considerable proportion of gravimetric determinations the experimental error is considered to be about 0.02 per cent, in terms of the whole amount. I n the present case, in addition t o the errors due to measuring out and distributing the liquid, four independent weighings are required, which might introduce discrepancies in the final result. These would readily account for the slight differences noted and we may conclude that errors due t o the incomplete evaporation of the liquid or to the decomposition of the solids even after prolonged heating may be considered as negligible when the gauze dish is used with sucrose solutions. RESULTS O N PROLOXGED HEATING-on heating the residues of Serios A on the gauze dish for a prolonged period, the total averaging 42 hrs., an average weight representing 23.368 per cent was obtained, which differed from the average constant weight representing 23.417 per cent by only 0.049 per cent. This is equal to an average loss after constant weight has been reached of slightly more than 0.001 per cent per hr. I n Series B (Table I) practically the same condition is to be noted. After an average constant weight on the determinations a t or below 125" C. representing 15.930 per cent was obtained on the gauze dish, continued heating totaling an average of 54 hrs. showed an average weight representing 15.891 per cent, differing from the average constant weight by 0.039 per cent. I n this instance the average loss of weight after constant weight has been reached corresponds to slightly less than 0.001 per cent per hr. These figures may be compared with those secured in Series A, where determinations in parallel with the gauze dish were run in all cases on pumice dehydrated under the conditions under which evaporation was conducted. Here after heating for an average of 42 hrs. the residues in the six determinations, which on the gauze dish equaled 23.368 per cent, equaled 21.907 per cent on the pumice stone. THEEND-POINT O N THE GAUZE DISH-AS has been noted, the exact point of time when constant weight has been reached can be only approximately estimated. Since on the gauze dish the rate of loss per hour of the solids on prolonged heating after the complete evaporation of the moisture is so slight that it may be considered negligible, the length of the heating period on the basis of which this is eatsblished may be extended considerably. I n this way fewer weighings are usually needed to determine whether constant weight has been reached and the weighings can be arranged according to the convenience of the worker. I n any case, the 6 Although the percentage figures given here and in the tables should e considered as correct only to the second decimal place, the third place in the decimals is given since in m a n y instances the difference between the Percentage figures occur in t h e third place and could only be indicated by showing this.
739
difference between the rate of loss due to evaporation and that'due to decomposition is very much accentuated and an end-point that is quite distinctive is secured. Thus, in the curves (Fig. 1) for the gauze dish in contrast with those for the pumice, a turn occurs a t the true percentage, after which the curves hardly deviate even on prolonged heating. Constant weight is therefore established with accuracy and certainty.
lo
20
FIG.I-SERIES B (99'-125'
30
40
50
60
C . ) . SP. GR.,15.93 PER CENT
TEMPERATURE GIVINGMOSTFAVORABLE RESULTSWITH GAUZE DISH-There is an advantage in using the highest temperature that has no decomposing effect in that the quickest results with the sharpest turning or end-points are secured in this way. At 98" to 100" C. prolonged heating is often needed to secure complete dehydration of strongly viscous material such as sucrose, particularly when the concentration is above 20 per cent. However, correct results can always be obtained with the gauze dish if the solution is not too concentrated and if the precaution is taken to distribute the material widely. At considerably lower temperatures (50 " to 60" C.) required in the presence of levulose, a longer time is needed for the evaporation, but there is no tendency to retain slight traces of water and constant weight once reached is in agreement with the correct results. This is not the case with pumice, where high results are frequently obtained at low temperatures. I n the absence of readily decomposable material a temperature of 120" to 125" C. is best with the gauze dish, while in the presence of material slightly more subject to decomposition a temperature of 110" C. should be used, and, as will be shown later, if the material is readily decomposable a temperature below 70" C., preferably 50" to 60" C., should beused. HIGHTEMPERATURE WITH GAUZE DISH-A study of Numbers 7, 8, 9, 10, and 11, Series B, Table I, will show that at 155" C. the correct result for solids can be obtained in approximately one-half hour, and that t h e subsequent loss at the same temperature during half-hourly periods averages between 0.03 and 0.04 per cent. On pumice the corresponding loss per half hour was 1 per cent and the correct weight was not indicated in any way. With the gauze dish at this temperature the end-point is sharply indicated and the constant weight attained remains fairly constant, so that the results are still approximately correct within an hour after this. It is therefore possible by means of the gauze dish to secure rapid and accurate results on substances of the nature of sucrose, by using high temperatures of about 155" C. I n this way results can be had in about one-half hour, and subsequent weighings to determine whether constant weight has been reached should be made at half-hour intervals. THE
INDUSTRIAL A N D ENGINEERING CHEMISTRY
740 DISTRIBUTING
MEDIUM
TABLE 11-SERIES C , LSVULOSE SOLUTION Solids, calculated from specific gravity = 23.02 per cent
METHODO F DEHYDRATING
....................
