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
748
under field conditions is shown b y t h e following table i n which are t h e analyses of soils from four old potato rows t o which sulfdr was applied last year. Samples were taken approximately one year after t h e application. The rows were 1400 feet long a n d borings 0-3 inches were made about every twenty feet. The sulfured rows r a n consecutively b u t 1 2 rows were skipped before samples on t h e check row were taken. T h e sulfured rows were t w o feet eight inches apart. Rate of sulfur application Lime requirement in Lbs. per acre lbs. CaO per acre 1 ...................... 600 3187 300 2590 2 ...................... 3 ...................... 600 3025 4 ...................... 300 2247 5 . . .................... None (check) 883
Row
Rows I a n d z received ammonium sulfate in the fertilizer last year, while rows 3 a n d 4 received sodium nitrate, a fact which would account for a difference i n t h e acidity. Laboratory experiments with a heavy clay loam soil a n d a sandy loam soil show a much more rapid oxidation of sulfur with t h e latter soil. Soils made u p t o water content ( 2 0 per cent) once each week a n d allowed t o dry did not give as rapid oxidation as those kept covered so as t o maintain optimum moisture content. T h e influence of texture,, moisture a n d bacterial content will serve as a basis upon which further work on t h e influence of sulfur on soil reaction will be carried out. NEW JERSEY AGRICULTURAL EXPERIMENT STATION NEW BRUNSWICK
THE FERRIC ALUM ESTIMATION OF CASEINEI By H. V. ARNY AND H. H. SCHAEFER Received May 20. 1914
At t h e j 3 r d ( 1 9 0j ) meeting of t h e American Pharmaceutical Association, one of us, with T. M. P r a t t , presented a paper on a rapid estimation of caseine in milk based on precipitation of t h e casein with a n excess of a solution of ferric alum of known titer, estimation of t h e unused iron in t h e filtrate a n d then calculating t h e amount of iron used by t h e caseine i n t h e act of precipitation. This paper was published in the Proceedings of t h e American Pharmaceutical Association, 53 ( 1 9 0j ) , 2 7 5 , a n d in t h e American Journal of Pharmacy, 78 (1906), 1 2 1 , b u t as our attention has been called t o t h e fact t h a t these two publications are not always conveniently accessible t o t h e general chemical worker a n d as t h e paper was frankly a preliminary one, i t seemed advisable t o report further work on t h e topic a n d t h a t in a strictly chemical journal. I n t h e previous paper was presented t h e method of manipulation and figures showing influence of temperature, of presence of f a t (butter), of presence of sugar a n d , lastly, a few parallel experiments comparing t h e ferric alum figures with t h e Kjeldahl figures of t h e same sample of milk. These latter figures, t h e a t t e m p t t o express t h e value of I cc. of ferric alum solution in terms of nitrogen a n d of caseine, were not sufficiently exact nor were t h e number of assays performed sufficient for 1 Presented a t the 48th Meeting of the American Chemical Society, Rochester, September 8-12, 1913.
