No. 80 drill (0.013-inch diameter) ; it has a volume of approximately 0.0025 ml., which is the proper size to give the desired pressure of sample in our mass spectrometer. B and C are unpacked valves which close off the two arms of the T. The l/s-inch diagonal channels, D and E , serve to admit the liquid sample and to flush out the remains of the previous sample before starting the next run. The bonnet, containing valve A , is constructed from I-inch hexagonal freecutting brass 2'/s inches long. The 5/8-in~hthreaded lower end screws into the valve body against a Teflon gasket, while the '/s-inch threaded upper end accommodates packing, packing glands, and a packing nut. (A bellows-type valve could be used here to eliminate packing.) A l/S-inch pipe thread at the side takes the fitting through which attachment is made to the mass spectrometer sample system. The stems for valves A , B , and C are constructed from l/z-inch diameter Type 303 stainless steel, 33/4 inches long for A and 3l/4 inches long for B and C. The needle end includes an
angle of 14 to 15 degrees and the other end is square-cut to accommodate a suitable handle. Operation. Valves B and C are closed and valve A is opened. This permits the line and the small Tvolume to be evacuated through the mass spectrometer sample system. When the T-volume is completely evacuated, valve A is closed and a small quantity of liquid sample is admitted to the cavity around the seat of valve B by a dropper through channel D. Valve B is then opened one turn and immediately closed again. This permits the liquid to be sucked into the evacuated T and to fill it completely. The sample system is now isolated from the pumps by the appropriate sample system stopcock and valve A is opened. The small trapped liquid volume in the T flash-vaporizes into the sample system and may be admitted directly to the expansion chamber. Valve A is then closed and the unused portion of liquid is removed by opening valves B and C and rinsing through the channels with a suitable volatile solvent.
Air is then blown through to remove the solvent. This liquid sampling device has performed satisfactorily in the analysis of mix-tures of alcohols. When used for chlorosilanes, the entire device must be shielded by a dry atmosphere to prevent hydrolysis of the chlorosilanes. Based on measurement of peak heights, sample size is reproducible to 1 1 . 5 % and as little as 2 drops of liquid fills the small evacuated volume completely without trapping air. LITERATURE CITED
(1) Caldecourt, V. J., ANAL. CHBM.27, 1670 (1955). (2) Friedel, R. A., Sharkey, A. G., Jr., Humbert, C. R., Ibid., 21, 1572 (1949). ( 3 ) Friedel, R. A., Sharkey, A. G., Jr., Humbert, C. R., Summary of Consolidated Engineering Corp. Group Meeting, New York, 1949. (4) Huston, Charles, ASTM 4th Annual Meeting, Cincinnati, Ohio, May 1956. (5) Purdy, K. M., Harris, R. J., ANAL. CHEM. 22,1337 (1950).
A Special Adaptation of the Volhard Method for the Analysis of Manganese Oxide Ores V. L. Garik' and L. M. Silber, Microwave Research Institute, Polytechnic Institute of Brooklyn, Brooklyn, for deterA mining the manganese content of certain compounds in terms of di- and N
ANALYTICAL METHOD
quadrivalent manganese is presented. The method is an adaptation and simplification of the standard Volhard method ( I ) ; it is applicable to compounds such as oxides, which, in addition to manganese, contain cations which are inert to the oxidizing agent used. It has the advantage of being faster and of yielding a more readily seen end point The analytical scheme deals with the analysis of impure oxide ores in which M n is present in the plus two to the plus four state. Unstable higher valence states are not included in this scheme. The oxides which may be considered to be of most frequent occurrence are MnO, MnO?, hInzOa, and Mn304. I n practice, one presents the results of the analysis in terms of di- and quadrivalent manganese, omitting trivalent, for the following reasons. Previous investigators found that in general the plus three state is quite unstable in solution with respect to the disproportionation into the di- and quadrivalent states ( 2 ) . I n a weakly acidic medium, such as 1 N H2S04, this reaction goes to completion with the precipitation of MnO2. I n fact, the plus three state of manganese appears Permanent address, Iona College, 6 e w Rochelle, N. Y.
