VOl. 2, No. 4
AiVALY TICAL EDITION
380
ture, Both methods apply to rubber a t the same stage of its life, and it should be interesting to compare results from the same sample. I n general it h a y seem that their method is of somewhat higher accuracy, while the pyrrole test requires less time and labor and is more informative aa t o the actual processes of oxidation.
Literature Cited (1) Bruni. I n d i a Rubber J., 63, 814 (1922). (2) Bruni and Pelizzoia, I b i d . , 63, 415 (1922).
:ii ~ n ~ ~ e s ; , ~ . c ~ , ~ f ~ ~ ~ X , ~ ~ ~ 2 ~ , l
(1925), (5) Rossem, and Dekker, Kautschuk, 5 , (6) Whitby, India Rubber J., 63, 742 (1922).
(1929).
Determination of Nitrogen and Acetyl Content of Cellulose Nitroacetate' Wyly M. Billing and John S. Tinsley HERCULES POWDBR COMPANY, EXPERIXENTAL STATION, KENVIL, N. J.
HE increasing wide-
A method has been devised for determining both the slowly rises even Over a period spread use of cellulose acetate and nitrate content of cellulose nitroacetate. of several days. The present esters has made it seem This method is based on the usual nitrometer procedure miters found that where nide&able to investigate fully for nitrogen. The sulfuric acid residue from the nitrate was present the sapo& the mixed esters of Some of trometer after adjustment with secondary sodium phosfication value varied with the phate to pH 3.7 is steam-distilled in a special flask. time and temperature of the the more commonly used types, particularly cellulose Titration of the distillate gives the acetyl content. treatment w i t h t h e a l k a l i The maximum variation on known samples was solution. nitrate. I n s t u d y i n g the 2 per cent, while the mean average error was *0.5 Attempts to d e t e r m i ne preparation and properties of per cent. the cellulose nitroacetates it the nitrogen by the nitrometer and the acetate by diswas found that a more exact analytical procedure was necessary. tillation of the nitrometer residue gave results that were always high in acetate and due undoubtedly to cellulose Previous Work decomposition. A closer investigation into this general prinAn analysis of cellulose nitroacetate was first published ciple, together with necessary modifications, led to a very by Berl and Smith (2) and later Odd0 ( 7 ) used this method, satisfactory procedure. pointing out its drawbacks. Atsuki (1) and Kishida (6), Theory of Method Adopted in their work on nitroacetates, passed over the analvsis with just a statement that they had kmployed the Lunge-nitromeWhile the treatment of cellulose esters with concentrated ter for nitrogen and the Ost (8) method for acetate. sulfuric acid may be held to be quite drastic, it must be The Berl and Smith procedure, as more fully detailed borne in mind that, should the cellulose be decomposed by it, by Oddo, consists in a nitrogen determination by a nitrome- carbon dioxide would be evolved. If carbon dioxide is ter and a steam distillation of a separate sample after liberated from nitrocotton in any appreciable quantities, hydrolysis with sulfuric acid. The distillate gives, by then the whole principle of the Lunge nitrometer is wrong. titration, the total acidity, from which the acetate is calcu- The writers made no attempt to prove or disprove this point. lated by difference. On the other hand, data are lacking on the effect of the I n attempting to test this procedure by using known nitrometer reaction on cellulose acetate and nitroacetate. mixtures of cellulose nitrate and cellulose acetate, total The experiments on cellulose acetate show that no gas acidity values were obtained which were always much too is evolved from it on shaking in the nitrometer. With nitrolow. The precaution of an all-glass apparatus, as suggested acetates the gases evolved were bubbled slowly through by Berl and Smith, did not eliminate the error. From the limewater without forming a precipitate. It is therefore work of Carpenter and Babor (3) on the vapor pressure of concluded that the acetates are no more subject to demixed acids it is not surprising that the nitric acid could not composition than the nitrates. Obviously, then, the residue from the nitrometer should be completely distilled, because of its very low vapor pressure be free of decomposition products, and these must develop over the weak sulfuric acid content of the distilling flask. To increase the sulfuric acid