Vol. 6, N o . 9
a positive statement of t h e ferric alum factor. While t h e process in t h e empiric commercial milk testing work for which i t was devised proved entirely satisfactory, proof of its scientific accuracy was yet t o be shown; a n d this has been t h e aim of our experiments during t h e past winter. Since t h e publication of our prior paper, VanSlyke & Bosworthl have published a titration method based on use of tenth-normal alkali. This plan has not proved as satisfactory in our hands as t h e ferric alum method b u t t h e fact has n o bearing on our investigation, since there is room for more t h a n one simple estimation of caseine. T H E F E R R I C ALUM E S T I M A T I O K
Since t h e publication of our previous paper, several modifications of t h e process as originally published have suggested themselves a n d have been tried out. I n all t h e work we have used as t h e standard iron solution, a ferric alum containing 4 8 . 2 2 2 4 g. F e ( N H 4 ) (SO4)2.12H2O t o t h e liter, standardizing this solution by t h e usual method of volumetric ferric estimation; adding potassium iodide and hydrochloric acid a n d titrating t h e liberated iodine with tenth-normal sodium thiosulfate. One cc. of our standard solution, therefore, liberates 12.692 mg. iodine which in t u r n requires I cc. tenth-normal sodium thiosulfate for decolorization. I n our previous work, in a n a t t e m p t t o dispense with t h e use of t h e rather costly potassium iodide, we have repeatedly tried estimation of t h e iron solution b y titration with tenth-normal alkalis, noting end of reaction b y use of phenolphthalein as indicator. I n this case t h e ferric alum solution ( 4 8 . 2 2 2 4 g. t o 1000 cc.) was standardized b y titration against tenth-normal sodium hydroxide, I cc. of ferric alum solution taking 3 cc. tenth-normal aIkali for complete neutralization. This, we found a n easier procedure t h a n would seem a t first glance, as after a little experience, i t is not difficult t o note t h e red t i n t of t h e alkaline phenolphthalein combination in the supernatant liquid about t h e ferric hydroxide precipitate. The figures of such a line of assays in which t h e excess of ferric alum was estimated with N / I O alkali are given below. COMPARISON
OF
THIOSULFATE
ASSAYS
U-ITH
XORMAL
ALKALI ASSAYS
I n this and all other experiments described in this paper, t h e ferric alum solution employed was one containing 48.2224 g. FeNH4(S04)212H20 t o the liter and I cc. of this solution represented I cc. N sodium thiosulfate solution or 3 cc. N sodium hydroxide solution a n d in t h e rest of this paper this solution will be called t h e standard ferric solution. Five cc. of milk were mixed with j cc. of this standard ferric alum solution, t h e mixture filtered, t h e precipitate washed free from water-soluble iron and amount of iron in t h e filtrate a n d washings were estimated either with tenth-normal thiosulfate or with tenth-normal alkali. The results of such parallel assays from same samples of milk are here tabulated. ( a ) Five cc. milk with j cc. ferric alum a n d t h e 1
THISJ O U R N A L , 1 (1909), i 6 8 .
Sept., 1 9 1 4
T H E J O U R N A L OF I N D C 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
filtrate titrated with N j ~ o thiosulfate. Five cc. ferric alum = j cc. N / I O thiosulfate. ( b ) Five cc. milk with j cc. ferric alum a n d t h e filtrate titrated with N / I O NaOH. Five cc. ferric alum = I j cc. N , / I o N a O H . Cc. thio by used filtrate 1 A .... 2 . 5 I B. . . . 2 . 5 2 A .... 2.6 2 B . . . . 2.7 3 A. . . . 2 . 6 3 B.... 2.6 4.4 . . . . 2 , 7 4 B .... 2.6 S A. . . . 2.1 5 B.... 2.;
