Determination of Malic Acid in Plant Tissue Simultaneous Determination of Citric and Malic Acids GEORGEW. PUCHER,HUBERTBRADFORD VICKERY, AND ALFRED J. WAKEMAN Connecticut Agricultural Experiment Station, New Haven, Conn. extraction with petroleum ether. acid is one of the Malic acid, when oxidized by potassium perThis solvent does not extract the most widely d i s t r i b manganate in the presence of potassium bromide, oxidation product of malic acid. W uI t eC d organic acids is converted into a bromine compound that is uolaProducts that yield precipifound in plants, and frequently tile with steam. This substance combines with tates with dinitrophenylhydraoccurs in relatively high concendinitrophenylhydrazine in acid solution to yield zine are formed when carbohytration in them. A method for drates or c e r t a i n amino acids its a c c u r a t e determination is a product insoluble in water which can be filtered are subjected to oxidation with therefore a matter of consider08 and dissolved in pyridine. The pyridine potassium permanganate. Rut able importance in plant physisolution, when diluted with water and made no such compound is formed ology; unfortunately, however, alkaline with sodium hydroxide, promptly defrom either malic or oitric acid, none of the methods that have velops a blue color suitable for spectrophotounless potassium bromide is also been proposed hitherto are suitpresent d u r i n g the oxidation; able for the estimation of small metric measurements, strictly proportional to the consequently it is p o s s i b l e to quantities. quantity of malic acid taken over the range 0.1 estimate malic acid in the presMethods that d e p e n d upon to 2.5 mg., and stable f o r several hours. The ence of t h e s e interfering subprecipitation of the acids as lead same product is formed f r o m both optically active stances if two parallel oxidations or barium salts (4, 5, 9 ) , with and inactive malic acid. are conducted, one with and one subsequent preparation and fracwithout bromide, and the resulttional distillation of the esters, A method, based upon this reaction, has been ing solutions are extracted with can be successfully applied only developed to determine the malic acid content petroleum ether to remove pento large quantities of material. of dried leaf tissue, or of extracts from leaves, tabromoacetone. The difference The m e t h o d most commonly and has been applied to tobacco leaf. Prelimiin the yields of dinitrophenylused in recent y e a r s is b a s e d nary experiments have shown that it can readily hydrazine derivative then repreupon the high optical rotation sents with considerable accuracy of malic acid in the presence of be adapted to the investigation of the malic acid the q u a n t i t y of m a l i c a c i d uranium ( 2 ) or of molybdenum content of muscle, blood, and urine, and also present, inasmuch as the bro(1) salts. This method is speto the determination of aspartic acid after this mide has no effect upon the oxicific only in the absence of other has been deaminized with nitrous acid. dation of the carbohydrates. It optically active acids; furtheris generally more convenient, more. the uossibilitv that dlmalic acid may som&imes be present in plant tissues (8) in- however, to eliminate the amino acids and carbohydrates as a troduces an element of uncertainty into the result as a meas- preliminary step by extracting the organic acids with ether from the material under investigation. urement of the total quantity of malic acid present. Although considerable study has been given to the nature The method described in the present paper depends on the observation that malic acid, when treated with potassium of the oxidation product of malic acid, a detailed explanation permanganate in the presence of potassium bromide under of the chemical reactions that occur has not yet been obthe proper conditions, is converted into a bromine compound tained. It seems better therefore a t present to describe the that is volatile with steam. This substance yields an ex- reaction from an entirely empirical point of view. tremely insoluble condensation product when heated with REAGENTS dinitrophenylhydrazine in acid solution. The ratio between the amount of this product and of the malic acid submitted Sulfuric acid, 50 per cent: a mixture of equal volumes of conto oxidation is constant, and the quantity