Time, hrs Pressure. in . . . . . . . . . . . . . . . . . . Temperature 70' C. B'hrs., 70" C., 28 in. Pumice Heated to redness
.................. ..............
Number 1 3 27
3 27
23.017 23.086
22.796 23.047
22.619 22.950
6 15
2 29
4 29
23.888 23.090
22,921 22.887
22.847 22.855
Time hrs.. . . . . . . . . . . . . . . . . . . Tem$erature, 70° C. Pumice 15 hrs., 70' C. Gauze dish Heated to redness
31.957 23.074
..................
Time hrs.. . . . . . . . . . . . . . . Temp'erature, 50' C. Pumice 4 hrs., 50" C . Gauze dish Heated to redness Time hrs.. Temp'erature, 50' C.' Pumice Continued Gauze dish Continued
4.5
2 28,700 22,946
.
......
4
2 24.310 22.969
22.469 22.898
.... ....
3 28
....
2 22.850 22.760 2
4
22,938 22.945
22.841 22.861 Number 4 2
2
22.599 22.664
22.441 22.581 Number 5 2
2
30.568 23.373 4
23.164 23.047
23.009 22.943
22.985 22,923
Time, hrs.. . . . . . . . . . . . . . . . . . . Temperature, 65' C . Gauze dish Heated to redness
23.147
4
4
6 24.205 23.268
2 23.246 2 22.978
2 23.270 23.132
25.263 23.320
23.063 22,943 Number 6 2
2
3 22.754 22.806 2 22.314 22,519 2
TEMPERATURES O F 70" AND ABOVE-when sohltions containing readily decomposable levulose were dehydrated according to the official method of the Association of Official Agricultural chemist^,^ which requires evaporation in vacuo at 70" C. from pumice previously heated to redness, it was only occasionally possible to obtain results t h a t were in agreement with those indicated by the specific gravity. I n many instances, however, this was not the case. Where the pumice had been previously dehydrated by heating in vacuo a t 70" C. (Table 11,Nos. 1 and 2) there is a continuous loss in weight, and the +rue solid content is not indicated in any may. A similar condition exists with pumice when evaporation is carried on a t 70" C. under atmospheric pressure. With the gauze dish somewhat similar results are obtained under these conditions. At a higher temperature (Table 11, No. 4) the rate of decomposition is very much accelerated, slightly more so with the pumice than with the gauze dish. TEMPERATURE GIVING MOST FAVORABLE RESULTSWITH GAUZEDISH-In order t o obtain a satisfactory constant weight a lower temperature than 70" C. must be used for levulose solutions. When the gauze dish is used as the distributing medium and a temperature of 50" to 60" C. is applied, a distinct decrease is evident in the rate of loss a t a point indicating the completion of the dehydration and corresponding to the true solids. The weight from this time on remains practically constant, particularly a t 50 " C. and it is in close agreement with the correct percentage as shown by the specific gravity. This is not equally true with 5.) the constant weight pumice. At 50" C. (Table 11, 1.0 obtained with pumice is considerably too high, caused, no doubt, by the retention of moisture due t o the adsorptive power of the pumice a t this temperature. At temperatures slightly higher than 50" C . , the results with pumice are not
23.054 23.027
2 22.211 22.484 3
.... ....
.... ....
....
2
22.948
2
3 22.717 22.773
23.427 23.183
22.948
23.152 23,068
22.797 22.730
24.107 23.287
22.930
22.925 Number 8 4
15 min. desic
22.645 22.730
23.041 22.992 h'umber 7 2 2
LEVULOSE SOLUTIONS
c.
22.563 22.934
22,709 22.758 Number 3 4
40.491 23.498 40
23,428
................
4 27
6 28
56.041 23,667 3
Time, hrs.. . . . . . . . . . . . . . . . . . . Temperature, 60' C . Gauze dish Heated t o redness
Time, hrs.. Temperature 57" C. Pumice 4 hrs., 570 C . Gauze dish Heated to redness
....
4 27
Number 2
Time, hrs.. Pressure, i n . Temperature 70' C. Pumice 7'hrs 70° C 28 in. Gauze dish Heatzd t o rezness
Time hrs.. Temp'erature, 90' C. Pumice 3 hrs., 90" C. Gauze dish Heated t o redness
PERCENT SOLIDS--------------------
_ ~ _ . _ _ _ _ - - -
6 15
2
2 22,955 22,956
....