1
b _ _ 7
-
RepreRepresenting Cc.alum ferric Cc. NaOH senting ferric used by ferric Cc, ferric sol. alum sol. consumed filtrate solution consumed 2.5 2.5 7.7 2.57 2.43 2.5 2.5 8.4 2.80 2.20 6.9 2.3 2.70 2,c 2.4 2.J 2.3 6.1 2.03 2.97 2.6 2.4 7.6 2.53 2.47 2 .6 2.4 8.0 2.67 2.33 2.7 2.3 6.0 2.0 3.0 2.4 6.1 2.03 2.97 2.6 2.i 2.3 7.9 2.63 2.37 2.7 2.3 6.1 2.03 2.97
F r o m t h e above tabulation i t is seen, as might have been expected, t h e assay of t h e iron filtrate with potassium iodide. a n d tenth-normal thiosulfate gave more accurate results t h a n titration with alkali, even when t h e milk is strictly fresh. But in some cases the assay with alkali is very convenient. Our next work was a comparison of t h e ferric alumthiosulfate estimation with Kjeldahl-Gunning nitrogen estimations of t h e same milk. T h e ferric alum estimation was conducted with about 5 g. milk, carefully weighed in a Kjeldahl digesting flask, 2 0 cc. standard ferric alum solution added, t h e filtrate a n d washings treated with 3 cc. 3 1 per cent hydrochloric acid a n d 3 g. potassium iodide, t h e solution digested in water b a t h a t 40' for 30 minutes a n d then titrated with tenth-normal sodium thiosulfate which h a d been standardized against C . P. potassium dichromate. T h e nitrogen assays were performed on t h e caseine iron precipitate adhering t o t h e Kjeldahl flask, a n d t h a t portion which was washed upon a filter, t h e entire precipitate being carefully washed free from soluble iron a n d the nitrogen of filter paper being controlled b y using similar paper in the blank experiments which were run with every set of milk used. As none of t h e blanks required more t h a n 0.1 cc. of N/IOacid t o neutralize distillate, t h e y are n o t figured in t h e tables which follow. T h e Kjeldahl-Gunning method employed was t h a t described in U. S. Department of Agriculture, Bureau of Chemistry, Bulletin I07 ( I ~ I I ) , a n d the distillations were conducted in a Sy apparatus Certified pipettes a n d burettes were employed in all of these assays. T h e results of t h e first batch of these comparative assays are tabulated below. COMPARISOiY
O F F E R R I C ALUM-THIOSULFATE KJELDAHL
t h a t i t is due t o t h e use of a thiosulfate of too great a strength. One cc. i V i r o thiosulfate = 0.0482224 g. ,iron. Hence 0.1 cc. of N / I O thiosulfate = 0.00482224 g.
b
a 7
WITH
FIGURES
( a ) Milk (about j C C . ) mixed with 2 0 cc. standard ferric solution a n d titrated with &V/IO thio. Twenty cc. standard ferric solution = 2 0 cc. N / I O thiosulfate. ( b ) Precipitate from same milk assayed for nitrogen by Kjeldahl-Gunning process. One cc. N / I O H2S04= o . o o 1 4 0 1 g. nitrogen = 0.00893838 g. casein. Examination of t h e figures show so wide a variation in t h e ferric alum-csseine factor as t o suggest their absolute worthlessness, b u t a more careful s t u d y shows t h a t t h e discrepancy is more apparent t h a n real,
749
Milk sample 1 A , .. . . . I C...... 3 C. . . . . . 4 A , .. . . . 4 B.. .... 4 C...... 5 A , .. . . . 5 B...... 6 A , .. . . . 6 B ..... 6 C...... 7 A,,....
7 C...... 8 A , .. . . . 8 B.., .., 8 C. . . . . . 9 A. ..... 9 C . ,.. , . 10 A , . . . . . 10 B . , . . . . 10 C . , . . . . 11 A . . . . . . 11 B . . . . . . 12 B . . . . . . 12 C . . . . . . 13 A , . . , , . 13 B . . . . . .
Wt. milk 5.0569 5.56 5.342 5.9801 4.923 6.1 5.003 4.9861 5.2963 4.06 6.105 4.991 1 5.924 5.3691 6.0136 4.09 6.1001 5.369 4.9931 5.871 4.8654 4.065 5.1131 6 . 113 5.0365 6.0156 4.998