can be easily esti- centrated acid and water. Potassium bromide, 1 M: 11.9 grams diluted to 100 cc. mated by dissolving the product in pyridine, diluting with Potassium permanganate, 1.5 N: 47.4 grams diluted t o 1000 water, and making the solution alkaline with sodium hydrox- cc. ide. The intense blue color formed is in every way suitable Bromine water: saturated aqueous solution. Sulfuric acid, 2 N: 28 cc. of concentrated acid diluted t o 500 for measurement in a Pulfricb spectrophotometer. The specificity of this reaction with respect to the common cc.Hydrogen peroxide, 3 per cent: ordinary commercial product. organic acids is remarkable. A quantity of 0.2 mg. of malic Petroleum ether: boiling point, 35" t o 50' C. acid gave a distinct orange precipitate after heating the oxidaSodium sulfide, 4 per cent: 4 grams of crystalline sodium sultion product with an acid solution of djnitrophenylhydrazine, fide diluted t o 100 cc., prepared fresh every 2 to 3 days. Hydrogen peroxide (halogen-free): 4 grams of sodium eroxbut 25 mg. each of acetic, lactic, fumaric, succinic, pyruvic, ide dissolved in 50 cc. of water, cooled, and faintly acidited t o tartaric, maleic, glycollic, and glyoxylic acids failed to give Congo red with 50 per cent sulfuric acid (7 to 8 cc. required); any precipitation when oxidized and treated in the same prepared fresh every week. Silver nitrate, 0.02604 N: 4.4231 grams of pure silver nitrate way. Citric acid is converted into pentabromoacetone by to 1000 cc. 1 cc. = 1.0 mg. of citric acid. the oxidation procedure employed, and this substance yields diluted Ammonium thiocyanate, 0.02604 N ; 1.98 grams diluted to a precipitate with dinitrophenylhydrazine. No difficulty 1000 cc. and standardized against the silver nitrate. is thereby presented, however, since pentabromoacetone, if Ferric ammonium sulfate (ferric alum) : 3b grams dissolved present, is readily removed from the oxidation mixture by by warming with 100 cc. of water. 288
July 15, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
Dinitrophenylhydrazine: 5 grams of Eastman's dinitrophenylhydrazine are ground in a mortar with several successive portions of a mixture of 200 cc. of concentrated hydrochloric acid with 800 cc. of water. The whole is then transferred to a beaker and boiled for 1 to 2 minutes with vigorous stirring; after being cooled, the solution is diluted to 1000 cc. with water and filtered through hard paper. The reagent is filtered again just before using. Sodium sulfite, 20 per cent: 20 grams of crystalline sodium sulfite diluted to 100 cc., prepared fresh every 2 to 3 days. Pyridine: Eastman's pyridine (pract.) is redistilled. Sodium hydroxide, 5.0 N : 200 grams of c . P. sodium hydroxide diluted to 1000 cc.
PREPARATION OF ORGANIC ACIDFRACTION Two grams of the dried and powdered tissue are acidified to pH 1 with sulfuric acid, mixed with 3.5 grams of asbestos, and extracted with ether according to the technic of Pucher, Vickery, and Wakeman ( 7 ) . The organic acids are transferred to aqueous alkali and the solution is made to 100 cc. If extracts of tissue are employed, a portion equivalent to 2 grams of the tissue is evaporated to a sirup (not to dryness), acidified to pH 1 with sulfuric acid, and mixed with the asbestos; the mixture is then extracted with ether in the same way. OXIDATIONOF MALICAND CITRICACID An aliquot part of the organic acid fraction that contains 0.2 to 2.0 mg. of malic acid (5 cc. are usually sufficient) is diluted to 20 CC. with water, and 3 cc. of 50 per cent sulfuric acid are added. The mixture is boiled gently for 8 to 10 minutes to expel traces of ether, cooled, and 1 cc. of bromine water is added. After 5 minutes the solution is filtered with gentle suction through asbestos in a Gooch crucible into a beaker marked at 35 cc.; interfering impurities are thereby removed. The precipitate is washed with small quantities of water until the filtrate reaches 35 cc., when 2 cc. of 1 N potassium bromide are added. The temperature of the solution is then adjusted to 20" to 22" C. and 5 cc. of potassium permanganate previously brought to the same temperature are added. The mixture is allowed to stand in a water bath a t 20" to 22" C. for 10 minutes with occasional stirring, and is then chilled to 5" to 10" C. and decolorized with 3 per cent hydrogen peroxide added dropwise with vigorous stirring.
