Acidity Titration of Low-Grade Rosins W. C. SMITH,Bureau of Chemistry and Soils, United States Department of Agriculture, Washington, D. C.
I
N DETERMINING the acid number of rosin it is the general procedure to dissolve 1 to 2 grams of the sample in 50 cc. of neutral 95 per cent alcohol, add phenolphthalein as indicator and titrate with 0.5 N standardized aqueous potassium or sodium hydroxide solution. The acid number is the number of milligrams of potassium hydroxide required to neutralize 1 gram of sample. With the higher grades of rosin little difficulty is experienced in determining the end points, although the solution becomes slightly redder as the end point is neared. However, with low-grade rosins the red color of the solution almost completely masks the end point, resulting in unreliable acid numbers. By using a modification of the Albert method, Coburn (3) was able to duplicate results very closely in the titration of low-grade rosin. His method consists in dissolvini a 1- to 2-gram sample in 25 cc. of 2 to 1 benzene-alcohol mixture, adding 25 cc. of saturated sodium chloride solution, and several grams of sodium chloride, and then adding an excess of 0.5 N aqueous alkali. The uncombined alkali goes into the brine layer and is titrated back with standardized acid solution. Phenol-
grade rosin because t h e end point was masked by the red color of the salt solution, caused by the soap of the highly oxidized portion of the rosin, which could not be salted out. An examination of the absorption spectra of phenolphthalein and rosin solutions suggested a method of detecting the end point of the titration, By using a small directvision hand spectroscope, the first appearance of the phenolphthalein absorption band in a certain thickness of a solution of low-grade rosin in alcohol furnished a more sensitive criterion for the end point of the titration than attempting to note a color change. It has been shown by Brode (2) that phenolphthalein in solutions of 8.4 to 10.0 pH possesses a relatively narrow absorption band in the green with maximum absorption a t wave-length 553 mp, the more alkaline solutions showing stronger and broader absorption. Muller and Partridge (4) have made use of the characteristic absorption bands of phenolphthalein and other indicators in titrating with photoelectric cells. Brice ( I ) has shown that the absorption of light by rosins is practically continuous, the region of complete absorption shifting toward the red as the grade of the rosin becomes lower. For example, G rosin type (0.875 inch (2.22 cm.) thickness of rosin) absorbs all wavelengths shorter than about 530 mp, while F rosin type absorbs completely all wave lengths below about 560mp. The abw
FIGURE1
% $
sorption spectra of rosin solutions are similar to those of rosins, and to show any effect due to the phenolphthalein the rosin solution must have such a concentration and/or thickness that its color is not redder than about a G rosin. With the concentration fixed by other considerations, the optimum thickness of solution for observing the presence or absence of the phenolphthalein absorption band is 0.5 to 1 inch (1.26 to 2.5 cm.). The feasibility of using the first detectable appearance of the phenolphthalein absorption band as the end point in titration of rosins was first tested out on a solution of a highgrade rosin, so that a comparison could be made with the end point as determined by color change. Five grams of WG rosin and 1cc. of 1 per cent alcoholic solution of phenolphthalein were dissolved in 100 cc. of 95 per cent alcohol in a 300-cc. Erlenmeyer flask, and 0.5 N aqueous sodium hydroxide was added until a faint pink indicated the end point. The solution was then examined with the hand spectroscope, the sky being used as a background. The phenolphthalein absorption band, though detectable through about 3 inches (7.5 om.) of solution, could not be seen through a 1inch (2.5-cm.) layer of solution. However, the addition of 0.1 cc. excess alkali made the absorption band visible even