Jan., 191j
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G G H E M I S T R Y
with t h e apparatusdescribed above, 9 1 t o 9 2 per cent of the total formic acid added t o ketchup can be collected in an hour and a half, when t h e amount of distillate t h a t has passed through the apparatus is about 1000 cc. Thanks are due Mr. H. C. Lythgoe, chemist of t h e State Board of Health Laboratory of Massachusetts, for the suggestion which led t o this work. DEPARTMENT OB GENERAL A N D AGRICULTURAL CHEMISTRY MASSACHUSETTS AGRICULTURAL COLLEGE,AMHERST
THE VOLUMETRIC FEHLING METHOD USING A NEW INDICATOR By A. M. BRECKLER Received October 21, 1914
The volumetric Fehling method has been looked on very unjustly as inaccurate and unreliable. An inspection of t h e work of A. W. Peters’ shows t h a t under t h e most carefully controlled conditions of t h e gravimetric method, the ability t o duplicate reducing sugar determinations is limited t o a n accuracy of about one part in 2 0 0 . His dextrose copper ratios fluctuate about a mean value of 0 . 5 2 2 in t h e range of 2 5 t o I O O mg. of dextrose. The highest value is 5 2 5 and t h e lowest is 5 2 0 , the value in each case being a mean of 4 t o 6 determinations. The work of Munson and Walker2 a t times shows departures from t h e average which are fully as great. I n seeking a method of reducing sugar estimation, therefore, an accuracy of one part in 2 0 0 is as great as can be expected. The method here described is dependent on a cons t a n t volume a t t h e close of t h e titration, a constant time of boiling, and the use of sodium monosulfide solution as indicator. DESCRIPTION OF METHOD
The Fehling solution used is made up as follows: A. FEHLING’S COPPER s o L U T I O N 3 - 3 4 . 6 3 g g. Of carefully selected crystals of pure copper sulfate dissolved i n water and diluted t o exactly 500 cc. B . FEHLING’S ALKALINE TARTRATE SOLUTIOK-I 73 g. of Rochelle salts and 50 g. of sodium hydroxide are dissolved in water and diluted t o exactly j o o cc. The solution of sodium monosulfide is made up by dissolving 4 g. of crystalline sodium: monosulfide in I O O cc. of water. It should be made fresh every day. The test is conducted in a test tube X 3 / 4 in. A wooden clamp for holding this tube should be provided. A spot tile with six or more cavities, a I O cc. pipette, some straight pipettes, and a burette for the sugar solution completes the outfit. To make a determination, I O cc. of the mixed Fehling solution are pipetted into the test tube. The sugar solution, which should contain between zoo and 400 mg. of dextrose or an amount of other sugar equivalent in reducing value t o these amounts, is then run in, starting with 8.5 cc. The solution is then boiled one minute, counting from the time a bubble of steam first traverses t h e entire length of liquid. After the addition of 8. j cc. and boiling, t h e color is noted. If = J .A . c. s.. 34, 928. Ibid., 38, 663. 3 Leach, “Food Inspection and Analysis,” p. 591. 2
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t h e solution is very blue, t h e sugar solution may be added 2 cc. a t a time, boiling 1 5 seconds after each addition. When t h e solution in the tube is only a faint blue, a drop of it is added t o two drops of sulfide solution on t h e tile. The tile is given a slight rotary shake and the color of t h e spot noted. It will be seen t h a t the suspended cuprous oxide turns black and settles a t once, while the supernatant llquid turns a more or less intense yellow. The sugar solution is now added t o t h e test tube in smaller portions, depending on t h e intensity of t h e yellow coloration, boiling 1 5 seconds after each addition. It is usual in this laboratory t o use 0.4 cc. a t this point. As the color becomes fainter, less is added each time, until finally successive additions are only 0 . 1 cc. each. As soon as no color is perceived immediately upon t h e settling of the cuprous compound, t h e result is read from the burette and t h e mean of this reading and the previous one taken as t h e number of cubic centimeters required t o reduce the ten cc. taken. The experiment is then repeated, adding enough water t o make t h e final volume up t o 3 0 cc., and 97 t o 98 per cent of t h e sugar solution required in t h e previous trial. This is then boiled 1l/2 minutes and t h e exact amount adjusted, which can usually be done in two additions. This final determination should require from four t o six minutes from t h e time t h e solutions are mixed until the estimation is finished. It will rarely be found t h a t these t w o determinations differ more than I per cent in value from each other. The second determination is the one t o be used. When t h e approximate amount of sugar is known, as it is in most cases, t h e first approximation can be made much closer and the two titrations given equal weight.
