ANALYTICAL EDITION
June 15, 1941
393
of dilute nitric acid (1 to 2). Boil for 5 minutes, cool t o room temperature, add 50 ml. of ammonium molybdate solution ( I ) , COW~ R I S O X OF MANGANESE DETERMINATIONS BY and let stand for 0.5 hour with occasional shaking. Finish as DIFFEREXT METHODS usual by filtering, dissolving the well-washed precipitate in Manganese Found excess alkali, and titrating the excess with standard nitric acid A. S. T M. method HClO~-NazCrz01-NaCl solution. Sample E30-39 ( I ) method A standard iron of an analysis similar to the unknown sample 70 should be carried through all the steps in order to standardize 1 60 the nitric acid. 0 79
TABLEI
1 12
Caution
0 85
0 14 0 23
TABLE11.
O F PHOSPHORUS DETERMINATIOSS BY DIFFERENT METHODS
C'O\lP.4RIsOS
Phosuhorue Found A 8 T. M.method HC104-NazCrzOv-SaCI
Sample Pig lion Pig iron Pig iron Pig iron
%
method %
0.205
0.206
E30-39 ( 1 )
A , 4.57" C
B , 4.57" C C, 4.5% C D, 4.5% C
0.198 0.177 0.073
0.197 0.174 0.072
sulfate solution, and boil for 30 seconds. cool and titrate with arsenite solution (3.75 grams of sodium arsenite per liter), standardized against a standard steel or iron.
Procedure for Phosphorus To a 1.000-gram sample in a 500-ml. IMenmeyer flask add 0.2 gram of sodium dichromate, 10 ml. of dlilute nitric acid (1 to 2), 5 ml. of dilute hydrochloric acid (1 to l),and 25 ml. of 72 per cent perchloric acid. Heat until dense white fumes issue from the mouth of the flask and volatilize the chromium by adding 0.5-gram portions of sodium chloride as directed in the procedure for manganese. Remove the flask from the hot plate and allow it to cool. Add 40 ml. of water and 10 ml. of 20 er cent sodium hydrogen sulfite solution, twirl the flask, and a d s 10 ml.
Perchloric acid must be considered a hazard and handled with caution at all t.imes. If a bit of paper, oil, cloth, or other organic matt'er finds its way into fuming perchloric acid i t may cause a disast'rous explosion. So far the authors have encountered no difficulty when using these methods.
Conclusions Chromic acid in fuming perchloric acid oxidizes graphite completely. Xitric acid added a t the beginning of a perchloric acid phosphorus determinat'ion prevents the loss of phosphorus as hydrides. The solution of iron is hastened and the graphite is readily removed by the use of perchloric acid and a soluble chromium salt. Application of these methods to the determination of manganese and phosphorus in irons containing graphite gives results agreeing with the older methods and effects a time saving of 40 to 50 minutes.
Literature Cited (1) Am. Soc. Testmg Materials, "Methods of Chemical Analysis of t h e Metals", 1939. (2) Smith, F. W., IND. ENG.CHEY.,Anal. Ed., 10, 360-4 (1938).
A Simple Sintered-Glass Salt Bridge H. .4,, LAITINEN, University of Illinois, Urbana, 111.
Method of Construction
that of Bright and Miller ( I ) , often have the disadvantage of complexity of construction and tend towards high electrical resistance. bridge described here can be made of 3-mm. tubing if desired, is simple, has a low electrical resistance, and effectively eliminates siphon action. Thus i t is unnecessary to maintain
CT
Pyrex glass is ground to a h e powder in a porcelain mortar. The 3- to 10-mm. Pyrex tubing to be used is sealed at one end and filled to a length of a few centimeters with the powdered glass. The glass pov,-der is warmed gently to allow expansion of the gases in it, and the glass is tapped to settle the powder in the end of the tube. The tubing is now heated with rotation, a t some distance from the end, in a very small oxygen flame. As soon as the tubing begins to soften it is withdrawn rapidly from the flame and suction is applied with the mouth partially to collapse the tubing. The end of the tube is cut off, and the excess powdered glass is removed. The sintered-glass membrane can be tested by suction. it should be dense enough so that water can barely be sucked through with the mouth. With practice the membranes can easily be constructed; the main difficulty is in preventing complete fusion of the powdered glass. The bridge is completed as shown in Figure 1. Either one or both ends may be provided with the sintered-glass diaphragm, and the stopcock may be omitted. The bridge is cleaned and filled by applying an aspirator or suction pump to the vertical arm, shutting off each end of the bridge in turn, if necessary, by means of R piece of rubber tubing provided with a clamp.
Literature Cited (1) Bright, W. M., a n d Miller, E. L., IND. ENCI.CHEII.,Anal. Ed., 9, 346 (1937). (2) Irving, G. W., and Smith, N. R., Ibid., 6. 480 (1934).
