Determination of Crude Fiber Potassium Sulfate as a Filtration Aid A . &l.NEUBERT, FRED VANAMBURGH, AND J. L. S T . JOHN Division of Chemistry, Agricultural Experiment Station, State College of Washington, Pullman, Wash.
D
IFFICULTY in filtering the crude fiber of occasional mixed-feed samples has led the authors to search for a method for hastening the filtration process. Because of the hydrated colloidal state of many of these hard-to-filter fiber samples, the use of a strong electrolyte as a precipitant was suggested. Potassium sulfate was selected because it has no water of crystallization and is not decomposed a t temperatures employed in ashing the fiber ( 2 ) . Thus, there would be no difference in TTeight of samples because of water loss due t o the temperature differential between the drying and ashing steps. Although many devices have been used to aid fiber filtration, a search of the literature revealed no modification employing the principles involved in the method presented in this paper. Experiments designed to show whether the use of potassium sulfate in the crude-fiber method alters the value obtained were conducted on mixed-feed samples that were filtered with difficulty by the usual method. Mixed-feed samples that' filtered rapidly by the official (1) method and feces samples, which often present the greatest difficulty in filtration, mere also analyzed by the two methods.
Methods Employed The official method of the Association of Official Agricultural Chemists (1) for crude fiber in feeds was employed, n-ith the exception that polT-dered potassium sulfate was added to the fiber sample, suspended in hot vater, before filtering through a Gooch crucible. The sample was transferred from the digestion flask to the filtering cloth after the sodium hydroxide digestion, the alkali was permitted to drain off, and the fiber was washed with hot water. If difficulty was encountered at this point in the filtration, powdered potassium sulfate was sprinkled on the sample. The sample was then washed TT-ith about 100 cc. of hot water into a 250-1212.beaker and powdered potassium sulfate (10 to 20 grams) was added with stirring to the hot suspension. The presence of undissolved potassium sulfate at this point does not interfere. When the sample was permitted to stand for a few seconds, the suspended hydrated fiber precipitated. The supernatant liquid could be readily decanted from the fiber and the fiber TTashed into the Gooch crucible with hot 5 per cent potassium sulfate solution. Filtration was very rapid in all samples, and it was not necessary to decant the sample to attain rapid filtration. The difficultly filtered colloidal state of the fiber had been transformed into an easily filtered condition. To wash the sample in the Gooch with hot water successfully, only small amounts of water could be used. When the potassium sulfate in the fiber had been reduced below a certain concentration by washing, the fiber resumed its hydrated form and stopped the filtration process. Permitting the suspended fiber to cool before treatment with potassium sulfate decreased the efficiency of the method. The fiber was washed with 10 t o 25 cc. of hot 5 per cent potassium sulfate solution and then with 10 cc. of 95 per cent ethyl alcohol. The crucible was rinsed on the outside n-ith water and wiped dry to remove any potassium sulfate adhering to the outer and bottom surfaces of the Gooch crucible before drying to constant weight at 110" C. After weighing, the sample was ashed at from 600" to 800" C., cooled, and reweighed.
method, but were filtered and washed within 5 minutes by the modified method. Results with easily filtered feeds show very close agreement between the standard and modified method. Because of the ease of filtration there was a possibility that all of the potassium sulfate might be washed from the Gooch in the modified method. T o assure the presence of potassium sulfate in the fiber during drying and ashing, solid potassium sulfate was added to the Gooch crucible after washing. TABLEI. DETERMIXATION OF CRUDEFIBER Sample No.
1
Officlal Method
c
2
*
o
1
A V
%
7
0
7
o
Comparison of Methods 70
7.38 5,57 3.96 2.19 5.75 12.23
-0.06 +0.02 -0.01 +0.01 +0.21 f0.12
17.70 20.87 17.86 16.67 17.13 15.32
-0.40 +0.14 $0.30 0.00 +0.18 +0,15
22.92 19.93 43.63 42.74 42.81 33.83
-0.71 -1.36 -0.24 -0.93 -1.18 -0.71
Modified RIethod
Av.
2
0
"
C
"
Easily Filtered Feeds
!.gs
1 2 3 4 5 6
7.35 5.54 3 99 2.12 5.61 12.16
3.95 2.23 5 47 12.06
7 8 9 10 11 12
18.19 20.87 17.49 16.56 17.07 15.08
18.09 20.59 17.63 16.78 16.82 15.26
13 14 15 16 17 18
23.64 21.00 43.45 44.42 44.18 34.73
23.61 21.58 44.29 42.91 43.80 34.34
3-30
7.44 5.55 3.97 2.18 5.54 12.11
7.38 5.58 3.98 2.18 5.81 12.27
7.37 5.56 3 93 2.19 5,68 12.18
Difficultly Filtered Feeds 18.14 20.73 17.56 16.67 16.95 15.17
17.56 20.81 17.90 16.66 17.10 15.35
17.83 20.93 17.81 16.68 17.16 15.28
Difficultly Filtered Feces 23.63 21.29 43.87 43.67 43.99 34.54
23.05 19.86 43.51 42.64 42.70 33.70
22.79 20.00 43.74 42.84 42.92 33.115
There was close agreement between the two methods for feeds which were difficult to filter, except for sample 7 . This sample contained considerable quantities of hempseed meal and was very difficult to filter by the official method, requiring filtration b y suction overnight. It was readily filtered by the modified method, but gave results somewhat lower than by the official method. Table I shows consistently lower values but much closer agreement between duplicates for t'he modified method than the official method as applied to feces. The authors believe that the values obtained by the official method on samples which are difficult to filter are subject to question because of the long delay in the filtering process. It may, therefore, be assumed t'hat the modified method is more nearly correct for these samples.
Conclusions The use of pot,assium sulfate in the filtration of the crude fiber after alkali digestion reduced the filtration and washing time to the normal on all difficultly filtered samples studied. The modified method gave the same per cent of fiber except in the case of feces and in the sample containing hempseed meal. Results by the modified method in these cases were lower but are believed to be more nearly correct. Duplicates filtered rapidly by the modified method show closer agreement than those filtered slowly by the official method.
Discussion of Results Results are presented in Table I. Samples 1 to 6 represent six mixed-feed samples which could be filtered and washed within 5 minutes b y either the official or the modified method. Samples 7 to 12 represent six feed samples which required from 20 minutes to 24 hours to filter by the official method, but were filtered and washed within 5 minutes by the modified method. The six sheep feces samples required from 1 to 24 hours to filter through the Gooch crucible by the official
Literature Cited (1) Assoc. Official Agr. Chem., "Official and T e n t a t i r e Methods of Analysis". 4 t h ed.. p. 330, 1935. ( 2 ) Hodgman, C. D., "Handbook of Chemistry and Physics", 21st ed., p. 435, Cleveland, Ohio, Chemical Rubber Publishing Co., 1936-37.
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