Determination of Reducing Sugars Application of Shaffer and Hartmann Iodometric Cuprous Titration G. L. MARSHAND M. A. JOSLYN,F r u i t Products Laboratory, University of California, Berkeley, Calif.
S
HAFFER and Hartmann (6) have reported that the iodometric cuprous titration could be adapted to the determination of reducing sugars by the use of FehlingSoxhlet solution and the amounts and conditions of heating prescribed by Munson and Walker (3). ,Using the Munson and Walker tables for the conversion of copper into terms of sugar, they have obtained substantially correct values for the sugar present when the equivalence factor of 6.36 mg. of copper per cc. of 0.1 N sodium thiosulfate was used. However, from previous investigations conducted in this laboratory, by others as well as the writers, this theoretical factor seemed too low to give complete recovery of sugar. Therefore we have undertaken to determine if such is the case, and, if possible, to determine the cause. Munson and Walker have demonstrated the accuracy of the direct weighing of cuprous oxide so that the chief source of difference between the results obtained by the procedure as carried out by Shaffer and Hartmann and that prescribed by Munson and Walker apparently is due to the difference in conditions of reduction used. Shaffer and Hartmann did not strictly adhere to the conditions of heating prescribed by Munson and Walker. The solutions were heated in a 300- or 400-cc. Erlenmeyer flask covered with a small beaker instead of in a 400-cc. beaker covered with a watch glass. Quisumbing (5) and others ( 1 , 2, 7 ) have shown that the type of container used for reduction affects the results obtained.
The solutions were made to volume a t room temperature (22.5" C. average), and a record was kept of the temperature. The chemicals used were the purest that could be obtained on the open market. Stock solutions were made up in only 4liter lots according to the procedure recommended by Munson and Walker, and Shaffer and Hartmann. The thiosulfate solution was adjusted to exactly 0.1 N at 22.5" C. against recrystallized potassium dichromate. It was stored in a 10liter Pyrex stock solution bottle and was preserved with a few cubic centimeters of carbon disulfide. Blank determinations on the stock solutions were frequently run and showed only slight variations. The 500-cc. Pyrex Erlenmeyer flasks and 400-cc. Pyrex beakers used in these tests were carefully selected to have the same distribution of glass by choosing flasks or beakers of the same weight. It was found that, as reported by Shaffer and Hartmann, it was necessary to dissolve the cuprous oxide precipitate completely in the presence of the iodate-iodide solution after acidification, and subsequently to dissolve the cuprous iodide formed in order to obtain close agreement between duplicate or triplicate determinations. It was found that the cumous
100
TABLEI. COMPARISON OF LATERAL AREA AND TOTALAREA OF CONTAINERS OF VARIOUSTYPES LATERAL
HT.OF
CONTAIN~R 400-cc. 300-cc. 400-cc. 600-cc.
beaker Erlenmeyer Erlenmeyer Erlenmeyer
AREA TO
TOTAL SURFACEAREA OF
OF
44.2 69.5 66.6 73.9
100.8 85.8 95.2 97.1
LIQUID LIQUID AREA LIQUID LIQUID IN EXPOSEDOF EXPOBEDEXPOBED CONTO BOTTOM TO TO DIAM.TAINBR GLASS SURFACE AIR GLASS Crn. Crn. Sq. em. Sq. cm. Sq. cm. Sq. om.
5
7.6 8.7 9.2 9.7
2.4 2.1 1.9 1.7
56.6 50.6 55.6 55.2
44.2 35.3 39.6 41.9
G
$
Quisumbing shows "that when the action of air is excluded, reduction is proportional not to total but to lateral area of liquid exposed to the glass." He believes that this relationship is due to the fact that as the reduction proceeds the cuprous oxide formed drops from the sides of the container to the bottom, thereby decreasing the accelerating action of the glass along the bottom, leaving the free surface along the sides for catalytic action. A comparison between totaI area and lateral area of a number of containers is shown in Table I. It is seen there that the lateral area of the liquid exposed to glass is approximately the same in the 400-cc. beaker and the 500-cc. Erlenmeyer flask, so that if this is of the flask for the the controllhgfactor the beaker should not markedly affect the results. However, owing to the increased surface area exposed to air in the flask, there is more danger from loss of cuprous oxide by oxidation.