concentration in the distilling in the distillation. The effect of the acidity and the temperaflask would lead to decomposition of the cellulose residue, ture can be minimized by dilution of the residue with water, with the accompanying formation of volatile acids. steam distillation to maintain a dilute solution, and further Determinat,ion of the composition of the distillate by double by conversion of the sulfuric acid of the residue to sodium indicator methods was unsatisfactory, owing to the high sulfate by the use of sodium phosphate. dilution of the acids in the distilled portion. A satisfactory application of these factors haa been deSimilarly, a direct saponification of the original nitro- vised and put to use in the following method. icetate, or mixture of nitrate and acetate of known compoApparatus sition, by the Knoevenagel (4) procedure for acetates was The essential feature of the apparatus is a steam generator found unsuitable. Cellulose acetate is completely saponified in a very short time and the reaction proceeds no further. connected t o a special distilling flask of the type shown. With nitrocellulose a saponification value equivalent to its This flask is 1 liter capacity and is provided with a sealed-in nitrate content as determined by the nitrometer method thistle tube for admitting the sample and the steam supply. is reached only after 1 or 2 hours, and from that Doint it The vapor Dasses through a suitable sealed-on trap which effectively prevents entFainment. The condenser is fitted Received April 26, 1930. Presented before t h e Division of Cellulose to the t'rap by a ground-glass joint, thus eliminating mbber Chemistry a t the 80th Meeting of t h e American Chemical Society, C i n c h connections. nati, Ohio, September 8 t o 12, 1930.
T
October 15, 1930
I S D C S T R I A L AND Eh'GINEERI.%7G CHEXISTRY
A 2-liter flask serves as a suitable receiver and is provided with a soda-lime tube to exclude atmospheric carbon dioxide. Materials
Concentrated sulfuric acid. Anhydrous .Sodium Phosphate, used t o buffer the oht ti on for distillation, must be free from carbon dioxide. The anhydrous material is less likely to contain objectionable volatile acidity. The saturated solution should be freshly prepared, as the hydrated salt is less soluble and precipitates on standing. Bromophenol bEu8 indicator is used to adjust the hydrogenion concentration to the proper value of pH 3 6-3.8. 0.2 N or 0.1 N alkali. If more concentrated solutions are used, the volume used is very small, thus multiplying the error in reading the buret. Steam supply. Precautions should be taken to insure free from carbon dioxide. Distilled Eater containing a small amount of sodium hydroxide is recomnlended, The stopper should be of cork and any rubber connections of gum rubber tubing.
381
lected (time, 3 to 31/2hours). This is titrated using phenolphthalein and the per cent acetyl calculated according t o the follom-ing formula: N F of alkali X cc. used X 0 043 0.2
X 100
=
per cent acetyl
The in the distillation flask be kept at about 30g-350 cc, If the volume or pH of the solution varies, the rate of distillation of acetic acid changes, It is therefore important to adjust the solution carefully in the distilling flask to pH 3.6-3.8 and to distil at ]east the volume recommended. of the cellulose from sulfuric By changing the to phoqphoric acid, a much lower hydrogen-ion concentration can be maintained, preventing undue hpdrolysis. Applicability
I n order t o determine the accuracy and of the proposed method, analyses have been made on mixturps of known composition of cellulose acetate and cellulose nitrate. The acetate and nitrate were both carefully analyzed. CELLULOSE ACETATE Per cent acetyl Knoevenagel method 36.63 36 25 Proposed method 36.37 36.13 CELLULOSE NITRATE Per cent nitrogen Sitrometer method 11.93 11.94 171TROMETER
RESIDUEF R O M NITROCELLULOSE B Y PROPOSED hfE'IHOD Alkali required Equivalent as acetyl G, CC. /O 0.07 0.34 0.49 0.10 0.15 0.73
W t . of sample Gram 0.1 0.2 0.3
It is apparent that there is a small amount of volatile acidity from the pure cellulose nitrate, which varies with the size of sample, or the per cent of nitrogen. For this reason we suggest that where greater accuracy is desired a
- X per cent r\' , or 0.041 X per cent (Oi:i9 N,on a 0.2-gram sample be subtracted from the per cent ace t y1.