Cc. S I 1 0 Per H ~ O I cent consumed N 16.4 0.454 18.3 0,461 17.7 0.464 19.3 0.466 16.0 0.455 19.8 0.455 16.7 0.468 16.5 0,464 17.1 0.453 13.1 0,452 19.3 0.443 16.9 0.474 19.i 0,466 17.9 0.467 19.9 0.463 13.3 0.455 19.6 0.450 16.9 0,441 17.3 0.486 19.6 0 468 16.3 0.469 12.6 0.434 16.2 0,444 20.0 0.458 li.0 0 473 20.3 0.473 0.490 17.5 I
a 1
No. Per Cc. ferric cc. cent sol. con- thio caseine sumed used 2.895 2.5 17.7 2.941 2. I 17.3 2.96 17.3 2.7 2.973 3.2 16.8 l7,i 2.903 2.3 17.0 2.903 3.0 17.5 2.986 2.5 2.96 2.5 17.5 2.89 17.6 2.4 2.884 18.1 1.9 17.0 2,826 3.0 3.024 2 2 17.8 2.973 16.8 3.2 2.979 17.7 2.3 16.8 2.954 3.2 2,903 18.0 2.0 2.871 3.0 17.0 17.6 2.814 2.4 3.101 17.7 2.3 2.986 17.1 2.9 2.998 17.7 2.3 18.1 2. i69 1.9 17.6 2.833 2.4 2.922 16.8 3.2 17.6 3.018 2.4 16.7 3.018 3.3 3.126 17.4 2.6
-
Value of 1 cc. ferric
sol. i n g . caseine
iron, a n d t h e difference in our thiosulfate figures of only 0.1 cc. when used in above calculations means a discrepancy in t h e ferric alum-caseine figure greater t h a n is permissible in careful analytical practice. For instance, a difference of 0.1 cc. N / I O thio solution would be equivalent t o a difference of approximately 0.006 g. casein which is in excess of permissible error when we figure t h a t we used about j cc. milk in each experiment containing approximately 0 . 1 2 j g. caseine ( 2 . j per cent) which would mean t h a t a n error of 0.006 g. caseine (or 0.1 cc. N / I O thio solution) represents 4.8 per cent of t h e total caseine present. This defect was remedied by t h e simple expedient of changing t h e amount of standard iron solution used a n d employing a fiftieth-normal instead of a tenthnormal thiosulfate solution. Therefore, in t h e final run of comparative assays, we employed about j g. milk (exactly weighed), j cc. standard iron solution ( 4 8 . 2 2 2 4 g. to liter) and after adding t o t h e filtrate 3 cc. 31 per cent HC1 a n d 2 g. potassium iodide, we titrated with X / j O thiosulfate solution, each cc. of which represented 0.00964448 g. iron. The results are tabulated below. Examination of the following table shows t h a t under precautions observed, t h e relationship of the amount of ferric alum absorbed b y t h e milk a n d the nitrogen (and deduced caseine) value of t h e milk by the Kjeldahl-Gunning method is practically within t h e limits of analytical deviation. I t will be noted t h a t in t h e 46 assays reported, ten showed t h a t I cc. standard iron solution represents j 8 t o j 9 mg. of caseine, 3 4 show j 9 t o 6 0 mg. caseine, while only two show deviation from these figures. Hence we feel justified in striking the average of our results tabulated above a n d stating t h a t I cc. of standard iron solution [48.2224 g. of F e N H t ( S O ~ ) ~ I a H zt o O 1000 cc.] precipit a t e s from milk 0.0 j 9 3 4 g. casein. As t o manipulatire details, t h e assay we published in 1 9 0 5 h a d the one disadvantage t h a t separation of
T H E J O U R Y i l L OF I i V D r S T R I A L A N D ENGINEERING C H E M I S T R Y
750
the precipitate from t h e filtrate a n d particularly b y washing t h e precipitate free from soluble iron, was a te'dious process, covering a t least a half hour under the most favorable conditions of filtration. Efforts have been made t o reduce time of operation a n d considerable success has been attained. T h e Kjeldahl assay