ANALYTICAL PROCEDURE The dextrose used in these determinations was obtained from the Bureau of Standards. It was weighed against brass weights in air, and transferred to calibrated volumetric flasks.
4b
4 MiNmzs
0
'*
5
6
RATE OF TEMPERATURE OF loocc. OF SOLUTION IN ERLENMEYER FLASK
Flame adjusted to bring solution t o boil in (1) 3 min., (2) 4 min., (3) 6 min.
iodide precipitate dissolved more readfly in the presence of a large excess of iodine than when but a slight was present. menthe solutions were cooled to from 350 to $0" C., the solution of both precipitates was rapid, but a t about 20" C. the DreciDitates dissolved with difficultv. The end point in the titratibn of solutions cooled to tempkratures below 35" C. were somewhat obscured by a precipitation which occurs, apparently composed of tartrates.
368
October 15, 1932
369
INDUSTRIAL AND ENGINEERING CHEMISTRY
DISCUSSION BOILINGCONDITIONS.Quisumbing
EFFECTOF (4)has shown that changes in barometric pressure affect the amount of copper reduced by reducing sugars when the Munson and Walker technic is used, He has also shown that the temperature of boiling under their conditions is not constant, ranging from 101' to 105" C, depending on conditions of heating.
caused by increase in the surface of the liquid exposed by the more vigorous boiling. Where the glass beads are used, there is a vigorous agitation of the cuprous oxide precipitate causing it to rise and drop back to the bottom of the flask. In the uncovered flask without beads in which the liquid boils without superheating, such agitation does not occur. The boiling is less vigorous, the cuprous oxide precipitate does not rise to the surface of the liquid, and surface oxidation is reduced. TABLE11. EFFECTOF BOILINQ CONDITIONS ON OXIDATION OF DEXTROSE BY FEHLING'S SOLUTION (100 mg. of dextrose per determination) THIOSULFATE
TREATMENT Uncovered 500-00. Erlenmeyer containing 20 glass beads 500-cc. Erlenmeyer covered with 50-mm. funnel with 20 glaes beads Uncovered 500-cc. Erlenmeyer 500-00. Erlenmeyer oovered with 50-mm. diameter funnel 500-00. Erlenmeyer covered with 50-cc.
TITRATION"
cc
.
Cu
REDUCED
MQ.
DDXTRORE RE. COVERED
Ma.
6.07
197.86
99.76
5.92 5.84
198.81 199.32
100.21 100.47
5.81
199.51
100.58
200.40 101.08 5.67 0 Figures represent average of at least three determinations which varied by not more than 0.1 cc. haaker
Glass beads, although alleviating superheating, are objectionable for other reasons. During the titration they tend to increase the crystallization of tartrates. Although this does not interfere with the reaction, it does interfere slightly in securing a sharp end point. For reasons given, the remaining tests reported for Erlenmeyer flasks were carried out in uncovered containers. COMPARISON OF BEAKER AND FLASK. A comparison of the relative extent of recovery of dextrose when reduction was carried out in Erlenmeyer. flasks and in covered beakers is shown in Table 111. The Shaffer and Hartmann procedure was followed, using both the flask and beaker with the same dextrose solution, The determinations were carried out a t the same time so that the conditions of heating would be identical for the two. Quadruplicate determinations were run in the tests, while L'blanks" were run in duplicate a t the concluMINUTES sion of each test. FIGURE2. EFFECTOF HEATINQ CONDITIONS ON RECOVERY The analyses show that consistently higher results were obOF DEXTROSE tained with the beaker, also in all instances except the 50-mg. sample, the milligrams of dextrose found are higher than the Flame adjusted to bring solution t o boil in (1) 3 min., (2) 4 min., (3) 5 min. actual amount contained in the sample. The lower amount I n carrying out the reduction both in Erlenmeyer flasks and of dextrose recovered in the flasks was probably due either to in beakers superheating was found to occur with such regu- increased surface oxidation of the cuprous oxide preci itate, larity that it is almost impossible