REMOVAL OF PEXTABROMOACETONE The oxidation mixture contains the pentabromoacetone derived from any citric acid present; this is removed by extraction with petroleum ether according to the technic described by Pucher, Vickery, and Leavenworth (6). The petroleum ether is washed once with 3 cc. of water and the citric acid equivalent to the pentabromoacetone may be determined as described below.
DETERMINATION OF MALICACID The aqueous solution together with the 3 cc, of wash fluid from the petroleum ether is diluted to 100 cc. and a 25-cc. aliquot is transferred to a 300-cc. Kjeldahl flask. About 25 cc. of water and a few angular quartz pebbles are added. To insure the destruction of hydrogen peroxide about 0.5 cc. of potassium permanganate is then added and the solution is decolorized by the addition of 2 cc. of sodium sulfite solution. The flask is fitted with the distillation tube described by Folin and Wright ( 3 ) . A 250-cc. wide-mouth Erlenmeyer flask charged with 10 cc. of freshly filtered dinitrophenylhydrazine solution and 20 cc. of water is used as a receiver; the end of the distillation tube is dipped beneath the surface of the reagent in the receiver and the flame of a microburner is applied to the flask. Distillation is continued at a rapid rate without cooling the receiver until the volume of solution remaining in the Kjeldahl flask has been reduced to somewhat less than 10 cc. A mark previously placed on the flask with a wax pencil at the 10-cc. point is of assistance. The distillation requires from 12 to 15 minutes. The distillation tube is then rinsed off and the receiver is cooled to room temperature. The orange precipitate is transferred as completely as possible with water to a small (No. 2) Gooch crucible furnished with an asbestos mat, and is then dried for a short time at 100" to 110" C. Meanwhile the receiver is thoroughly drained, and the last traces of precipitate are taken up in hot pyridine used in several successive portions but not more than 3 to 4 cc. in all. The pyridine washings are transferred to a 25-cc. volumetric flask. The crucible is then fitted into a rubber stopper carried on a cylindrical funnel which is attached to a test tube equipped with a side arm
289
and marked at 20 cc. Boiling pyridine is added in small portions to the crucible, and the contents are gently triturated with a glass rod, after which suction is applied and the pyridine is drawn through each time; three or four washings are usually sufficient to dissolve and transfer all the precipitate to the test tube. The pyridine solution is then added quantitatively to the 25-cc. flask used for the rinsings of the receiver. After being cooled to room temperature the solution is made to volume with pyridine and mixed; it is then allowed to settle or is filtered, if necessary, through soft paper to remove shreds of asbestos. Either a 2-cc. or a 5-cc. aliquot part of the pyridine solution is transferred to a 100-cc. flask and 50 cc. of water are added, followed by 8 cc. of 5 N sodium hydroxide. The solution is then diluted to the mark and the blue color is read in a Pulfrich spectrophotometer using color filter s-57 and a cell length that gives readings between 30 and 80 per cent. In general a 1-cm. or a 3-cm. cell is suitable. The extinction coefficient is calculated and the malic acid equivalent is read from a calibration curve constructed as described in the next section. The method is satisfactory for the estimation of from 0.1 to 2.5 mg. of malic acid