to be stable in only a restricted number of complex ions and compounds. It is convenient to consider for analytical purposes both MnzO3 and Mn,04 as manganous manganites-Le., to assign them the structures MnO-MnOz and (MnO)2-MnO2, respectively. Thus, the practice has developed in analytical work to report the oxide compositions of samples in terms of the ratio of MnO to MnOn-i.e., as (Mn0)z(MnOz)y. The R4n0 component of samples of manganese oxides can readily be leached out with warm dilute sulfuric acid. Thus, by dissolving the sample in warm dilute sulfuric acid, manganese in the original sample as A h f 2 will go into solution, and manganese present in the original sample as Mn+3 will undergo disproportionationi nto di- and quadrivalent manganese. The MnO content of the sample, or the divalent manganese, can be determined by various standard procedures from the Mn+2 concentration of the acid extract. It has been shown and verified in this laboratory, that under the same conditions the MnOz component will not be affected in any analytically relevant measure. The ratio of the MnOz component can then be determined from the difference between total M n content and the Mn content assigned to MnO in the sample. The total M n content, which is necessary to be known, may also be determined by various standard procedures. Usually, to obtain the latter, the sample is treated with con-
N. Y.
centrated HCl, and the total M n is brought into solution in the divalent state. The Volhard method of M n analysis generally proves to be very practical and simple in application once certain inherent drawbacks in the procedure are overcome. These consist primarily in adjusting analytical conditions over fairly narrow and sensitive operating ranges so that the reaction between manganous and permanganate ions proceeds stoichiometrically, and in the developing of practical methods for discerning the end point in the resulting suspension of zinc-acid manganite. It was empirically found that addition of a small amount of alkali (for instance, 4 to 5 N sodium carbonate) in excess to a solution of sample to be titrated, to elevate the pH to a pH of about 7.5 to 8.0, yields titration results which fall by 2 to 3% under the expected stoichiometric ratio. In the present adaptation of the Volhard method, this relationship is utilized to determine the approximate end point for the titration of an aliquot. A fixed volume of solution containing a known weight of sample in a known volume is adjusted to a pH of approximately 7.5 to 8.0, and titrated with MnO4- reagent to the nearest 0.5 ml. by adding the reagent in milliliter quantities until a definite color change is obtained. The addition of the alkali moves the apparent end point 2 to 3% ahead of equivalence, and makes it very readily discernible. VOL. 33, NO. 2, FEBRUARY 1961
319
The volume of reagent consumed at this end point is then added to two or three aliquots of the same stock solution, measured out to the same volume as the pilot run, using a buret. The solutions are heated on the hot plate, after p H adjustment t o neutrality with dilute acid, with occasional stirring for 10 to 15 minutes at 85’ to 95’ C. Then t h e titrations are carried on to a definite end point. Usually only a small amount of reagent, between 2 and 3 ml., is required. For this titration, the desired p H level of 7.0 + 0.2, can be conveniently obtained by checking with p H indicator paper, such as Alkacid No. 3, a short range indicator between p H values from 6.0 to 8.5, supplied by the Fisher Scientific Co. For accurate results, the absolute p H value lvithin the defined range does not appear to be as critical as obtaining consistency with respect t o a definite value. I n general outline, the procedure can be condensed as follows. The Rfn titer (in milligrams of M n per milliliter of Mn04- reagent) of the potassium permanganate reagent is first determined in two separate standardization runs,
utilizing a weighed sample of reagent grade MnO,, converted t o the divalent chloride, and a weighed sample of reagent grade manganous sulfate. The titer from both runs should agree within approximately 0.5%. The end point of the titration for the total manganese content is matched with the end point of the standardization based on l\ln02, and the end point from the manganese fraction from the dilute warm acid leach is matched with the end point from the standardization run utilizing manganous sulfate. With respect to the determination of the total M n content, this adaptation of the Volhard titration is based on the fact that a sample solution containing a low concentration of chloride ions, matched with a properly selected blank, will not show any appreciable errors on application. Utilizing the same permanganate solution, several samples may be run on the basis of the same standardization results, thus eliminating duplication of effort and rendering this method expedient where sets of analyses are required.
