V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2 particularly convenieiit, when analyzing samples coiitainiiig :ipyreciable carbon such as iron ores. In t'he present method, the carbon is collected on the filter paper along with the nickel dimet,hylglyoxime. This carbon is left' on the filter when the preripitate is dissolved with hydrochloric acid. The addition of an excess of dimethylglyosime reagent up to seven times the theoretical amount required had no unfavorable effect on the results. No harm is done if the reagent contaminates the precipitat'e. Therefore much less care need be exercised in the addition of the proper volume of reagent than in most other methods. In addit,ion,a much larger excess of the reagent than is ordinarily permissible has the effect of slightly reducing the solubility of the nickel precipitate. The standard Versenate solution is easilj- prepared and is stable. It, does not require rest,andardization a t frequent intervals, as does the standard solution used in the cyanide method.
1063 LITER4TLRE CITED
(1) Beta, J. D.,and Noll, C. -4,, J A m . W a t e r Works 9 s s o c . 42, 49 (1950). (2) Biedermann, W., and Schwaizenbach, G., Chirnia (Suck ), 2, 56 (1948). (3) Brunok, O.,2. angew. C h e m , 20, S34,1844 (1907\. (4) Diehl, H.,Goetz, C. A., and Hach, C. C., J. Am. Water Works Assoc., 42, 40 (1950). ( 5 ) Furman, N. H., and Flagg, 3. F., IND. ENG.CHEM.,A N ~ LED.. 12, 738 (1940). (6) Hartley, W. N., J . Chem. Soc., 87, 1791 (1905). (7) Pomerantz, I. I., Zavodskaya Lab.,4, 966 (1935). (8) Schwarsenbach, G., Biedermann, W., and Bangerter, I-., H e l i . Chzm. Acta, 29, 812 (1946). (9) Schwaizenbach, G., and Gysling, H., Ibid., 32, 1314 (1940). (10) Tougarinoff, 11. B.,4nn. m c . BCZ. Bruxelles, 54B,314 (1934). RIGEI\E D for rex iew Januarj 2 , 1 9 2 .
-4ccepted February 14, 10.52.
Modified Signer Molecular Weight Apparatus LAWRESCE RI. WHITE
AND
RICHdRD T. MORRIS
F'estern Regional Research Laboratory, .ilbany, Calif.
IIE Signer inethod for the determination of molecular weights r h a s been shon-n to be accurate and applicable to many types of organic compounds ( 1 , 2 ) . The method is based on the principle that, when tJvo isothermally insulated solutions are placed with their vapors in contact, distillation take? place until the solutions have the same vapor pressure. .kt the isopiestic point the solutions are equimolar. The apparatus used for the determination consists essentially of two bulbs connected by a vapor bridge. Ope bulb holds the reference solution and the other holds the unknown solution. Provisions are made for introducing the solutes and solvent,, for evacuating the apparatus before it is sealed, and for measuring the volumcs of the solutions. The chief deterrents to the use of this excellent method are that the apparatus must be returned to the glass blower after each second or third determination to have new filling tubes sealed on and t h a t the filling tubes must be sealed off with a torch after the apparatus is evacuated. The sealing of the filling tubes must take place in such a manner that no moisture is introduced into the apparatus and that neither of the solutes nor the solvent is decomposed in any degree by the heat of sealing. The sealing of t.he evacuated apparatus is a difficult manipulation for an inexperienced glass blower and is potentially ha.zardous when the solvent is flammable. This operation can be avoided through the use of the apparatus described herewith. r
APPARATUS AND PROCEDURE
The apparatus, shown drawn to scale in Figure 1, is similar to the one previously described ( I ) , except that the filling tubes that had to be sealed off for each determination have been replaced by a re-usable closure consisting of a spherical joint and a concentric-sleeve stopcock. As these joints cannot be lubricated, they are placed within a mercury well to effect a completely airtight seal. The details of construction and the function of each part are evident from the figure and the description of the filling of the apparatus. Although the dimensions are not critical, five details should be kept in mind n-hile construct,ing the equipment. 1. The angle of the two arms of the vapor bridge, Figure 1 , A , should permit, the introduction of micro weighing tubes and pipets directly into the bulbs. 2. The volume of mercury in the well, the upper part of A , should be kept a t a minimum to provide better stability and to decrease the weight of the apparatus. For this reason the joints a t the tops of the mercury well and inner sleeve were made from sawed-off joints of t.he sizes indicated in the figure.