2 22.898
.... .. .. .. .. .. .. .. .. .... .... 3
.... .... .... ....
.... .... .... 3
.... .... .... .... ....
.... .... 3
.... .... .... 4
22.649 22.731
22.639 22.725
22.233 22.554
.... ....
.... .... ....
....
....
.... ....
.... ....
3
....
.... 3 23.258 23.097
.... .... ....
2 22.873
3 23.214 23.047
23.228 23.047
.... ....
.... ....
....
.... ....
....
2 22.864
....
.... 17 22.766
17
....
....
....
....
.... ....
....
.... ....
.... ....
7
.... .... ....
22.694 22.740
22.711
22.707 22.768
....
.... .... ....
....
....
.... .... ....
.... .... .... ....
as high, but the actual dry residue is not definitely indicated by a sharp end-point or a satisfactory constant weight. With the gauze dish there is also a continuous loss in the case of levulose solutions a t 60" C., but it is a t a lesser rate than t h a t due to the dehydration and the true solids are indicated fairly accurately. However, a t 50" C. there is practically no loss in weight once the true solids are indicated, and this would therefore seem to be the most favorable temperature for determining levulose on the gauze dish. SOLUTIONS CONTAINING BOTH LEVULOSEAND SUCROSEI n Table I11 are recorded the results secured on a solution (Series D) containing levulose and the more viscous sucrose in equal amounts. Owing to the presence of levulose a temperature between 50" and 60" C. must be used. It will be noted (Table 111, Nos. 1, 2, and 3) t h a t where the gauze dish is used as the distributing medium constant weights are obtained which are in fair agreement with each other and with the weight indicated by the specific gravity, and that these weights remain constant throughout a prolonged period of heating. At the same time the checks obtained are not as close as those obtained in some other instances with sucrose solutions. It was also found t h a t a solution containing the two sugars in equal proportion, but which was more concentrated than in the present instance, did not yield, either with the gauze dish or the pumice stone method, results showing a sharp end-point or a satisfactory constant weight. Pumice, however, did not yield correct results even with a dilute salution, as can be noted both in No. 4, Table 111, where t h e determination was carried on according to the official method of the A. 0. A. C., and in No. 1 where the pumice was previously heated a t 59 " C. and therefore possessed less tendency to gain weight. This accords with the results previously obtained a t low temperatures on viscous material such a s sucrose distributed over pumice. With the gauze dish results
July, 1923
IiVD USTRIAL AND ESGILYEERING CHEMISTRY
which are entirely correct can be obtained a t a temperature of 50" to 60" C., provided only the solution issufficiently dilute and the liquid widely distributed in a thin layer. GENERAL ORGANIC SOLCTIONS MoLAssEs-The solids of an unrefined sample of canesugar molasses (Series E) that was dark brown and turbid even in a dilute solution were determined according to the official method in vacuo a t a temperature of 70" C. on pumice previously heated to redness. A uniform rate of loss was observed after 15 hrs. heating, when the losses indicated 75.64 and 75.55 per cent of solids, averaging 75.60 per cent, in the original material. When the determinations were conducted on the gauze dish without vacuum and a t temperatures of 70" and 60" C., the results for solids in the original material were 74.99, 74.91 and 74.91, 74.95 per cent, respectively, averaging 74.94 per cent. The results obtained a t the two temperatures on the gauze dish were in quite satisfactory agreement, but a t 60" C. the rate of loss per hour was practically negligible, showing that they indicate the correct percentage. The difference between the average results on the gauze dish and on the pumice stone amounts to 0.67 per cent, or almost 1 per cent of the actual solids present. SIRUPS-In so-called table sirups both sucrose and levulose are present, but the proportion of levulose to sucrose is considerably greater than is the case in molasses. With the gauze dish the average solids present in the original material (Series F) were 78.57 per cent (maximum difference of 0.14 per cent). Temperatures of 60" and 70" C. but without vacuum were used. When three determinations were conducted on pumice stone according to the official method, each batch being prepared separately, considerable variations in percentage (maximum difference 1.34 per cent) were obtained so that no significant average could be established. The difference in results between the two methods varied from a minimum of 0.04 to a maximum of 1.30 per cent which latter amounted to about 1.7 per cent of the actual solids present. Both the better checks obtained with the gauze dish and the smaller loss per hour on prolonged heating prove that the results obtained with it are the correct ones. SwmrENm CONDENSED M1LK-h Series G on sweetened condensed milk the official method,' involving the use of 26 to 30 g. of sand, was followed on an approximately 25 per cent solution, the temperature varying from 99" to 106" C. The results obtained in six determinations varied from 82.91 t o 84.20 per cent. A uniform rate of loss was to be noted a t any percentage between these figures, and had the evaporation been continued for a longer period even lower results might have been obtained. With the gauze-dish two series of determinations, yielding 83.55 and 83.59 per cent, were carried on a t 105" and 110" C., respectively. These results were in practical agreement and remained constant on prolonged heating. The results obtained by the sand method were about 0.6 per cent above and below those secured by the gauze dish, equal to about 0.8 per cent of the actual solids present. Rapid and satisfactory results (83.43 and 83.59 per cent) were obtained on the gauze dish at temperatures of 140" to 150" C. in 30 to 40 min. EVAPORATED MILK-When a 40 per cent solution of evaporated milk was dehydrated according t o Bigelow and Fitzgerald8 in an ordinary dish without sand or pumice and the results were compared with those obtained with the gauze dish, the average of five determinations with the gauze dish (old modification) was 25.646 per cent, while with the ordi8
Assoc. Official Agr. Chem., Methods, 1920, p. 231. Nat. Cannevs Assoc., Bull. 5 (1915), 14.