to decide when to begin to since they were uncovered, or to the superheated con? ition I reckon the time of boiling. It was impossible to obtain trip- prevailing in the covered beakers, causing an increased yield licate determinations agreeing within 0.6 mg. of copper on of reduced copper. It can safely be concluded from the resamples when some had superheated to varying degree and sults that the uncovered Erlenmeyer flask can be used in the extent during boiling, and when some had not superheated. method to give yields which are very close to the actual It was thought that glass beads (4mm. in diameter) placed amount of dextrose present. in the flask would reduce the danger of superheating. In TABLE111. COMPARISON OF BEAKER AND FLASKAS addition to covering the flask with a 50-cc. inverted beaker, it RECEPTACLES was thought that covering with a small funnel might allow DEXthe release of steam pressure more freely than the heavier TROSE PER -BEAPERB"~ E R L E N X E YFLASKEb-ER water-sealed beaker. Slight improvement resulted from the 25-cc. ThioThioALP sulfate Cu Dextrose sulfate Cu Dextrose use of the funnel alone, but superheating still persisted, makQUOT titration reduced recovered titration reduced recovered ing it very difficult to obtain checks with triplicate deterMo. Cc. MQ. Mo . CC. M Q. Mo. 101.63 49.75 21.13 minations. Leaving the flask uncovered, however, almost 49.87 101.83 20.87 50.0 199.07 100.34 5.81 100.73 199.77 100.08 5.40 completely stopped the superheating. The glass beads pre290.08 150.47 29.17 151.17 291.35 150.12 28.92 15.82 374.99 201.29 200.20 376.89 200.16 15.47 vented superheating when used in either an uncovered or 400 00. capacity, 7.5 cm. in diameter. covered flask. b 500 00. capacity. The results shown in Table I1 indicate that superheating RATEOF REDUCTION OF FEHLING'S SOLUTION BY DEXis of considerable importance in the determination. The determinations in which superheating occurred gave values TROSE. It was observed that during the heating process which exceeded the actual amount of dextrose present. The there was a definite time at which the reduction of Fehling's covered flask containing 20 glass beads was very close to the solution by dextrose became visible, as noted by a change actual, whereas the uncovered flask containing glass beads from the clear dark blue of the unheated liquid to a cloudy was below the actual, probably owing to surface oxidation bluish red liquid. The time at which this change occurred 0
ANALYTICAL EDITION
370
Vol. 4, No. 4
TABLEIV. RATEOF REDUCTION OF FEHLING'S SOLUTION BY DEXTROSE OVER FREEFLAME ADJU~TED TO BOILIN TIMEINDICATED
TIME Min. 1 2 3
4 5
6I a
-BOILINQ Thiosulfate titration"
cc.
33.67 19.69 7.09 6.35 5.81 6.34
IN
cu
3 MIN.Dextrose recovered Ma. 10.93 64.66 96.30 98.81 100.50 102.24
reduced Ma. 23.02 111.30 191.44 196.14 199.58 202.57
(100 mg. of dextrose Der determination) IN 4 MIN.-BOILING Thiosulfate cu Dextrose titration' reduced recovered
8 2.5 8.5 Figures represent averages of triplicate determinations.
-BOILING Thiosulfate titration5
reduced 13:03 31.67 109.20 185.78 193.27 196.59 199.77 202.44
CC.
Mg.
Ms.
cc.
35.56 24.30 10.01 7 09 6.37 5.84
10.30 81.92 172.80 191.37 195.95 199.32
4.72 39.90 86.48 96.26 98.70 100.47
4.93 17.07 7.34
205.11 127.90 189.78
103.85 63.10 95.42
35.16 32.23 20.04 8.00 6.87 6.30 5.80 5.38
varied slightly with the amount of dextrose in the sample; the temperature was about 70" C. The rate of reduction of Fehling's solution by dextrose was determined a t minute intervals on 100-mg. samples with the flame of the gas burner adjusted to cause boiling in 3, 4, and 5 minutes. The rate of heating of the dextrose-Fehling's solution mixture was determined by the use of a mercury-inglass chemical thermometer with a range of 10" to 110" C,, graduated a t 2" C. intervals. I n most cases ten runs were made a t each flame adjustment using the different flasks employed in the determinations.
IN
5 MIN.-
cu
Mu.