3. The shape of the cover elioultl pcniiit the well to be conipletely filled without forming air pockets. If the filling tube is off center (toward t'he back in the vien- s h o m in Figure l), it facilitates the removal of the mercury with a capillary suction tube after the determinat'ion is complete. 4. illthough ideally calibrated tubes for making the mcasuriiig arms are not conimerrially available, satisfactory arms ran be made from 2 X 0.1 nil. serological DiDets x-ith the t i m sealed. -The graduation numbers should he changed to read from the tip u p instead of from the top down. 5 . The concentric-sleeve stopcock is made by drilling a 1-nim. hole perpendicular to the asis of an assembled 7/15 joint and sealing the small end of the outer 7/16 joint. The stopcock is turned nith a wrench made from a short lengt,h of aluminum alloy tubing, 0.5 inch in outside diameter with '/,s-inch wall, that fit,s the q u a r e shoulder. Tl-eighed samples of t'tie reference and unknown compounds, sufficient of each to make 0.05 to 0.1 molal solutions a t equilibrium, are placed in their respect.i\-e bulbs with micro weighing tubes. One to 1.5 ml. of solrent is added to each bulb. The inner sleeve is seated in the stopper n-ith tjhe drilled holes in alignment and the assembly is placed in the base. The handles are used :o hold t,he stopper in position. -%vacuumline (a good water aspirator is adequate) terminating in an outer 5 / 6 joint is inserted through the wrench and attached TO the 5 / e joint on the sleeve. Immediately after the vacuum connection is made, mercury is poured into the well until the Figure 1. Sketch of ioint is covered. Evacuation is Apparatus ;,ontimed until about 0.25 1111. A . Base B . Stopper ni solvent has distilled from each C. Inner sleeve nulb, then bhe sleeve is turned D . Cap about 90". ThevacuumconnecE . Steel spring F . Cover iion and the wrench are removed, 6. Filling tube (offcenter) h e cap is placed on the open H . Square shoulder end of the sleeve, and the spring J . Handle K . Serological pipet is placed over the cap. The i'orer, with the joint lubricated, Standard taper and spherical joint sizes are shown on is seated in the base, so that the right. Numbers in parentension of the spring will hold theses show. original joint The cap in position. Springs or sizes
ANALYTICAL CHEMISTRY
1064 rubber bands are put in place to hold the cover securely and mercury is added through the fillin tube until the well is filled. The filling tube is sealed with a rub%er policeman. After the apparatus is filled (an operation requiring about 5 minutes), it is completely immersed in a water bath or other container where the bulbs will be a t the same temperature. The volumes of the solutions are determined at appropriate intervals by temporarily rotating the apparatus so that the solutions drain into the measuring arms (1). When the volumes become constant, the molecular weight is calculated from the formula
where M , V , and W are, respectively, the molecular weight, volume of solution, and weight of the reference compound, and Mi, VI, and W1 are the corresponding values of the unknown. RESULTS
The data in Table I illustrate the accuracy obtainable with the apparatus. Errors of the order of magnitude shown in the table could be caused by an uncertainty of 0.01 ml. in reading one of the solution volumes. The apparatus has also been used successfully for the determination of the molecular weights of research compounds, with chloroform, tetrahydrofuran, and methylene chloride used as
Table I. Apparent Molecular Weight of Ambenzene Reference Compound Benzoic acid,
T ~ ~ Molecular ~ . , Weight C. Found Theory
Weight, Mg.