)
correction factor of
A p p a r a t u s for D e t e r m i n a t i o n of h-it r o g e n a n d Acetyl C o n t e n t of C e l l u l o s e h'itroacetate
Technic of the Method
A 1.OO-gram sample of cellulose nitroacetate is dissolvecl in 80 per cent sulfuric acid by the regular nitrometer procedure. The solution should be allowed to stand several hours, as hydrolysis is complete only after long standing. The nitrogen is determined by the Lunge nitrometer in the usual manner. Details of this procedure are found in most books on technical analyses ( 5 ) . The residue in the nitrometer decomposing bulb is forced out into a 500-cc. flask containing about 250 cc. of distilled water. The mercury in the nitrometer bulb is washed twice with 10-cc. portions of 50 per cent sulfuric acid. These are also added to the flask. The flask is cooled and filled to the mark with water, making allowance for mercury present. After the mercurous sulfate has settled, a 100-cc. aliquot is placed in a beaker, 1 to 2 cc. of phosphoric acid added, and the pH adjusted to 3.6-3.8 with the freshly prepared sodium phosphate. By using the phosphoric acid before neutralization there is little chance of rendering the solution alkaline and thus adsorbing carbon dioxide. The buffered solution is transferred to the distillation flask and steam-distilled until a volume of 1800 cc. is col-
T a b l e I-Results KKO&rK
of A n a l y s e s b y M e t h o d Described
AfIXTC'RE.5 OF CELLULOSE ACETATE
CELLC'LOSE &-ITRATE
.4ND
~
Cellolose Cellulo:e acenitate trate
R
Found
0
0 25 25 50 50 75
100
7;I J
50 50 25 25 0
Cor.
Calcd.
7; % * % 36.37 . . . 36.44 36.13 36.44 27.47 2 7 : 3 5 27.32 27.24 27.12 27.32 i 8 . 6 6 1 8 . 4 2 18 22 18.55 18.32 18.22 9 40 9 03 9 10 9.15 9.10 9 51 0.49 ... ...
%
100
KITROGEN
ACETYL
75 100
Found Calcd
Error
% -0.19 -0.84 +0.10 -0 72 j1.08 + 0 54 -0.77 +0.55 .
1 %
.
c/u
Error
%
'..
".
.'.
2.91 2.84 5.57 5.83 8.87 8.88 11.93
2.98 2.98 5.86 5.86 8.94 8.94
-2.35 -4.70 -4.94 -0.51 -11.78 -0.67
...
.
I
.
NITROACETATE ~~
Samnle
a
I
Acetvl found
1
Sitrogen found
This result on nitrometer residue after standing, diluted, for 3 days.
Table I shows the results on the various mixtures of acetate and nitrate. The final per cent error is corrected for the nitrogen present. Data are also included on some typical
ANALYTICAL EDI T I O S
382
cellulose nitroacetates which show that concordant results can be obtained by the method described. The analysis on nitroacetate sample 53 is another point of value-via., that there is no apparent change in the nitrometer residue after dilution even on standing for several days before distillation. From the data shown it can be seen that in the acetate analysis the greatest variation from the calculated acetyl content is 4-1.08 and -0.84 per cent. The mean average error is 1 0 . 5 per cent. The nitrogen determination by the nitrometer is not so accurate on mixtures as on standard
1-01. 2, s o . 4
materials. The per cent error seemingly iiicreases with tlic decreasing nitrogen content. This is to be expected, since the amount of nitrogen is proportionately srnitller. Literature Cited (1) Atsuki, J. F U C I L I Eng., ~) T o k y o I m p , U n i r , 15, 311 1 ~ 1 9 2 5 ~ . (2) Berl and Smith, B e r . , 40, 003 (1007). (3) Carpenter and Babor, Trans. . i m , Insl. Chem. E n g . , 16, 1 1 1 8,1924i. (4) Knoevenagel, Z. o n g e v . Chem., 27, 597 (1914). (5) Law& "Glycerol and the Glycols," p . 401, Chemical Catxlog, 1023. (6) P\-ishida. liwnststqrie, 4, 141 (1914). . 7 ) Oddo, G a z z . chim. itd., 49, 140 (1019:i. '81 Ost, Z . unge;.'. C h e m . , 19, 995 (1906) 29, 1467 t1912;. ~
Sampling of Apples for Arsenical Spray Residue Determinations' J. R. Keller AGRICCLTURAI, EXPERIWE\T SrArro\ P L II
\I \ \ ,
\VI=H
