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 b y 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 M n 0 4 - 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 b e h h d 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
