Stabilization of the Potassium Reagent:
Sodium Cobaltinitrite
Sodium hexanitmcobaltateillI).e~,mmnnlyknown assodiummbaltinitrite, is usedasareagent for thequalitativeanalysis of K+ in many qualitative analysis schemes (e.g.. see (1)-(8)). The reagent is usually prepared as a 0.2M.25 M solution of NaCoiNO.)a. rometimea with aceticacid added. These solutions exhibit an unfortunate instabilitvand n e d to be te~laced or oftener when stored a t room temoerature. The Co(NOl)c3- ion decomooses biaauation of one 0; more ever"-2-3we'ki . NO; ligands (9). followed by a slower redox reaction between Coillll and NO?- ipossibis still bound a s a ligand) to pve Co" and NO2. Stoppered hottles develop the red color of NO2 above the solution and gas preswre often hu~ldaup suIliriently to blow the stopper out of the bottle. In contrast to this usual behavior, we found one preparation of025 M Na3Co(NO& ta be extremely stable--no evidence of decomposition was exhibited over a period now in excess of one year. Solutions prepared from the same source of solid Na3C~(N02)6 decomposed at the normal rate, indicating that an external impurity was involved. The trace metal content of the stable solution was found bv emission soectroscouv to he identical to that of the unstable material. Infrared spectra of a chloroform extract indicated h o detectahie ouanti&s of extractable oreanic materials Since the allezed imouritv was nrobablv,introduced at the time the s o l u h was oreoared.. one droo of various nossible impurities una added to 10-ml portions of n freshly prepared unntahle erlution. The aolutim was indeed stabilized to varying extents by methanol, glycerol, acetone, and ethanol in approximately decreasing order, but not at all by soap solution. One drop of the year-old stahle ~olutronWRP also found to completely stabilize the new solution. The year.old stable solution was analyzed by gas chromatography and was found to contain about 15 ppm of methanol, along with minor amounts of two unidentified volatile constituents. Since one drop of this solution was able to stabilize 10 ml ofa new solution, amounts of methanol on the order of 0.01 ppm appear to he effective. The mechanism of this inhibition process is not known, but prohahly involves trapping of fre; isdie& to terminate a long chain reaction, since such small quantities are sufficient for stabilization. We recommend the addition of 15dropsofmethanol per liter tostabilize NaCo(NO& reagent solutions. The authors are grateful to Dr. Joe Fulton for carrying out the gas chromatography analysis.
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Literature Clted
(9)
Bkk. J. P..unpublished expedments.
Arizona State University Tempe, 85281
James P. Birk Charles Q. Buekwalter. Jr. Michael L. Parsons
Volume 54, Number 7, July 1977 / 419