Precision and accuracy of this procedure are satisfactory for most analytical purposes, the precision being 2 to 3 parts per thousand and the absolute accuracy =tO.257,, provided the results are based on blank runs. The procedure is discussed in greater detail in a report (S), which may be referred to for further information. ACKNOWLEDGMENT
The authors thank the Rome Air Development Center, Air Research and Development Command, for support of this work under Contract AF-30(602)1816. LITERATURE CITED
(1) Fales, H. A., Kenny, F., “Inorganic Quantitative Analysis,” p. 446, Appleton. New York. 1939. (2) hieyer, Julius, Kanters, Robert, 2.anorg. chem. 185, 177-83, (1930).
(3) Microwave Research Institute, Polytechnic Institute of Brooklyn, Rept. Nb. R-774-59, PIB-702.
A Simple Micro Volumetric Combustion Polarographic Cell R. M. Parkhurst, number
Stanford Research Institute, Menlo Park, Calif.
methods have been for the polarographic determination of trace elements in combustible materials such as biological samples (1, 6), foods (4, plant tissues (2, 6),organic salts, and metal organic chelates (3). The procedures usually require combustion in air or oxygen or digestion with concentrated acids to destroy organic material before polarography. These procedures can be simplified by carrying out the weighing, combustion, dilution, and polarography in a single piece of apparatus, amicrocell. The use of a number of these strong, inexpensive, microcells for routine work would save the time required to transfer material and lessen the chance for errors due to nonquantitative transfer of very small samples. The microcell is constructed from borosilicate or Vycor glass, depending on the use, as shown in Figure 1. A few milligrams of sample are placed in the bowl and weighed. Any remaining solvent is removed by passing a slow stream of clean dry air down the stem. The sample is combusted in air or oxygen by passing a slow stream of the gas through the stem section while the bowl is being heated. After combustion and/or digestion with acids, the excess acid is evaporated by continuing the gas stream through the stem and over the liquid surface. Then the electrolyte is added and the oxygen removed by passing nitrogen down the of
A described
320
ANALYTICAL CHEMISTRY
stem and through the liquid in the conventional way. The cell is tipped so that the liquid runs back into the stem section for measurement of the volume. Evaporation during removal of oxygen is not especially important since the volume measurement is made last. Mercury is allowed to run down the stem behhd the liquid and form a mercury pool electrode in the cone section. A wire placed down the stem makes electrical contact with this electrode; the dropping electrode is inserted into the solution in the bowl. Various types of caps with or without openings br side arms may be useful, depending on the analytical procedure Caps with side arms as shown in the diagram may be used to
CAP WITH
collect the gases evolved during combustion of the sample. The sample must be heated slovly and evenly to obtain complete combustion nithout loss of material; an electric heater is most convenient. LITERATURE CITED
(1) Ames, S. R., Dawson, C. R., ASAL. CHEM.17,249 (1945). (2) Hinsvark, 0. N., Houff, IT. H., Wittwer, S. H., Sell, H. pvl., Ibid., 26, 1202 (1954). (3) Menzel, R. G., Jackson, hl. L., Ibid., 23, 1861 (1951). (4) hfonier-Nrilliams. G. W.. “Trace Elements in Food,” p. 53, Wiley, New ~
( 5 ) Page, J. E., Analyst 71,52 (1946). ( 6 ) Reed, J. F., Cummings, R. W., ANAL. CHEM.13,124 (1941).
-
End Curved
Up A n d F l a r e d For A d d i n g M e r c u r y
\
For
Figure 1. Polarographic cell
SUP
Pan During We ig h i n g
Dropping Mercury Electrode Liquid Sample Mercury Pool Electrode
WI TH J A1-IONS