through a 0.5-inch (1.26-cm.) layer. Similar results were obtained on N and M rosin. This procedure was then extended t o solutions of low-grade rosins, for which the color end point is masked but for thin layers of which the phenolphthalein absorption band is not masked. Five grams of B rosin (rosin redder than D type) were titrated as above until the phenolphthalein absorption band became just perceptible when 0.5 inch (1.26 cm.) of the solution was viewed through the hand spectroscope. The amount of alkali added was assumed to be not more than 0.1 cc. in excess. The flask was stop ered and allowed to stand overnight, so that the excess alkag would be taken up b the esters in the rosin. No absorption band was then visibz through a layer of solution 0.5 to 1 inch (1.26 to 2.5 cm.) thick. However, on the addition of 0.1 cc. of alkali the absorption band reappeared in 1 inch (2.5 cm.) thickness. The appearance of the absorption band is illustrated by the spectrograms in Figure 1 taken on a spectrograph of moderate dispersion, using a tungsten lamp as the source of light. The low dispersion of the hand spectroscope is more favorable for detecting the presence of the faint absorption band than a higher dispersion instrument, and in the visual method the band appears narrower and more distinct than in photographs. The spectra are for 0.5 inch (1.26 cm.) thickness of the B rosin solution containing phenolphthalein (1) with no free alkali present; (2) a t the end point [band appears in 1-inch (2.5-cm.) layer but not 0.5-inch (1.26-cm.)] ; (3) 0.1 cc. alkali in excess [band appears in 0.5-inch (1.26-cm.) layer];. (4) 0.2 cc. alkali in excess; (5) tungsten lamp; and (6) mercury arc. The following procedure is proposed for determining the acid number of low-grade rosin: Place 100 cc. of 95 per cent alcohol and 1 cc. of 1 per cent alcoholic solution of phenolphthalein in a 300-cc. Erlenmeyer flask. Add 0.5 N standard aqueous alkali solution until the absorption band appears in 1 inch (2.5 cm.) thickness. This requires about 0.05 cc. excess. Add 5 grams of sample, stop er and allow the rosin t o dissolve at room temperature. i d d rapidly about 1 cc. less than the expected or theoretical quantity of 0.5 N alkali. After the addition of each successive 0.05 t o 0.1 cc. of alkali, hold the flask in an inclined position towards a source of light, preferably dayli ht, and view 0.5 to 1 inch (1.26 to 2.5 om.) of the solution tirough a hand spectroscope. The end point has been reached when the absorption band becomes just perceptible.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
March 15,1934
Typical results obtained on some low-grade rosins by this proposed method are shown in Table I.
123
method. The acid number can therefore be reported to the first decimal place.
SUMMARY
TABLEI. TYPICALRESULTSONILOW-GRADE ROSINS SAMPLBSAMPLESAMPLESAMPLESAMPLB
Individual determinations
1' 2a FF wood B gum 151.6 161.5 151.3 161.5
ii::: i::
With the aid of a direct-vision hand spectroscope for observing the end point, the acid number of the lowest grade (reddest) rosin can be determined with a degree of i,"!::: accuracy equal to that ordinarily obtained with high-grade 157.1 or yellow rosin.
3 4 5 B gum D gum E gum 156.0a 161.0a 157.2a 156.3" i61.3a 157.2a
Average acid number 151.4 161.4 166.0 a Determinations made on 5-gram portions. b Determinations made on 5.5-gram portions. 0 Determinations made on 4.5-gram portions.
::;::: 161.0
The probable error in titration by this method is not greater than 0.1 cc. of 0.5 N alkali, corresponding to an error of less than 0.6 in acid number when a &gram sample is used, This is equal to the accuracy ordinarily obtained in titrating a 1- to 2-gram sample of high-grade rosin by the conventional
LITERATURE CITED (1) Brice, Naval Stores Rev., 43, No. 30 (Oct. 21, 1933); Drugs, Oils & Paints, 48,380 (1933). (2) Brode, J. Am. Chsm. SOC.,46,581 (1924). (3) Coburn, IND.ENQ.CHEM.,Anal. Ed., 2, 181 (1930). IND. ENQ. CHEM*,20, 423 (1928). (4) Mfiller and RECEIVED October 31, 1933.