No. cc. N/10 Per Sample Wt. HzS04 cent NO. milk consumed N 1 A . . . . . . . . . . 5.913 1 9 . 1 0.452 1 B . . . . . . . . . . 5.4451 1 7 . 5 0.448 1 c. . . . . . . . . . 7.26 23.5 0.453 2 A . . . . . . . . . . 6.1381 2 0 . 1 0.458 2 B . . . . . . . . . . 5.212 16.8 0.451 4.6891 1 5 . 2 0.454 1 8 . 0 0.448 5.621 3 B . . . . . . . . . . 5.1312 1 7 . 9 0.489 3 c. . . . . . . . . . 6.12 19.8 0.453 7.131 23.2 0.456 4.1219 1 3 . 3 0.452 6.421 2 1 . 1 0.460 5 . I312 1 6 . 8 0.459 6 A . . . . . . . . . . 7.1691 2 2 . 9 0.447 6 B . . . . . . . . . . $51 2 0 . 9 0.450 6 C. . . . . . . . . . ,8131 25.2 0.452 7 A . . . . . . . . . . 4.9361 1 6 . 0 0.454 5.121 16.7 0.457 4.8262 1 5 . 6 0.453 6.131 2 0 . 1 0.459 8 C . . . . . . . . . . 4.9361 1 6 . 1 0 45i 447 9 A . . . . . . . . . . 5.426 1 7 . 3 0..... 9 B . . . . . . . . . . 6 , 4 2 8 9 20.7 0 . 4 5 1 7.1101 2 2 . 6 0.445 5,1324 1 6 . 7 0.456 6.1203 1 9 . 7 0 . 4 5 1 10 . . . . . . . 4.9132 1 6 . 1 0 , 4 5 9 11 A . . . . . . . . . . 5 , 6 5 4 1 8 . 2 0.451 11 B . . . . . . . . . . 6.3521 2 0 . 4 0.449 1 2 B. . . . 1 9 . 8 0.453 6.12 12 c . . . . 5.9916 1 9 . 3 0.451 13 A . . . . . . . . . . 4.3571 1 4 . 2 0.457 13 B . . . . . . . . . . 4.1361 13.5 0.457 13 c . . . . . . . . . . 7.013 22.7 0.453 14 A , . . . . . . . . . 6.1251 1 9 . 6 0.448 14 B . . . . . . . . . . . 5.989 1 9 . 3 0.451 15 A . . . . . . . . . . 6.1231 1 9 . 6 0.448 15 c . . . . . . . . . . 5.1369 1 6 . 6 0.452 1 6 A . . . . . . . . . . 4.9812 1 6 . 2 0 . 4 5 5 16 C . . . . . . . . . . 6 . 7 1 21.8 0.455 17 B . . . . . . . . . . 5.0091 1 6 . 1 0.450 1 4 . 2 0.459 17 C . . . . . . . . . . 4.327 18 A . . . . . . . . . . 5.6631 18.1 0.448 1 6 . 1 0.452 19 A , . . . . . . . . . 4.986 19 B . . . . . . . . . . 4.3521 1 4 . 2 0.457 1 4 . 1 0.457 19 C . 4.32
,
c...
.........
The ferric alum and thiosulfate assay
No. cc. ferric sol. Per cent concaseine sumed 2.884 2 . 8 6 2.858 2 . 6 2 2.89 3.54 2.922 3 . 0 4 2,877 2.54 2.897 2 . 2 8 2.858 2.7 2.68 3.12 2.98 2.89 2.909 3 . 5 2.884 2.02 2.935 3 . 2 2.928 2 . 5 8 2.852 3 . 4 2.871 3 . 1 2 2.884 3 . 7 4 2,897 2 . 4 4 2.916 2 . 5 2 2.89 . 2 . 3 4 2.928 3 . 0 6 2.916 2 . 4 2 2.852 2 . 6 2.877 3 . 1 2 2.839 3 . 4 2.909 2 . 5 2 2.877 2 . 9 8 2.928 2.4 2.877 2 . 7 6 2,865 3 . 0 8 2.89 2.98 2.877 2 . 0 9 2.916 2 . 1 4 2.916 2.02 2.89 3.44 2.858 2 . 9 2 2.877 2 . 8 8 2.858 2 . 9 6 2.884 2 . 5 2.903 2 . 4 4 2.903 3 . 2 6 2.871 2 . 4 4 2.928 2.14 2.858 2 . 7 2 2.884 2 . 4 4 2.916 2 . 1 4 2.916 2.10
Cc. thio N/50 10.7 11.9 7.3 9.8 12.3 13.6 11.5 11.6 10.1 7.5 14.9 9.0 12.1 8.0 9.4 6.3 12.8 12.4 13.3 9.7 12.9 12.0 9.4 8.0 12.4
Value of 1 cc. ferric solution In gs. caseine
10.1
13.0 11.2 9.6 10.1 10.5 14.3 14.9 7.8 9.4 9.6 10.2 12.5 12.8 8.7 12.8 14.3 11.4 12.8 14.3 14.5
0.05920 0.05950 0,05893 0.05930 0.05999
One way t o save time t h a t has been employed i n other caseine assays was by t h e titration of a n aliquot p a r t of t h e filtrate, rather t h a n t h e entire filtrate a n d washings from t h e precipitate, b u t t h e plan, while quick, did not appeal t o us on t h e score of accuracy. I n order t o see whether titration of a n aliquot p a r t would be as exact as titration of t h e entire filtrate, a line of comparative assays from t h e same milk was performed. In each case I O cc. of milk were mixed with I O cc. standard ferric iron solution a n d t h e filtrate t i t r a t e d for ferric iron by t h e usual KI-thiosulfate method, described above. B u t in one case t h e entire filtrate a n d washings were titrated, while in t h e other case t h e milk-iron mixture was brought t o a definite volume (100 cc.), a n d this filtrate, representing half of this volume (or j o cc.) a n d presumably representing half of t h e milk used (or 5 cc.), was titrated. The figures given below show what might have been expected, t h a t such aliquot jo per cent p a r t does not represent 5 0 per cent of t h e milk employed, since in such reasoning no consideration is taken of t h e bulk of t h e precipitated casein of all t h e milk, which remains in t h e untitrated portion.