in the 25 cc. of pyridine. If more than 2.5 mg. are found the distillation should be repeated on a smaller aliquot part of the oxidation mixture, but if less than 25 cc. are employed it is necessary to add 1 or 2 cc. of 50 per cent sulfuric acid before distilling, and in any case the aqueous volume in the Kjeldahl flask must be from 40 to 50 cc. CALIBRATION CURVE Pure commercial dl-malic acid is recrystallized twice from water and dried to constant weight in a vacuum desiccator; exactly 200 mg. of the product are dissolved and diluted to 100 cc. with 1 N sulfuric acid. This solvent preserves the malic acid indefinitely. A 5-cc. aliquot (10 mg.) is oxidized under the conditions described; the oxidation mixture is made to 100 cc. omitting the extraction with petroleum ether, and aliquot parts are distilled into the dinitrophenylhydrazine reagent. Readings of the blue color are obtained using both 2- and 5-cc. aliquots of the pyridine solution, and the extinction coefficients are plotted against the number of milligrams of malic acid in the aliquot of pyridine taken. Several determinations should be made of each point and these should agree within 5 per cent of each other. The average values should give a straight line. CONSTRUCTION O F
TABLEI. CALIBRATION DATAFOR DETERMINATION OF MALIC ACID WITH PULFRICH SPECTROPHOTOMETER (Light filter S-571 EXTINCTION COEFFICIENT MALIC.kCID K Mo. 0.016 0.030 0.020 0.039 0.024 0.047 0.032 0.060 0.040 0.078 0.116 0.060 0.156 0.080 0.200 0.100 0.315 0.160 0.395 0.200 0.473 0.240 0.320 0.628
Table I gives the data of the calibration curve obtained in this laboratory. It represents the average of a large number of closely agreeing determinations obtained in several different ways: (1) From 0.1 to 5.0 mg. of malic acid were separately oxidized and the extinction coefficient was obtained with and without steam distillation of the product of oxidation. When distillation was omitted the dinitrophenylhydrazine was added and the mixture was heated for 10 minutes in a boiling water bath. (2) Ten milligrams of malic acid were oxidized and the resulting solution was diluted to 100 cc.; of this 0.5- to 25-cc. aliquots were analyzed both with and without distillation. (3) Similar experiments were conducted in which from 0.3 to 6.4 mg. of citric acid were added before oxidation, and the pentabromoacetone was subsequently removed by extraction with petroleum ether. I n all cases five and usually ten readings of the spectrophotometer were taken and averaged, and readings were usually obtained with
ANALYTICAL EDITION
290
two cells of different lengths. I n these observations the extinction coefficient observed for a given quantity of malic acid never varied more than *8 per cent, and duplicate determinations usually agreed within *3 per cent. Consistent results were obtained whether the oxidation products were distilled or not, and the presence of citric acid during the oxidation had no effect. The data in Table I give the extinction coefficient, K , for the number of milligrams of malic acid represented by the 1Oo-cc. volume of blue solution upon which readings are taken. The calculation of the result of an analysis is made as follows: Grams of malic acid in 2 grams of tissue = mg. of malic acid in PY
100 x 25 - x 100 Py 02 x E x
1
where Py = aliquot of the 25 cc. of yridine solution Ox = aliquot of the 100 cc. oroxidation mixture Fr aliquot of the 100 cc. of organic acid fraction To convert into percentage the result is multiplied by 50.