Dextrose recovered Ma. 6.05 16.05 53.55 93.30 97.25 99.06 100.72 102.17
the flame, allowed to remain there for a given time interval, removed, and plunged into a cooling bath of running water at a temperature of 15.6" C. The flasks were rotated in the cooling bath for 30 seconds and then allowed to stand in water for 2 minutes. It was found that the temperature of the boiling solution could be reduced to 45" C. a t the end of the 30-second interval. Each member of a complete series was heated and cooled before the samples were subjected to the regular iodometric titration procedure of Shaffer and Hartmann. The results of these determinations are tabulated in Tables IV and V and are shown graphically in Figures 1, 2, and 3. TABLE V. RATEOF HEATING OF DEXTROSE-FEHLING'S SOLUTION MIXTUREOVER FREE FLAMEADJUSTEDTO BOIL IN TIMESINDICATED (100 cc. of solution per 500-00. Erlenmeyer flask)
TIME Min. 0:oo
30 1:oo 30 2:oo 30 3:OO 30 4:OO 30 5:boO
30
6:OO
30 7:OO 30 8:OO
TE MPERATUUL,
"c
FIGURE3. EFFECTOF TEMPERATURE ON AMOUNTOF DEXTROSE RECOVERED DURING HEATING Flame adjusted t o bring solution t o boil in (1)3 min., (2) 4 min., (3) 5 min.
The method used in determining the rate of reduction of the Fehling's solution consisted of adjusting the flame so that boiling began a t the times indicated. The dextrose solution containing 100 mg. per 25 cc. of aliquot was pipetted, over as short a time interval as possible, into enough Erlenmeyer flasks so that triplicate determinations could be made a t minute intervals from the start of heating up to and including 3 minutes of boiling. After adding 25 cc. of distilled water and 25 cc. of Fehling's A and B, the samples were placed on
BOILINGIN 3 MIN.
BOILINGIN 4 MIN.
BOILINQIN 5 MIN.
22.0 38.2 46.7 60.7 73.7 85.5 96.2 103.5 103.5 103.0 102.0
26.7 26.7 44.4 55.6 66.1 76.7 86.1 93.9 100.6 104.2 104.4 105.6 103.0 103.0 103.0 103.0 103.0
22.8 31.2 38.7 47.5 55.5 63.5 72.2 79.0 85.0 91.2 97.5 102.2 103.0 108.0 103.0
c.
c.
c.
Figure 1 shows the rate of heating of the solutions under the conditions specified. The curves follow the averages of the points obtained. It will be seen that there is a change in slope in each of the curves a t approximately 70" C., corresponding to the time a t which the reduced cuprous oxide first becomes visible. This is probably owing to a change in the rate of heat transfer caused by the cuprous oxide formed. Figure 2 shows the milligrams of cuprous oxide formed with the flame adjusted to cause boiling a t 3, 4, and 5 minutes, respectively. It is seen that enough cuprous oxide is formed in the preliminary heating to boiling to give 96 to 99 per cent recovery of dextrose regardless of the time of the heating period. At the end of 2 minutes of boiling, complete recovery is obtained regardless of the time required to heat the solution to boiling. The curves also show a steady increase in the amount of copper reduced to cuprous oxide by dextrose with increase in the time of boiling. Apparently the essential factor in the procedure of heating used by Munson and Walker is not the time necessary to bring the solution to boiling, but the time elapsed from the start of boiling until the flask is removed and cooled by immersion in a cold water bath. Munson (8) and Walker evidently realized this to be the case, but did not give data to support the selection of their particular procedure. I n his preliminary work, Munson showed that the reduction was from 99 to 99.5 per cent complete a t the time boiling starts, using the amounts of copper recovered after 2 minutes of boiling as 100 per cent. His
October 15, 1932
I N D U S T R I A L A N D E N G I N E E R I N G C-H E M I S T R Y
data also show that reduction proceeds a t a steady rate for a t least 4 minutes after boiling starts. From our data it is safe to conclude that the preliminary heating period is of minor importance and that the time should be reckoned from the start of boiling and continued for exactly 2 minutes in order to secure accurate results by the use of Munsori and Walker tables. At sea level the time of boiling should be reckoned from the time the solution first reaches 102.5' C. Figure 3 shows the relation of milligrams of dextrose recovered to the temperature. It will be seen that the reduction of Fehling's solution by dextrose is a function of both the time and temperature with the flames adjusted to cause boiling in the times indicated, Two interesting points are shown by the graph. All three curves intersect at a common point, showing approximately 50 per cent reduction at a temperature of 70' C. This is the point in the preliminary
.
