Table II. Temperature at Which Precipitate Formed by Cooling to 0" C. Redissolves Temper a t tire,
c.
Test Solution
1f i 1T
19 20 13 25
IIC1, SOTc datd. (2.3.11) RbC1, 0.3.11
S o precipitate n t 0" c. I1 20
the salt solution is mixed with 3 nil. of the reagent and allon-ed to stand. Smaller amounts can be detected with experience. The source of the sodium ion was sodium chloride in all cases. DISCUSSION
One diqadvantage of carrying out the test a t ice bath temperature is that precipitates are obtained with concentrated solutions of many alkali metal salts. However, they redissolve on stirring for 5 minutes a t higher temperatures. The data in Table I1 are for 3 volumes of the recommended reagent mixed with 1 volume of the test solution. With the modified reagent s a t n r a t d
n i t h the sodium acid salt a t 25' C. (see Table I,'), a small but definite precipitate of the sodium acid salt will form n h e n the test solution mixed lyith 3 volumes of reagent is cooled to 0' C. and maintained a t this temperature for several hours, even when the test solution is free of sodium ions. Becauqe this precipitate does not redissolw a t 20" C., a false conclusion about the presence of traces of sodium ion is possible if the reagent saturated with and filtered from the sodium acid salt a t 25" C. is used by the 0' C. procedure. K i t h the recommended reagent saturated a t 0" C., no precipitate formi under these condition.. In the preparation of the recommended reagent, trtramethylammoniuni hydroxide is used because of the good sensitivity of the resulting reagent and because no foreign ions are introduced into the solution. This latter makes i t possible to ash the filtrate from a sodium test and examine it for other elements. A reagent may alw he prepared by substituting a n equivalent amount of potassium hydroxide and water for the expensive tetramethylammonium hydroxide solution. With such a reagent, the sodium test is best carried out a t 20" C. The latter reagent can detect 0.6 and 1.0 mg. of sodium ion per ml. of test solution i n 60 and 10 minutes, respectively, when other salts are not present. This shone it t o lie slightly less qensitive than
the reagelit \ J ~ S C ~on tetrnniethylammonium hydroside. Another disadvantage of this modification of the reagent is that other alkali metal salts are less soluble in it than in the recommended reagent and, therefore, should not be preserit in as high a concentration in the t w t solution. ACKNOWLEDGMENT
The authors are grateful to IT-. A . Bonner of Stanford University, ~ h o prepared the D(-)- and L(+)-a-nIethosyphenylacetic acids, which were obtained from John TI-. Mitchell of the C . 8. Department of Agriculture. LITERATURE CITED (11 Bonner. IT.b.,J . Am. Chem. SOC.73, 3126 (1991). (2) Braun, J. v., Anton, E., Keissbach, I