Solvent
NBS.!40
15.02
Ethylether
25
181.8
NBS 141
18 84
Acetone
39
183.0
182.2
4.702
Acetone
39
181.4
182.2
Acetanilide, Acetanilide, NBS 141
182.2
solvents. The rate of distillation of the solvent appears to be the same in the new and in the original apparatus. The apparatus described is easily and quickly filled by technicians having no glassblowing experience, and its use causes no sacrifice in accuracy of results or time required to reach equilibrium. The type of closure designed for this apparatus should find use in other applications where a nonlubricated, vacuum-tight closure is required. LITERATURE CITED
(1) Clark, E. P.. 1x0. ESG. CHEM.,ANAL.ED.,13, 820 (1941). (2) Signer, R., Ann., 478, 246 (1930). RECEIVED for revierv December 26, 1951. Accepted February 18, 1952
Photometric Determination of Aconitic Acid with Potassium Permanganate SHERMAN R. DICKMAN, University of Utah, Salt Lake City, U t a h
N T H E course of a study of the enzyme aconitase it was deIcitric sirable to measure the formation of cis-aconitic acid from either acid or isocitric acid as substrate. A satisfactory chemical method for this purpose would be sensitive to microgram quantities of cis-aconitic acid and insensitive to relatively large quantities of either citric or isocitric acid. The procedure developed fulfills these criteria, is rapid, and makes possible the estimation of aconitic acid with but little interference from many other types of compounds. The method is based on the reaction of the ethylenic group with potassium permanganate in dilute metaphosphoric acid solution. The decrease in the absorbancy of the permanganate is determined with a spectrophotometer, and the concentration of aconitic acid is calculated by means of a standard curve. Although the reaction of ethylenic substances with potassium permanganate has been known for many years, difficulties in controlling the extent of the reaction have made quantitative applications almost impossible. Lauer and Makar ( 4 ) ,for example, were unable to utilize the partial oxidation of aconitic acid by acidified permanganate solutions because of the separation of manganese dioxide a t low temperatures and the fading of the end point a t temperatures up to 80". Stamm ( 6 ) devised an alkaline permanganate reagent containing barium ions for the complete oxidation of organic compounds. The very insoluble barium manganate is formed as a reduction product and thus the formation of manganese dioxide is prevented. Excess potassium permanganate is determined volumetrically. The photometric measurement of the absorbancy of permanganate solutions serves as a very sensitive index of permanganate concentration. Goldblith and Proctor (9)added potassium permanganate to solutions of hydrogen peroxide acidified with sulfuric acid, in the determination of catalase activity. The concentration of excess permanganate was determined photometrically. The absorbancy of the solution did not remain constant for over 1 minute, however, because of the rapid formation of manganese dioxide. In the present work it was found that 6.6 X lop5M solutions of cis-aconitic acid which contained sulfuric acid or trichloroacetic acid (TCA) were readily oxidized by 2.67 X 10-3 N potassium
permanganate. The reaction mixture retained the permanganate color, but a brownish discoloration, probably due to the formation of manganese dioxide, became noticeable almost immediately. M ) were not oxidized by this Citric acid solutions (6.6 X concentration of permanganate under these conditions, but on the addition of catalytic amounts of manganese ions to the reaction mixture, an induced oxidation of the citric acid was observed. If a mixture of aconitic acid and citric acid in the above N potassium listed concentrations was treated with 2.67 X permanganate, the solution was rapidly and completely decolorized even a t 0". These observations indicate that the lower valency states of manganese ions act as electron carriers in the induced oxidation of citric acid, whether they are added directly or formed in the course of the aconitic acid reduction of potassium permanganate. Similarly, Mew, Stafford, and Waters (6) have found an induced oxidation of alcohols by potassium permanganate in acid solutions containing ferrous sulfate. In order to prevent these induced oxidations of hydroxy compounds in the analysis of aconitic acid, the inorganic acid in the reaction mixture was changed to metaphosphoric acid, an excellent metal-complexing agent in acid solutions. Two effects of this substitution were noted: (1) An aconitic acid-potassium permanganate-metaphosphoric acid mixture remained a clear pink-Le., manganese dioxide formation was prevented (Table I, lines 3 and 5),and (2) the induced oxidation of citric acid m-as completely inhibited (Table I, lines 4 and 6). REAGENTS AND MATERIALS
Stock Solution of Metaphosphoric Acid. Metaphosphoric acid pellets, anal tical reagent grade (30 grams), are dissolved in io ml. of distilgd tvater with mechanical shaking. The solution is stored a t 4' and is prepared fresh every few days. Stock Potassium Permanganate. Potassium permanganate solution (0.1 N ) is prepared and stored according to Kolthoff . and Sandell ( 3 ) . Working Potassium Permanganate Solution. One milliliter of the potassium permanganate stock solution is diluted to 26.0 ml. with water. This solution is unstable and is prepared just Drior to use. The addition of 1 ml. of 10% metaphosphoric acid to the dilute permanganate solution greatly refards- manganese dioxide formation.