371
heating period where the reduction first becomes visible. The curves again come together a t a temperature between 102" and 103" C., corresponding to the boiling point of the solution a t which the reduction is from 96 to 99 per cent complete. LITERATURE CITED Kjeldahl, Compt. rend. trau. lab. Carlsberg, 4, 1-62 (1896). Munson, L. S., U. S. Dept. Agr., Bur. Chern., Bull. 73, 59-63 (1903). Munson and Walker, J. Am. Chem. SOC.,28, 663-86 (1906). Quisumbing, Ph. D. Dissertation, Columbia Univ., pp. 3-5 (1921). Quisumbing and Thomas, J . Am. Chem. SOC.,43, 1503-26 (1921). Shaffer and Hartmann, J. Bid. Chem., 45, 365 (1920). Vrecht, Ber., 15, 2687 (1882). RECEIVED July 18, 1932.
Use of Metallic Lithium in Analysis of Gases Determination of Nitrogen in Inert Gases J. H. SEVERYNS,E. R. WILKINSON, AND W. C. SCHUMB Massachusetts Institute of Technology, Cambridge, Mass.
H
ELIUM of approximately 97 per cent purity is produced in this country by the liquefaction of natural gas, the remaining impurities being mainly nitrogen, with small percentages of argon and oxygen, together possibly with some water vapor from the storage cylinder. Determination of the purity of the helium offers difficulties when a rapid and accurate one with more than one gaseous impurity present, is desired. If we aBsume a mixture of two gases only, a number of more or less satisfactory methods for determining the purity may be employed. Thus, the Edwards balance (1) measures the density of the gas mixture, but requires a trained operator and the use of an expensive precision instrument. The use of the viscometer (9)requires a thorough knowledge of the properties of the gases involved, and also an appreciation of the limitations of the apparatus; the device, furthermore, is not very sensitive. The katharometer (3) is excellent as a recording purity meter, but in practice it has been found that it cannot be completely trusted without periodical calibration. The charcoal adsorption apparatus as arranged by H. P. Cady is probably the most accurate and sensitive instrument in use. However, considerable time is required for each determination, and the apparatus is elaborate. Furthermore, a supply of liquid air, which may not always be available, in necessary. If the impure helium under test is known to contain oxygen as well as nitrogen, and the air-nitrogen ratio is not in a definite and known proportion, the problem becomes more difficult. Of the devices mentioned, only the Cady instrument does not require that the composition of the impurities be known. The others measure some property of the helium as modified b? the presence of a known impurity, and the purity of the helium is derived thereby. APPARATUS A device has been constructed which uses metallic lithium to form nonvolatile compounds with nitrogen, oxygen, and water vapor contained in a given volume of impure helium, and from the change in pressure produced by the removal of
these gases the purity is calculated. The device does not distinguish helium from the other rare gases, but if the contamination of the helium has been due to admixed air (resulting from its use in balloons) the determination of the nitrogen content becomes a matter of first importance rather than the detection of very small quantities of argon or neon which may be present. The apparatus, which has been quite satisfactory in analyzing samples, is shown in Figure 1. A 400-cc. roundbottomed Pyrex flask is supplied with three capillary stopcocks sealed to the bottom; one leads to a capillary manometer, another to a high-vacuum pump, and the third to the source of gas sample. Since lithium must be heated to the melting point before rapidly combining with the nitrogenowing to protective action a t room temperature of the surface film-a heating element is introduced, This consists of a small resistance coil similar to that in an ordinary automobile cigar lighter. The current is conducted through heavy wires supported by a Pyrex tube held firmly in a rubber stopper. The outer end of the tube is sealed gastight with de Khotinsky cement. The heating element serves the dual purpose of supplying heat and supporting a small shallow boat of sheet iron containing the lithium.
PROCEDURE A small piece of lithium is introduced into the boat and the stopper is tightly inserted. The flask is thoroughly evacuated and the tightness of the apparatus is confirmed by comparing the manometer reading with the barometric height. The sample of gas to be analyzed is next admitted slowly into the bulb. When the sample to be taken is part of a stream flowing constantly through a train of apparatus, it is convenient to allow the analyzer to fill slowly so that but a small fraction of the gas current is withdrawn for analysis, the rest being vented through a trap or bubble tube. When the flask has filled to the desired pressure, say 550 mm., the stopcock admitting the gas is closed and the manometer reading again taken. Current is then applied, conveniently derived from a small transformer. As the
