ANALYTICAL CHEMISTRY
620 minutes to dissolve the soluble salts. Add a few drops of phenolphthalein indicator (pH 8 t o 10) and then slowly add 0.5 N sodium hydroxide until the red color appears. Carefully addO.l N nitric acid until the red color of the supernatant liquid is just dischareed. Boil for a few minutes and filter. washing the residue wiFh hot water. The residue will consist of thginsoluble hydroxides of iron, aluminum, etc., and manganese dioxide, and the filtrate will consist of calcium and magnesium salts. Because the filtrate is neutral (pH X 8) and free from interfering radicals, the earlier portion of the original procedure given in the present paper should be omitted. The rest of the procedure, commencing with the addition of a known volume of standard calcium hydroxide solution in moderate excess to precipitate ma~neciiim. ..~.. ~~~. . ~is followed. ~ .~ ~... . ~ . , Hydrochloric arid is objectionable in the presence of the manga~~
~~
~
nese dioxide to be precipitated and sulfuric acid interferes with calcium hydroxide. Hence, nitric acid is recommended in this procedure. LITERATURE CITED
(1) Blum, W., J . Am. Chem. Soc., 38, 1282 (1916). (2) Kolthoff, I. M . , and Stenger, V. A,, “Volumetric Analysis,” Vol. 11, p. 178, New York, Interscience Publishers, 1947. (3) Scott, W. W., and Furman, N. H., “Standard Methods of Chemical Analysis,” Vol. I, 5th ed., pp. 532-4, New York, D. Vsn Nostrand Co., 1939. (4) Ibid.,Vol. 11, pp. 2062-3. (5) Williams, L. R., IND. EXG.CHEM.,ANAL. ED., 18, 542-4 (1946). RECEIVED February 19, 1948.
Determination of Pentosans in Cellulose Simplification of TAPPI Standard T 450 m 44 Method PIETER HENDRIK TEUNISSEN, N . V . Onderztoekingsinstituut Research ( A . K .U . and Afiliated Companies), Arnhem, Holland A deterniination of pentosan by an abbreviated form of TAPPI standard T 450 m 44 method is discussed. The method is shortened by not adding fresh hydrochloric acid during distillation. A change in the calculation formula is required. A correction factor for the presence of hydroxymethylfurfural is presented.
I
N T H E analysis of straw cellulose the author has used for a long time the method of Gierisch ( 1 )for the determination of
pentosans. According to this method a quantity of cellulose containing 150 to 200 mg. of pentosan is put with 210 ml. of 12% hydrochloric acid in a 500-ml. round-bottomed flask (Figure 1). The flask stands on a tripod with an asbestos ring in order to prevent superheating of the walls. In the cases of cellulose samples with a very low pentosan content, a large quantity of cellulose must be used for the experiment and consequently more 12% hydrochloric acid-e.g., 250 ml. for 10 grams of cellulose. The velocity of the distillation must be accurately adjusted a t 12 ml. per 5 minutes. In this way 180 ml. are distilled off in i 5 minutes. The volume is not kept constant by supplying fresh 12% hydrochloric acid as is the case with other methods. The furfural that distills over is determined by precipitation with barbituric acid. This has an advantage over the phloroglucinol method of Krober ( 2 ) because the barbituric acid does not precipitate hydroxymethylfurfural. Recently the bromide-bromate method of TAPPI ( T 450 m 44) has been used by the author. This TAPPI standard prescribes distillation while the volume is kept constant by adding fresh hydrochloric acid. The author found that the distillation can be shortened, when no fresh 12% hydrochloric is supplied during distillation. This requires a change in the calculation formula as described below. TAPPI STANDARD AND ABBREVIATED DISTILLATION METHODS
The two distillation methods were compared by applying them to Kahlbaum’s “xylose-reinst.” From this substance only furfural is formed and no hydroxymethylfurfural, so that a strict comparison of the determinations of furfural is possible (Table I). For the titration by means of bromate both the distillation according to the TAPPI standard and that according to the abbreviated method were applied. For the barbituric acid test only the abbreviated method was used, because when a more prolonged distillation is applied no additional furfural distills over. When for the bromate titration of the furfural VI ml. of thiosulfate are used (normality N ) and for the blank test 8, ml., this corresponds with N(V2 - Vi) X 0.048 gram of furfural. The
-
quantity of pentosan is then 1.58 X N(V2 VI) X 0.048 gram and the pentosan content when It’ grams of dry cellulose had been used is 1.58 X N(V2 - Vi) X 0.048 X 100
W
%=
7.57 X X(Vz - Vi)
W
%
In the TAPPI standard, the following formula is given for the pentosan content in cellulose: 7.5
x
lY(T-2
p
-
VI)
-
1.0%
-
The correction 1.0 is due to the presence of hydroxymethylfurfural which is formed from cellulose. This is not formed from
I: j
:
406
n Figure 1. Apparatus Used in Gierisch’s Distillation RIethod
V O L U M E 21, NO. 5, M A Y 1 9 4 9 Table I.