From a study of three different lots of apples that arsenical loads on these sa1nhad been washed with dilute hydrochloric acid, it was investigations relative ples were calculated to grains found that the average probable error of single samples . to the arsenical spray of arsenic as L%Szoper 3 pound consisting of six apples each was 8.20 while that of residues that are found on of fruit, as that is the method duplicate samples was 5.36 per cent of the total arsenic apples and pears a t harvest of considering tlie national on the fruit. time, much of this fruit is now and international toleranee Since the average probable error of analysis has subjected to chemical cleanlimits for arsenic in foodstuffs. been found to be 7.40 per cent, it is apparent that two ing processes for the removal Since the chief interest is in samples of six apples each constitute a sufficiently of a large part of the arsenic. materials that approach or accurate sampling. Fruit that may carry arseniexceed this i n t e r n a t i o n a 1 Since the washed apples in the present study-carried cal loads close to the regutolerance limit, an effort was an average arsenical load of 0.018 grain as As,Oa per latory allowance of 0.01 grain m a d e t o s e l e c t fruit that pound, the results given above may safely be applied of arsenic as AS203 per pound tended to exceed this tolerto apples carrying the tolerance limit of 0.01 grain or of fruit is subjected to analyance, on the theory that less of arsenic per pound. sis before it is offered for s a n i pI i n g information obIt is shown that, whereas the error of sampling is sale. This procedure necestained with fruit carrying a considerably reduced by using duplicate rather than sitates information concernheairy load of arsenic might single samples, the rate of reduction is much less for ing the nature of a sample safely be applied t o f r u i t triplicates, quadruplicates, etc. that may be expected to be c a r r y i n g less or not more representative. During the than the tolerance limit. inkstigations a t this station relative t o methods of cleaning Frequency of Distribution apples, this question of sampling has received considerable attention, the results of which are summarized herein. The ten saiiiples of lot 1 varied from 0.015t o 0.025 with Experimental Procedure an average of 0.019 grain of arsenic as As303 per pound. Early in the course of the spray-residue investigations it Forty-five pairs of samples are obtainable from these ten was tentatively decided that results should be based upon samples. The frequency of distribution of these forty-five the average obtained from two duplicate samples of six apples samples is shown as curve A of Figure 1. Since it is sonieeach. The extreme variation that may be expected in the times considered that forty-five individuals are not a large arsenical load of uncleaned fruits of the apple or pear is quite enough number from which to construct such a curve, a coniwell known and is well exemplified in a recent article by Barnes parison was made with the frequency curve of all possible pairs-namely, one hundred ninety duplicate analyses of ( 2 ) . It is to be expected also that the variations between individual fruits should be less after thorough cleaning than these ten samples starting with two analyses per sample, or before (S). Since the arsenical load on cleaned rather than twenty in all. This is shown as curve B in Figure 1 . As on uncleaned fruit is generally the more important item, spe- niay be expected, the base of curve B is wider and the apey cial attention was given to errors that may be encountered in higher than for curve A . As shown in Table I, the probable errors of these curves are not so different as might be expected the sampling of cleaned fruit. considering the large difference in the number of individuals During the course of cooperative experimental work with F. L. Overley of this station, a quantity of apples of uniform used. This may be due to the fact that each individual ic size (about 126 per box) as obtained from the Wenatchee an average of a pair of original or basic determinations. I n the fruit of lot 2, the average arsenic load in ten samples District were analyzed by the official Gutzeit method for arsenic (1). These apples were subjected to a commercial of six apples each was found to be 0.017, with the extremes a t cleaning process, using dilute hydrochloric acid (5). The 0.015 and 0.019 grain of arsenic as As203. The data of these samples plotted in the same way as for lot 1 gave the curves Puhlished e i t h the approval of the Direc1 Received April 30, 1930. of Figure 2. The base of the analysis curve, B , is broader tor of the Washington Agricultural Experiment Station as Scientific Paper than that of the sample curve, A . This is due to a considerNo. 165, College of Agriculture and Experiment Station, State College of able variation between some of the duplicate analyseq, some Washington.
A
S X result of extensive