741
nary dish the average was 25.503 per cent, the difference amounting to slightly more than 0.6 per cent of the actual solids present. NATURALMILK-comparative determinations made on 150 samples of natural milk by the dish method (without sand or other distributing medium) and the gauze method dish {old modification) showed that with the latter the results were uniformly higher by an average of 0.08 per cent (equal to 0.7 per cent of the actual solids present). TABLE 111-SERIES D, SUCROSE, 50 PER CENT-LEVULOSE, 50 PER CENT Solids, calculated from specific gravity = 14.53 per cent DISTRIBUTIXG METHOD OF MEDIUM DEHYDXATING -----PER CENT SOLIDSNumber 1 Time hrs . . . . . . . . . . . . . . . . . 6 4 3 Temierature, 59' C. Pumice 4 hrs.;5Q0 C. 14.871 14.645 14.604 Gauze dish Heated to redness 14.620 14.509 14.488 Number 2 Time hrs.. . . . . . . . . . . . . . . 15 4 40 TemGerature, 58' C. Gauze dish Heated t o redness 14.524 14.572 14.534 Numbev 3 Time, hrs.. . . . . . . . . . . . . . . . 19 2 .... Temperature, 65' C. Gauze dish Heated to redness 14,506 14.521 .... Kumber 4 Time, hrs.. . . . . . . . . . . . . . . 10 3 . . . Pressure, in.. . . . . . . . . . . . . . 15 26 Temperature, 70" C. Pumice Heated t o redness 14.773 14.733 .... 14.803 14.731 .... Number 5 Time, hrs.. . . . . . . . . . . . . . . 8 2 4 Pressure, in.. . . . . . . . . . . . 12 26 26 Temperature, 70' C. Pumice Heated to redness 14,662 14.646 14 606
---2 14.562
....
6 14.510
... ... .... .... .... 3 27 14,564
DIRECTIOM FOR GAEZEDISH The liquid should preferably be weighed by difference and distributed one drop a t a time in rows running the length of the gauze dish so that no two drops touch each other, care being taken that the ridges are aligned so that no two are in contact. About 2.5 to 3 g. of the solution should be distributed in about 100 drops over the gauze dish. In the case of highly viscous solutions containing readily decomposable material requiring a low temperature, the concentration should be between 10 and 20 per cent; otherwise, a solution having a concentration between 20 to 30 per cent may be used. I n determining the solids on the gauze dish the highest temperature that will not produce decomposition should always be used. For organic solutions containing material that does not readily decompose, a temperature of 110" to 125" C., depending upon the degree t o which it is affected by heat, should be used. Rapid results (in half an hour) may be obtained with substances of this nature by using comparatively elevated temperatures (140" to 1%" C.). For organic solutions containing levulose and other similar substances which are readily decomposed, a temperature of 50" to 60" C., depending upon the proportion of decomposable material present, gives the best results. No vacuum is needed in any determination with the gauze dish. Hygroscopic material should be weighed in a closed dish. The correct percentage of solids is indicated when the rate of loss per hour is uniform and a t a minimum which is maintained on prolonged heating. Since a t the proper temperature this loss due to decomposition is practically negligible, the heating may be prolonged well beyond the time when the evaporation has been completed. The weighings to establish the rate of loss and the attainment of constant weight should be made in intervals of 2 to 4 hrs., depending upon the temperature. The residue upon the gauze dish can be burned or dissolved off and the gauze dish used repeatedly.