RELATIOX
OF
TITRATION
OF
y01. 6 , S O .9
COMPLETE
WASHINGS
A N D O F ALIQUOT PARTS RELATION OF TITRATION OF COMPLETE
WASHINGS A N D OF
Added 10 cc. ferric solution to each Cc. S / 1 0 Used milk thio for total A 10 cc . . . . . . . . . . . . . . . . . . . . . . 5.1 B 10 cc . . . . . . . . . . . . . . . . . . . . . . 5.0 c 10 cc ...................... 5.2 D 10 cc . . . . . . . . . . . . . . . . . . . . . . 4.9 E 10 c c . , . . . . . . . . . . . . . . . . . . . . 5.1 F 10cc. . . . . . . . . . . . . . . . . . . . . . 4.9 G 1Occ . . . . . . . . . . . . . . . . . . . . . . 5.2 4.9 5.0 J 10 cc . . . . . . . . . . . . . . . . . . . . . . 4.8
ALIQUOT PARTS
Cc. thio for (50 per cent) aliquot part 2.9 2.7 3.0 2.8 2.9 2.7 3.0 2.5 2.7 2.9
Our present method of assay differs from t h a t published in 1905 only in respect t o t h e two important improvements of greater accuracy a n d less time req u i r e d . Accuracy is secured by use of Y / j o thiosulfate solution a n d time-saving is accomplished b y hastening filtration a n d washing. I n our prior work we had tried rapid (suction) filtration without much success, b u t in our recent experiments we have very materially lessened duration of t h e assay by use of quick filtration. At first we mixed t h e milk with purified talc. (U. S. P.) a n d with a pure form of kieselguhr ( 2 g. t o each j cc. milk) running blank assays t o see if t h e diluent employed had a n y influence in t h e iron figure. Finding t h e diluent of n o influence, by employment of a water suction p u m p , aspirating bottle, porcelain perforated filtering disc a n d small disc of filter paper we reduced t h e time of filtration and washing from 30 t o 60 minutes down t o 15 minutes. Later we found we could filter as rapidly with straight iron curd without t h e diluent, especially if t h e milk a n d iron mixture was allowed t o stand a n hour before filtration. Our present method of assay may, therefore, be described as follows: About j cc. (or g.) milk are mixed with j cc. standard ferric solution ( 4 8 . 2 2 2 4 g. per liter) and allowed t o s t a n d a n hour; t h e mixture is then filtered by use of filtering disc a n d funnel, aspirating bottle a n d water suction p u m p a n d t h e precipitate washed until washings are free from'iron as shown b y "spotting test" with potassium ferrocyanide. Freedom from iron is usually shown after zoo cc. of combined filtrate a n d washings are obtained a n d when suction filtration is employed t h e mrashing is finished within fifteen minutes. T h e filtered fluid is placed in a n appropriate glass stoppered flask, 3 cc. 31 per cent HC1 a n d 2 g. K I are added, t h e mixture digested a t 40' C. for 30 minutes a n d t h e free iodine is titrated with Y j 5 0 thiosulfate solution, starch paste being used as indicator after t h e fluid has been decolorized t o a faint straw t i n t . Our figures show t h a t if 6.12 g. milk take 2 . 9 8 cc. standard ferric solution, employment of t h e ironcaseine-factor deduced above indicates t h a t t h e 6. I 2 g. of t h a t milk contains 0.17683 g. (2.98 X O.Oj934) caseine; t h a t it, therefore, contains about 2 . 8 8 9 per cent caseine. Examination of t h e method just given shows t h a t t h e actual time consumed in t h e assay is 3 j minutes, distributed as follows: Llixing milk a n d standard ferric solution, j minutes; filtering a n d washing mixture, 15 minutes; adding K I a n d HC1. j minutes; final titration, IO minutes. I n addition an hour's
Sept.. 1914
.