621
Comparison of TAPPI and Abbreviated Distillation Method on Xylose Furfural Determination Using KBrOa KBrOa KBrOa KBrOa KBrOa KBrOa Barbituric acid Barbituric acid
Distillation Rlethod TAPPI 300 ml. TAPPI 300 ml. Abbreristed 180 ml. 180 nil. 180 nil. 180 ml. 180 ml. 180 ml.
Calculated Content,
% 101.2 100,6
.
99.0 100.4 99.7 100.4 99.2 100.1
xylose, so in that case no correction is necessary. As a consequence of this the formula is equal to that used by the author except that 7.5 is mentioned instead of 7.57 in this author's formula. In this special case, where the monomer xylose has been used, the result has to be multiplied by
The calculation of the pentosan content from the quantity of furfural found takes place by means of an empirical factor. The theoretical factor-Le., the ratio of the molecular weights of xylans and furfural-l32/96 = 1.38, was found to be too low. According to Krober 1.56, according to Schmidt (S) 1.58 must be taken. This means that the furfural yield is 1.38/1.58 X 100 = 87.5% of the theoretical one. This is also mentioned in the TAPPI standard. The quantity of pentosans then becomes 1.58 X 0.466 ( P
+ 0.0037) = 0.735 ( P + 0.0037)
When W grams of dry cellulose are used, the pentosans-content becomes: 0.735 ( P 0.0037) X 100 70 T.i'
+
Table I1 shows that the bromate titration always gives too high a value, which points to the presence of hydroxymethylfurfural. On an average this value is 0.6%, so that we shall have to correct our formula by this amount. Thus, the pentosan content in cellulose becomes
3
Molecular weight of xylose = = 1.136 molecular weight of xylans 132 This factor is valid for both the barbituric acid and the bromate method. Table I shows that, when the bromate method is applied to xylose, the difference between the TAPPI distillation method (300 ml.), the abbreviated method (180 ml.), and the barbituric acid method remains within the experimental error. This corresponds with the remark made above, that for xylose no correction for hydroxymethylfurfural is required. After this the bromate and barbituric acid methods were applied on straw cellulose Here a correction for hydroxymethylfurfural can be expected. In these determinations the abbreviated distillation method (180 ml.) was applied in all cases. The results of a number of determinations have been collected in Table 11. For the calculation of the pentosan content the previously mentioned formula was used: 7.57
x S ( V *-
VI) %
Table 11.