If preferred, calibration curves may be so drawn as to give the malic acid content of the entire 25 cc. of pyridine when a 2-cc. aliquot and a 5-cc. aliquot are taken. The data for these curves may be derived by multiplying the data in column 2 of Table I by 12.5 and 5, respectively. DISCUSSION The oxidation of malic acid with potassium permanganate must be conducted with careful attention to the temperature. I n a series of experiments carried out a t various temperatures with known amounts of malic acid, the yield of product, as measured by the blue color of the alkaline solution of its dinitrophenylhydrazine derivative, was 66 per cent a t 9" C., 100 per cent at 19" and a t 26" C., 70 per cent a t 32" C., and 32 per cent a t 40" C. Experience showed that an oxidation temperature within the range 20" to 22" C. gave consistent results and this condition was therefore adopted. The yield of product is also influenced by the amount of potassium bromide added, being considerably depressed if only 1 cc. of N bromide is added instead of 2 cc. A further moderate excess of bromide, however, did not influence the results. Variations in the amounts of sulfuric acid and of permanganate were without material influence, provided excess of both were present, but the conditions recommended with respect to these reagents have been found advantageous and should be observed. The removal of the excess of permanganate after the oxidation was found to present some difficulty. Ferrous sulfate was not satisfactory because oxidation mixtures prepared with this reagent gave a large blank when heated with dinitrophenylhydrazine. Sulfites were satisfactory if malic acid alone was to be determined, but if citric acid was present the pentabromoacetone was partly converted into substances that could not be removed with petroleum ether and that subsequently increased the apparent yield of malic acid. Hydrogen peroxide was satisfactory if carefully used. Undue excess must be avoided since, if present under the conditions adopted, a dinitrophenylhydrazine compound soluble in pyridine is produced which increases the amount of blue color observed. The addition of a small amount of permanganate to the solution before distillation provides for the removal of any ordinary excess of peroxide, and the subsequent addition of sulfite removes the excess of permanganate together with any bromine that may have been liberated. The distillation of the oxidation product with steam effects an important purification of the substance and eliminates the necessity for blank determinations, a t least in connection with the determination of malic acid in the leaf tissue the
Vol. 6, No. 4
authors have chiefly employed. Before the possibility of this step had been appreciated, it had been necessary to conduct parallel oxidations on equal aliquots of the oxidation mixture, one with and one without potassium bromide. The blank determination usually amounted to the equivalent of from 0.2 to 0.4 per cent of malic acid in the tobacco leaf tissue, but was undoubtedly due to the presence of some other substance. Blanks of a similar order of magnitude were secured when tissue was analyzed from which all the malic acid had been removed by thorough extraction with hot water. This difficulty was completely avoided by the steam distillation of the oxidation product. Another blank, although of very small magnitude, was eliminated by the aliquoting of the oxidation mixture. The reagents alone give a small but constant blank equivalent to from 0.025 to 0.04 mg. of malic acid in the entire pyridine solution when the whole of the oxidation mixture is distilled; a similar blank is obtained if organic acids are present but potassium bromide is omitted. The amount of color is unreadable, however, if an aliquot of 50 cc. or less of the oxidation mixture is employed, and does not influence the accuracy of the determination of even as little as 0.1 mg. of malic acid. The blank determination in the oxidation mixture does assume importance, however, when tissues very low in malic acid are studied. Such preliminary experiments as the authors have carried out with blood, urine, or muscle extracts have shown that a control oxidation in the absence of bromide is necessary in these cases. The selection of the proper aliquots to use must be founded on experience. The method is best adapted to the determination of quantities of malic acid not exceeding 2.5 mg. Larger amounts yield a precipitate with dinitrophenylhydrazine that is difficult to manage by the technic described. The oxidation product of malic acid is stable for many days in sufficiently acid aqueous solution