T H E JOC'RNAL OF I N D C S T R I A L A N D ENGINEERING C H E M I S T R Y
maceration of t h e ferric-milk mixture a n d 3 0 minutes digestion of t h e ferric-KI mixture before titration means a n assay occupies about two a n d a half hours. If we hasten, however, t h e maceration during a n hour can be omitted (as little as 2 0 minutes will suffice) a n d as cited above, where haste is more essential t h a n strict accuracy t h e filtrate can be immediately titrated for iron b y use of N I r o alkali, t h u s reducing t h e entire operation t o a m a t t e r of less t h a n a n hour. If titration with alkali is followed, i t is, of course, essential t h a t t h e natural acidity of t h e milk be taken into consideration. Recently we have endeavored t o apply t h e method t o d r y technical caseine, b u t so far without satisfactory results. The first work-maceration of t h e d r y caseine with a definite quantity of standard ferric solution a n d subsequent titration of t h e filtrate a n d washings with iV/'ro thiosulfate solution-gave no uniform results whatever; b u t when t h e caseine was dissolved in alkali before treatment with t h e standard ferric solution, fairly good results were obtained. I n t h e operation, approximately 0.6 g. caseine was carefully weighed, dissolved in 2 j cc. N,'Io DjaOH solution which had been previously standardized both against acid a n d against t h e standard ferric solution. Enough of this solution ( 8 . 3 cc.) t o combine with all of the alkali was added, followed by z j cc. of t h e standa r d ferric solution ( 3 3 . 3 cc. in all) after which t h e mixture was macerated for 3 0 minutes. It was t h e n filtered a n d t h e filtrate a n d washings were then titrated with N , IO thiosulfate solution with t h e following results: FERRIC ALCM-THIOSULFATE ESTIMATION O F D R Y C A S E l K E
One cc. standard ferric solution = 3 cc. N j r o S a O H solution = . .I cc. N / I O thiosulfate solution. Sample IIUmbcr 1. . . ~
WL,
caseme
. . . . . . . 0.6014
2 . . . . . . . . . . . . . 0.6002 3 . . . . . . . . . . . . . . 0.6130 4. 5.
Cc. thio used 12.7
. . . . . . . . . . . . . 0,6100 . . . . . . . . . . . . . 0.5991
14.2 11.2 12.1
11.3
Equivalent of Cc. ferric 1 cc. ferric solution sol. expressed consumed in e . caseine 12.3 10.8
13.8 12.9 13. I
0.0488 0.0555 0.0444
0.0472 0.0437
Average (all estimations were from batch of d r y caseine) 0.0479. These figures do not agree very well with t h e factor deduced above--I cc. standard ferric solution = o . o j 9 g. caseine-but t h e y are near enough t o suggest t h a t further work m a y bring results of practical value. COLLEGE O F P I i A R M A C Y ,