Sample K 97 K 119 K 66 K 121 K 122 K 123 K 123 K 124 A K 124 A K 124 B K 126 A K 126 A a cake K 126 A K 126 a cake K 126 B
Furfural Determination with Potassium Bromate and Barbituric Acid
Quantity of Substance, G. 1.5 0.6 1.0 1.0 1 .o 1.0 0.6 0.6 1 .o 0.6 1.0 0.6 1.2 0.8 1.0
-
Pentosan Content, % Using barbituric acid 10.8 21.3 18.1 23.4 25.2
Using KBrOs 11.3 21.6 18.7 24.0 25.6 25.4 25.4 26.1 26.2 25.0 26.6 26.6 7.1 27.0 8.5
24.4 25.4 24.6 25.9 6.5 26.4 7.9
Difference 0.5 0.3 0.6 0.6 0.4 1 .o 0.7 0.8 0.4
0.7 0.6 0.6 0.6
W The pentosan contents calculated by means of this formula are given in the third column of Table 11. In the fourth column the pentosan content is shown n-hen the furfural was determined by means of barbituric acid. I n the furfural determination with barbituric acid, the distillate is transferred into a 500-ml. beaker, after which the measuring cylinder is rinsed twice, each time with 10 ml. of 12% hydrochloric acid. I n the meantime 0.75 gram of barbituric acid has been dissolved by heating in 100 ml. of 12% hydrochloric acid; this solution is added to the furfural solution. A yellow color is immediately formed, and after heating to about 60" C. the yellow furfuralbarbituric acid double compound is separated. During the first hour, when the liquid is cooling, i t is stirred frequently. The next day the liquid is filtered through a Jena glass filter G 3 and the precipitate is washed free of acid by means of a saturated solution originating from an acid-free precipitate of a former determination. After drying 4 hours a t 100" C. the precipitate is weighed. The calculation takes place as follows Furfural 96
+ barbituric acid +double compound + HzO
+
128
+
206
+ 18
When the weight of the double compound amounts to P grams, the quantity of furfural is 96/206 X P = 0.466 P. A small correction is, however, still required because of the solubility of the double compound, which is 1.22 mg. per 100 ml. of 12% hydrochloric acid. The precipitate has been formed in 300 ml. (180 ml. of distillate, 20 ml. with which the measuring cylinder has been rinsed, and 100 ml., in which the barbituric acid has been dissolved). The quantity of furfural becomes
F
= 0.466 = 0.466
(P
+ 3 X 0.00122)
(P + 0.0037)
Table I showed that for xylose the same pentosan content is found when the TAPPI distillation (300 ml.) or the abbreviated method (180 ml.) is applied-viz., in both cases 100%. Table I11 gives the pentosan content of some samples of straw cellulose, determined according to three methods-viz., TAPPI (calculated with the TAPPI formula), the shortened method (calculated with the author's formula), and the shortened method in which the pentosans are determined by means of barbituric acid. From the results obtained (Table 111) it follows that with the shortened distillation method the furfural has distilled over quantitatively just as when the TAPPI method is applied.
Table 111. Pentosan Content in Cellulose Samples
Author's formula, % Using barbituric acid, T A P P I formula, %
Yo
Straw Cellulose 97 10.7 10.8 10.8
Straw Cellulose 119 21.0 21.3 21.2
Straw Cellulose 66 18.1 18.1 18.4
Some check experiments were performed to prove the correctness of the abbreviated method and to demonstrate the essential difference between this method and the TAPPI method. In the first experiment, after 180 ml. had distilled off according to the shortened method, 210 ml. of 12% hydrochloric acid were added again and distilled (without supplying fresh 12% hydrochloric acid during distillation). This second distillate (180 ml.) was divided into two parts, one part was used for the furfural deter-
ANALYTICAL CHEMISTRY
622 _______.-._I___._
~..
The results were:
Table IV. Continued Distillation without Supplying Fresh 1 2 7 ~Hydrochloric Acid during Distillation Pentosans,-% . . .. ._ Second 180 Ml. BroniideBarbituric bromate acid 0.3 N o precipitate 0.3 so precipitate 0.2 0.1 0.3 N o precipitate 0.3 Xo precipitate
First 180 mi. 27.5 25.8 17.5 10.8 27.5 28.5
Sample 146 151 I54 158 174 175
-
;
~
_____.__
mination with bromide-bromate, the other for that with barbituric acid. I n all cases there was no flocculation a t all with barbituric acid, thus, no more furfural distills over after the first 180 ml. With bromide-bromate a slight amount of oxidizable materials was found (Table IV); this result was expected because of the continued decomposition of the cellulose when boiled for a second time. The same amounts of oxidizable materials, which do not precipitate with barbituric acid, are found when, after 300 ml. according to the TAPPI method have distilled over, the distillation is continued and the following 180 ml. are collected. Pentosans, T A P P I Method I n following I n 300 ml. 180 ml. 27.5 0.3 G&
Sample 174
Applying the TAPPI distillation method the 300 ml. of distillate were divided into the first 180 ml. and the following 120 ml. In both fractions the furfural was determined by means of bromide-bromate. For the calculation of pentosans the correction 180 term 1.0 was divided into 300 X 1.0 = 0.6 for the first 180 ml. 120 and X 1.0 = 0.4 for the second (120 ml.) fraction. 300
Sample 174
In 180 ml. I n 120 ml. In 300 ml.