Quantitative Estimation of Furfural at 0" C. with Bromine ELIZABETH E. HUGHES AND S. F. ACREE,Bureau of Standards, Washington, D, C. HE quantitative estimation of pentoses and pentosans
T
is often required in the work of agricultural experiment stations, medical research laboratories, paper mills, and rayon and textile plants. The usual procedure involves the distillation of the material with hydrochloric acid to convert the pentose derivatives into furfural, which is then determined in the distillate. Among the best known reagents for this latter purpose are phloroglucinol, thiobarbituric acid, and dinitrophenylhydrazine used gravimetrically, and bromine or bromate used volumetrically, The amount of bromine taken up by the furfural has been estimated by electrometric titration by Pervier and Gortner (6),Kline and Acree (S),Magistad (4),and others, and with an excess of bromine and back-titration with potassium iodide and thiosulfate by van Eck ( I ) , Powell and Whittaker (8),Kline and Acree (S),and others. The gravimetric methods, especially with thiobarbituric acid, require a t least 24 hours and involve the usual difficulties in filtering and weighing. The present volumetric methods are more rapid but less accurate when performed a t room temperature. This is due to the fact that the rapid consumption of one mole of bromine per mole of furfural is followed by the slow addition of a second mole of bromine a t 20" to 30" C. The large temperature coefficient of the second reaction observed by Magistad (4) and the authors introduces considerable error when determinations are made a t variable room temperatures. Furthermore, it is found that considerably more than two molecules of bromine react with furfural a t 20" to 30" C. when the reaction time is prolonged. The authors have, therefore, carried out a series of experiments on the reaction of bromine and furfural at 0" C. to determine whether the reaction could be limited to the first rapid step a t this low temperature.
EXPERIMENTAL The furfural was extracted with alkali to remove acid and fractionated under reduced pressure. It had a boiling point of 161.6" C. at 760 mm. Freshly distilled furfural was sealed in small glass balloons holding about 1 gram, and weighed. They were placed under water in graduated flasks and crushed, the flask was filled t o the mark with water, and samples were pipetted into special Erlenmeyer flasks containing 200 ml. of 3 per cent hydrochloric acid. The 3 per cent hydrochloric
acid was used because it is rarely exceeded in the usual furfural distillates and liberates the bromine fully. The hydrochloric acid in 6 to 12 per cent concentrations (3,6, 6) decomposes the added thiosulfate. As it is very difficult to prevent the escape of bromine when standard 0.1 N potassium bromate plus potassium bromide is added t o acidified furfural in open vessels, groundglass-stoppered Pyrex Erlenmeyer flasks were fitted with two side arms t o hold measured volumes of 0.1 N potassium bromate plus potassium bromide and of 10 per cent potassium iodide solution. To prevent accidental tilting and mixing, lead horseshoes were placed around the bottoms of the flasks, which were suspended from the edge of the ice bath with heavy wires fastened about the necks. The flasks were cooled between 0' and 2" C. in an ice and water bath and the reaction started by tilting the flask to allow the bromate solution t o run into the furfural. The solutions were mixed and allowed t o stand until it was desired t o stop the reaction, when 10 ml. of 10 per cent potassium iodide in the other side arm was run into the mixture. The flask was removed from the ice bath and shaken vigorously to allow the enclosed bromine gas to react with the potassium iodide. The stopper was removed, rinsed as usual, and the contents were titrated with 0.1 N thiosulfate by using starch indicator. Under the appropriate conditions one mole of furfural reacts with one mole of bromine; hence one ml. of 0.1 N thiosulfate or 0.1 N potassium bromate is equivalent to 0.0048 gram of furfural.
TABLBI. REACTION OF BROMINE WITH FURFURAL AT o o C REACTION TIME Min. 10 sec. (approx.) 1 (approx.) 2 3 4 5 6 7 8 10 30
eo
90 240
PERCENTOB THEORETICAL AMOVNT OB FURFURAL FOUND 100-mg. 57-mg. 45-mg. 10-mg. sample
sample
sample
samplsO
%
%
%
% ... ...
... ...
... ... ... ... ...
99.6 99.5 100.0 100.2 100.3 100.1 100.1 100.4 iOi:3 103.0
... ...
... ... 1oo:o 99.9 99.7
...
99.8
31.5 99.3 100.1 100.2 100.0
...
99.1 100.0 100* 9 100 5
... ... ... 99.6 ... ...
... ... ... ... ...
100:3
100:3 100.1 101.8 103.0 105.5
16615
l00:5
1oi:0
ioi:4 102.6
...
... ...
... ...
...
103.4
iii.*s
... ...
a With small samples which use u p only 2 ml. the titration error is neoensarily large. 0.5 drop, or 0.02 ml. represents 1 per cent. This acoountn for the larger variations in the percentages.