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NOTES O N T H E METHOD
The time of boiling after ‘/z minute seems t o play a comparatively unimportant part. I n a series of experiments 99 per cent of t h e sugar (dextrose) required t o reduce a solution was added and the mixture boiled different lengths of time. The amount required for complete reduction was the same whether the first portion was boiled ‘/2 minute or I ~ / Zminutes. Too protracted boiling, however, is undesirable on account of evaporation and exposure t o air. Hence the total time of boiling should be kept within 1 3 / ~ t o 2’/4 minutes. The desirability of a constant volume in the gravimetric method during the reduction is well known; preliminary experiments showed this plainly. Under the conditions of this method I O cc. of Fehling solution with a final volume of 5 0 cc. required as a mean of several determinations 4 9 . 3 mg. of dextrose for reduction; I O cc. with a final volume of 3 0 cc. required 47.6 mg. for reduction. By making a preliminary titration, bringing the volume up t o a constant amount, and adding most of the sugar a t once, its reducing values become constant. Even if the reducing values of the 2 or 3 per cent subsequently added vary 2 0 per cent, the results are still within the limits of the method. AS can be seen from t h e difference shown above, a variation of z or 3 cc. in t h e final volume may be con-
*
T H E J O U R N A L OF 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 ,
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sidered negligible. T h e final volume was arbitrarily selected, as it was t h e greatest volume a t which 0.1 cc. of 0.2 per cent dextrose solution would make a distinct change in color. A test t u b e of t h e size given prevents much exposure t o t h e air, and its liability t o boil over when heated too rapidly, prevents excessive evaporation. Numerous trials were at first made with ferrous sulfocyanidel a n d iodide starch mixture,2 b u t it was found t h a t both have a common defect; namely, t h e y become colored on exposure t o air without addition of Fehling solution. Attempts t o obviate this destroy their sensibility t o a large extent. The writer had used sodium monosulfide for some time for colorimetric copper estimations and was impressed b y its sensitiveness. Furthermore, there ought t o be no tendency for t h e reoxidation of t h e copper on mixing with t h e indicator, as t h e medium is a reducing one. After a few minutes a yellow color will appear, owing probably t o solution of some of t h e copper sulfide. The absence of color, however, is permanent for sufficient time for a n observation t o be made, while when copper is unreduced, t h e appearance of t h e color is immediate. After t h e operator has become familiar with t h e end point, it can be judged very closely by t h e appearance immediately on addition of t h e drop. As long as unreduced copper is present, a yellowish color is perceptible without agitation, while when all t h e copper is reduced, t h e only shade is t h e reddish brown of t h e cuprous oxide. Proteins, a n d metals which give colored sulfides interfere with t h e end point. For t h e former, t h e writer boils a measured volume a n d adds alumina cream while still hot, then cools and makes up t o a convenient volume, adding sufficient water t o correct for t h e volume occupied b y t h e alumina. The solution can t h e n be allowed t o settle and t h e clear supernatant liquid used, or if time is a n object, i t may be filtered. This method works very well on most urines a n d colored solutions. For removing t h e metals, methods will doubtless suggest themselves. R E S U L T S OBTAINED
Various dilutions of a sugar solution were made u p and given t o a member of t h e staff (without information as t o their sugar content), who obtained t h e following figures. No. of cc. used in titration 23.30 15.65 11.70 9.35
M g . total dextrose present 47.60 47.75 47.60 47.75
It must be understood t h a t this was not pure dextrose, as t h e purpose was only t o see whether results were constant and independent of t h e concentration of t h e titrating solution over comparatively wide limits. The above results were each obtained with one titration besides t h e preliminary one. Impure maltose solutions were titrated in a similar manner. Ling Rendle and Jones, Allen’s “Commercial Organic Analysis,” edition. 2 F. F. Harrison, Sutton’s “Volumetric Analysis.” 10th edition. 1
4th
No. of cc. used in titration 23.2 15.5 11.6 9.25
Vol. 7 , NO.
I
Mg. total maltose present 92.8 93.0 92.4 92.5
The following results show t h e remarkable constancy of results over long periods. The titrations are on j cc. of mixed Fehling solution, which was the original manner of using t h e method, b u t i t was changed t o increase t h e accuracy of t h e end point. 6/11/1914 5 cc. Fehling reduced by 44.43 mg. crude maltose 8/26/1914 5 cc. Fehling reduced by 44.47 mg. crude maltose
For each value four concentrations were titrated. This constancy is not confined t o maltose, b u t is also t r u e of dextrose. The dextrose factor for j cc. in M a y was 23.6 mg. and in August 23.8 mg. The writer advises t h e standardization of t h e Fehling solution b y each user, a n d hence his factor should not be accepted as final. A mean of four determinations on pure glucose from two different sources in which t h e maximum variation from t h e average was * O . I S mg., gave 4 7 . j mg. dextrose as t h e value of I O cc. of a Fehling solution whose copper content had been accurately adjusted b y a sodium thiosulfate solution standardized with pure copper. As can be seen, t h e factors, once obtained, need be determined only at rare intervals. Pure dextrose, prepared by t h e Bureau of Standards, is now available for this purpose. SUMMARY
The writer describes a modification of Fehling’s volumetric method designed t o do away with some of its sources of error. The use of a new indicator is described which, so f a r as is known, is here first used for t h a t purpose. DISTILLERS LABORATORY 503 KENTUCKY TITLEBUILDING LOUISVILLE, KY.
A COMPARISON OF THE GUNNING-COPPER METHOD WITH THE KJELDAHL-GUNNING-ARNOLD METHOD FOR THE DETERMINATION OF NITROGEN B y OVP F. JBNSEN Received June 3, 1914
I n 1908 t h e Association of Official Agricultural Chemists officially adopted’ a modification of t h e Gunning method for t h e determination of nitrogen which prescribes t h e use of 0.1 t o 0.3 g. of crystallized copper sulfate in addition t o t h e potassium sulfate. Penny,* in his report on the determination of nitrogen in 1907, saps of this modification: “ T h e claims made for t h e catalytic action of copper sulfate seem t o be justified b y experience, a n d there is reason t o suppose t h a t this reagent will do much t o shorten t h e time of digestion.” Although possessing many advantages over t h e other methods in use a t t h a t time, i t was not generally adopted. More recently another modification known as t h e Kjeldahl-Gunning-Arnold3 method has been proposed. This method differs from t h e GunningCopper method, only in the use of metallic mercury instead of copper sulfate. I n a recent number of THIS JOURNAL,^ Trescot shows, very conclusively, 1 2 8 4
U. S. Dept. Agr., Bureau of Chemistry, Bull. 122, p. 183. I b i d . . 116, p. 43. I b i d . . 108, p. 15. Vol. 6 (L913). 914.