a t room temperature, b u t is unstable in alkali. The pyridine solution of the dinitrophenylhydrazine derivative can be kept for several days if necessary without change. The blue color produced by the addition of alkali is fully developed a t once and does not change appreciably for a t least 2 hours. SIMULTANEOUS DETERMINATION OF CITRICACID The citric acid equivalent of the pentabromoacetone removed by extraction with petroleum ether is best determined by the method recently described by Pucher, Vickery, and Leavenworth (6). Certain refinements of this method have been developed to deal with the small quantities of citric acid encountered when the chief object is the estimation of the malic acid. The petroleum ether extract, obtained as already described, is. washed three additional times with 3 cc. of water to remove inorganic halides, the washings being discarded. A 3-00. quantity of 4 per cent sodium sulfide solution is added to the funnel and, after vigorous shaking, the aqueous layer is drawn off into a 50cc. Erlenmeyer flask. The petroleum ether is then treated a second time with 3 cc. of sodium sulfide, and is washed three times with 3 to 4 cc. of water, the aqueous solutions being all received in the same flask. To this 2 cc. of 2 N sulfuric acid are added, a few angular quartz pebbles are dropped in, and the solution is boiled gently for 3 minutes to expel hydrogen sulfide. It is then cooled to room temperature, and sufficient 1.5 N permanganate is added to produce a red color (0.8 to 1.0 cc. usually) permanent for 15 to 20 seconds. The color is discharged by the addition of a small excess of halogen-free hydrogen peroxide, and 2 cc. of concentrated nitric acid, 3 cc. of standard silver nitrate, 1 cc. of ferric alum indicator solution, and 3 to 4 cc. of ether are added. The flask is shaken vigorously t o coagulate the silver bromide, and the solution is titrated with thiocyanate from a microburet t o a faint salmon-pink color. Multiplication of the number of cubic centimeters of silver nitrate used by the factor 1.12 gives the number of milligrams of citric acid in the aliquot of the organic acid fraction taken. If the solution should chance
291
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 15, 1934
to contain in excess of 3 mg. of citric acid the first drop of thiocyanate will produce a red color. In these circumstances additional silver nitrate is added and the titration is completed in the usual way.
RECOVERY OF MALIC ACID Table I1 gives the data of analyses of a solution of pure malic acid. The values of the extinction coefficient as observed in cells of two lengths are shown, and the malic acid equivalent to the average of these is given as closely as it could be read from the calibration curve.
malic acid by the polarimetric method of Dunbar and Bacon (8), as modified by Vickery and Pucher (9). The results in Table V show a satisfactory agreement between the two methods. The oxidation method in all but one case gave slightly higher results. The data illustrate the wide differences in malic acid content that may be encountered in this tissue, and provide evidence that little, if any, optically inactive malic acid occurs in the tobacco plant. TABLE V. COMPARISON OF OXIDATION AND POLARIMETRIC METHODS FOR MALICACID OXIDA-
OF MALIC ACID TABLE11. RECOVERY
TION
SAMPLE
(Light filter 5-57) MALICACID TAKEN
K
1-om. cell
K
3 - c m cell
Mg.
MALICACID FOUND RECOVERY
Me.
%
0.20
0.032 0.029
0.031 0.031
0.20 0.20
100 100
0.50
0.079 0.082 0.074
0.077 0.081 0.075
0.50 0.51 0.48
100 102
1.00
0.153 0.168 0.158
0.152 0.158 0.155
0.97 1.03 1.00
97 103 100
I n Table I11 are data on further analyses; the first five determinations were made on solutions that were subjected to the complete procedure-that is, the malic acid was extracted with ether in the customary way and every successive step was carried out. The next group of determinations was on aliquots of a standard solution to which 25 mg. of a mixture of oxalic, lactic, pyruvic, succinic, tartaric, maleic, glycollic, and fumaric acids in approximately equal amounts had been added. The last determinations were conducted in the presence of 6.4 mg. of citric acid. O F MALIC ACID TABLE 111. RECOVERY
MALICACID TAKEN
MQ. 2.00 4.00 100.0 100.0 100.0
MIXTUREOF OTHER CITRIC ACIDB ACID TAKEN TAKEN
Mg.
.. .. ... . ..
0.0 0.0 1.0 1.0 0.1 0.1
25 25 25 25 25 25
2.0 2.0
.. ..
MALICACID FOUND
%
... ... ...
2.10 4.00 104.0 101.0 106.0
105 100 104 101 106
100 10s
...
0.00 0.03 1.00 1.08 0.09 0.11
6.4 6.4
1.96 2.04
98 102
...
...
... ...
...
%
%
0.00 0.10 1.02 4.76 0.88 3.04 4.08 8.14 6.48 9.64
0.00 0.00
0.95
4.00 0.86 2.88 3.93 8.34 6.20 9.63
.
