Routine Analysis
OF
Sodium-Potassium Alloys
S. L. WALTERS AND R. R. MILLER Naval Research Laboratory, Office of Research and Inventions, Anacostia Station, Washington,
A volumetric method for the determination of potassium in sodiumpotassium alloys i s described. A sample of the alloy is obtained in an ampoule in such a way that contact with moisture and oxygen is prevented. The sample thus obtained is reacted with absolute alcohol under neohexane and subsequently titrated with standard acid. Alloys of higher than 92% potassium content are solid at room temperature and are analyzed b y means of their freezing points. A n equation i s given for the freezing point curve. Accuracy of *O.i% potassium is readily obtainable. Liquid alloys can be analyzed in less than 1 hour and solid alloys in about 5 minutes.
I
Iu A problem involving the production of metallic potassium,
an urgent need arose for a method of determining potassium when alloyed with sodium, m-hich could be used for routine control analyses. The usual gravimetric methods involving the formation of the chloroplatinate, cobaltinitrite, or perchlorate were investigated and found sufficiently accurate, but too slow. These methods m-ere used only when impurities were such that the volumetric method described below was inapplicable. The method reported here was designed for alloys of sodium and potassium n-ith little or no impurities. In the presence of foreign elements results are vitiated by multiplied errors in calculations. The alloys analyzed in this laboratory contained over 99.9y0 sodium and potassium. APPARATUS AND PROCEDURE
The most difficult part of an alloy analysis of this type is obcaining a clean, oxide-free sample of known weight. The apparatus shown in Figure 1 was developed to enable a weighed sample of clean alloy to be taken, avoiding contact with oxygen or moisture. A sample (approximately 30 grams) of the alloy was drawn into the sample bulb by evacuating through the stopcock and drawing the alloy from the original container into the sample bulb through a glass tube fitted with a ground joint. The sample bulb was then fitted t o the sampling device, which was clamped in such a position that it could be rotated to raise either end. A weighed thin glass ampoule was placed in the sampler; the system was evacuated through stopcock A and filled with nitrogen through stopcock B. The nitrogen was purified by passing over copper turnings maintained a t 425" C. and a drying tube filled with Anhydrone. The sample bulb and sampler were warmed with a flame to permit free flow of the alloy. The apparatus was then tipped so that the alloy could flow from the sample bulb into the capillary. B slight vacuum was applied to draw the desired weight of sample (approximately 1 gram) into the alloy ampoule. Xtrogen was admitted t o force the excess alloy out of the capillary back into the sample bulb. The sampler was returned to the horizontal position and the alloy ampoule was removed, quickly sealed in a gas flame, then cooled, and weighed. In this manner an accurate weight - of clean. oxide-free alloy was obtained. The weighed amooule of allov was Dlaced in a 32 X 300 mm. test tube aGd covered with neohexane: The neohexane used was purified by washing first with sulfuric acid, then with alkali, and distilling over sodium. The ampoule was broken with a glass rod and absolute alcohol added dropwise to the hexane, decomposing the sample. The alcohol was added slowly to avoid loss of sample by entrainment in the hydrogen evolved. The decomposition tube was kept in a water bath to minimize overheating. When the sample had been completely decomposed it was washed out with distilled water into a 250-ml. Erlenmeyer flask and titrated with 1 S hydrochloric acid, using phenolphthalein as indicator. At the end point the hexane was evaporated on a hot plate and the titration continued to the methyl orange end point. The use of phenolphthalein reduces danger of overstepping the methyl orange end point and speeds the titration.
D. C.
CALCULATIONS
Calculations were made in the following manner: Let z
w
= weight of potassium in sample in grams = weight of sample in grams
M E = milliequivalents of hydrochloric acid ME/1000 = ~/39.1f (W ~ ) / 2 3
-
Solving the equation for x gave
x
= 2.43 w
- 0.0559 3 f E
An operator with a little practice can analyze an alloy in lese than an hour with an accuracy of *0.1% potassium. Considerable care must be exercised in following the procedure to obtain this precision. The most frequent errors are those caused by impure hexane, oxide film, and loss of sample in transferring. Because of the hazardous nature of t,he materials involved, safety goggles should be worn at all times during sampling and decomposition of the alloy. SOLID ALLOYS
Alloys-containing more than 92% potassium are solid at room temperature; therefore a rapid and convenient method of determining their composition is by their freezing points. Since the published data are not well defined in this region (2, S), thie part of the freezing point curve has been determined at thie laboratory. Freezing points were taken directly in the sample bulb (Figure 1) by inserting a calibrated thermometer graduated in 0.2" C. The alloy was warmed until liquid, and the bulb placed in a beaker of magnesia and allowed t o cool slowly. The first thermal arrest was noted, the thermometer removed, and the sample analyzed as described above. When per cent potassium wm plotted against temperature of the freezing point a straight line was obtained in the region from 92 t o 100% potassium (by weight,). The equation of this line can be represented by the relationship y = 0.259 t 4- 83.5 where y is weight per cent potassium and t is the corrected temperature in degrees Centigrade. The freezing point method of analysis is not readily extendable
t o alloys below 92% potassium because of the decreased s h a r p ness of the first thermal arrest. The freezing point of pure potassium was found to be 63.7" C. This value is 1 . 2 O higher than the usually reported value, but ie TO N e
SYSTEM
Figure 1.
Sampling Apparatus =-E