Total
2 6 . 2 Yo Pentosan8 1.25% P e n t o s p 27.45% Pentosans
I t follows from these figures that, when distilling according to TAPPI, the furfural is not distilled over quantitatively with 180 ~ml., but~ that 300 ~ ml. $are necessary. ~ ~ ~ ~ But by applying the shortened method, 180 ml. are sufficient for quantitative distillation of the furfural. The essential difference between the TAPPI method and the shortened method is not the volume of distillate, hut the fact that in the abbreviated method the distilling proceeds without supplying fresh 12% hydrochloric acid, so that the hydrochloric acid concentration and the temperature increase during distillation. Because of this, less time or less distillate is required to distill the furfural quantitatively. Continued distillation gives in both cases a little amount of oxidizable materials but no additional furfural. Thus the conclusion is justified, that when distilling 180 ml. without supplying fresh 12% hydrochloric acid during distillation, the furfural distilled over quantitatively, ACKNOWLEDGMENTS
The author wishes to thank E. T. Schouten and R. F. M. Pilzecker for the practical execution of the various determinations, and Ward Pigman and Bert L. Browning of the Institute of Paper Chemistry, Appleton, Wis., for kind suggestions and valuable criticism in reviewing the article. LITERATURE CITED
Gierisch, W., Cellulosechemie, 6 , 61,81 (1925). (2) Krober, E., J. Lnndw., 48, 357 (1900). (1)
(3) Schmidt, E., Cellulosechemie, 11. 61 (1930). RECEIVED
Xovember 28, 1947.
Volumetric Determination of Nitrate Ion By Reduction with Chromous Ion JAMES J. LINGANE AND ROBERT L. PECSOK Hart-ard Unit-ersity, Cambridge 38, Mass. A method is described for the determination of nitrate ion based on reduction to ammonium ion by chromous ion in dilute sulfuric acid solution. With amounts of nitrate ion of the order of 20 to 50 mg. the method is precise and accurate to =tO.Z’& and the accuracy is about *270 with 2 to 5 mg. of nitrate ion. Large amounts of chloride do not interfere. Nitrite ion undergoes the same reduction.
0
F T H E various volumetric methods that have been proposed
for the determination of nitrate ion the best known is that of Devarda (S), based on reduction to ammonia in strongly alkaline solution by an aluminum-zinc-copper alloy. 3YOa-
+ 8AI + 5 0 H - + 2H20 = 3XH3 + 8.4102-
The ammonia is distilled and finally determined by acidimetric titration. As improved by Arndt ( I ) , who recommended a copper-magnesium alloy and reduction and distillation from saturated magnesium hydroxide SOlUtiOn, this is probably the most reliable of existing methods. Knecht and Hibbert ( 4 ) described a distillation method based on reduction of nitrate ion to ammonia by titanous hydroxide in strongly alkaline solution. Methods based on acidimetric titration of the ammonia produced by electrolytic reduction of nitrate ion a t a copper cathode
have been described by Vortmann ( I d ) , Bottger (S), and Szehelledyand Schall (fa). Szebelledy (9) developed a more or less empirical method involving direct titration in concentrated sulfuric acid with ferrous ion. The nitrate ion undergoes a 2-electron reduction to nitrosyl sulfuric acid, me++
so3-
f 3H+
+ H80r
= 2Fe++? f
HSOaNOz -t 2H20
and the end point is indicated by the red compound formed between the nitrosyl sulfuric acid and the first excess of ferrous ion The method of Kolthoff, sandell, and ~ ~ ~( 6 )utilizes k ~ the ~ fact that nitrate ion is reduced quantitatively to nitric oxide hv ferrous ion in hot hydrochloric acid medium. 3Fe++
+ NO3- + 4 H + = 3 F e + + ++ NO + 2H20
i
t
~