The oxidation method to determine malic acid has been chiefly employed in this laboratory in the study of tobacco leaf and stem tissue. Preliminary experiments with other plant tissues have given equally satisfactory results, however, and there seems no reason to suppose that the method cannot be generally applied. Successful analyses have also been conducted on blood, urine, and muscle extracts, and experiments with aspartic acid have shown that this amino acid can be accurately determined, if deaminized with nitrous acid previous to the oxidation.
ACKNOWLEDGMENT
R~COVERY
Mg.
...
METRIC
The expenses of this investigation were shared by the Connecticut Agricultural Experiment Station and the Carnegie Institution of Washington.
MO.
...
Fresh leaf I after extraction with water Fresh leaf F.L. after extraction with water Cured leaf 684 Cured leaf 594 Cured leaf 582 Partially cured leaf I Partially cured leaf H Partially cured leaf A Partially cured leaf E Fresh leaf F.L.
96
POLARI-
METHOD METHOD
... ,..
90
110
Table IV illustrates the recovery of malic acid added to tobacco leaf tissue. The malic acid content of each sample of the tissue was first ascertained by the present method; additions of malic acid were then made, and the complete analysis was conducted. The results are expressed in milligrams per 2 grams of tissue and show excellent recoveries and close reproducibility.
LITERATURE CITED (1) Auerbach, F., and Kriiger, D., Z. Nahr. Genussm., 46, 97 (1923).
(2) Dunbar, P. B., and Bacon, R. F., J. IND.ENG. CHEM.,3, 826 (1911). (3) Folin, O., and Wright, L. E., J. Biol. Chem., 38, 461 (1919). (4) Franaen, H., and Helwert, F., Z. physiol. Chem., 122, 46 (1922). (5) Franzen, H., and Keyssner, E., Biochem. Z., 135, 183 (1923). (6) Pucher, G. W., Vickery, H. B., and Leavenworth, C. S., IND. ENG.CHEM., Anal. Ed., 6, 190 (1934). (7) Pucher, G. W., Vickery, H. B., and Wakeman, A. J., Ibid., 6, 140 (1934). (8) Ruhland, W., and Wetael, K., Z . wiss. Biol., Abt. E. Planta, 7, 503 (1929). (9) Vickery, H. B., and Pucher, G. W., Conn. Agr. Expt. Sta., Bull. 352 (1933). RECBIVED dpril 12, 1934. The fourth paper of a series under the general title “Determination of the Aoida of Plant Tissue.”
TABLEIV. RECOVERY OF MALICACID ADDEDTO DRYTOBACCO LEAFTISSUE (Figures are milligrams per 2-gram sample) ORIQINALLY ADDED PRERENT FOUND RECOVEREDRECOVERY
Ma.
1MO.
Mg.
Mg.
%
80.0 80.0 100.0 100.0 100.0 100.0 100.0 2.0 4.0
20.8 15.0 26.4 26.4 26.4 26.4 26.4 26.4 26.4
103.5 91.2 135.0 124.0 126.0 130.0 123.0 28.3 30.4
82.7 76.2 108.6 97.6 99.6 103.6 96.6 1.9 4.0
103 95 109 98 100 104 97 95 100
As a further test of the reliability of the present method, a number of samples of tobacco leaf tissue were analyzed for
Preparation of Sintered Pyrex Glass Filters
SIR: Attention of research workers was called in your Analytical Edition of March 15, 1934, to the preparation of fritted glass filters by an article on page 154 of that issue by Kirk and his coworkers at the University of California. Unfortunately, Kirk neglected to mention that the filters he described are covered by patents throughout the world and, in the United States, by Patent 1,620,815 issued to Herschkowitsch and Prausnitz and by them assigned to this company. Obviously your readers should be appraised of this fact for their protection and our own. JENAGLASSWORKS,SCHOTT & GEN. J ~ N AGERMANY , MAY28, 1934
