The Qualitative Analysis orthe Alkali ~ e t Group1 h MARCUS E. HOBBS Duke U n i v e r s i t y , Durham, North Carolina H E qualitative analysis schemes used up to the presentZfor the analysis of the alkali, or soluble group have, in general, employed the procedure of baking the filtrate from the group immediately preceding. The baking procedure has two chief disadvantages, (1) the time required, and (2) the fact that, in the hands of inexperienced analysts, the baking is likely to be insufficient. In an effort to overcome these difficulties a simple modification of the procedure was tried. It consists essentially in separating the solution to be tested into two portions and then removing the NH4+ from one portion by means of the addition of NaOH and boiling. The resulting solution, which is free from NH,+, can be tested for the K+ by the addition of acetic acid until acid and then the addition of a solution of sodium colialtinitrite. The other portion can be tested for the Na+. This general procedure has been applied by sophomore qualitative analysis students on a semimicro scale, and by freshmen qualitative analysis students on the maao scale. Sentimicro Scale.-The following are essentially the instructions used by a random group of students in a second year qualitative analysis course in which Vosburgh'sa text is used. The procedure assumes that group precipitations of the HCl, HIS, (NHSaS, and (NH&20a groups have been carried out, and that no P04-Sor G 0 k - 2 are present. Other common negative ions do not inte~fere.~The test for NHd+ must be made on a sample of the original unknown.
T
SBMU6ICRO SCALE
Procedure for the Naf end i?+ Tests Evaporate the filtrate from the carbonate group in a 50-ml. beaker to a volume of approximately 3 ml. (This volume may be estimated by taking the clean beaker and pouring into it 3 ml. of water.) Remove one half of the solution (approximately 1.5 ml.) and save i t for the Nat test. Polassiurn Test.-Add 3 ml. of 6 M NaOH t o the 1.5 ml. of the (NH4)COsgroup filtrate remaining in the 50-ml. beaker, and boil the resulting solution, while stirring, until the volume is approximately one k l . Add 2 ml. of water drop by drop, being sure t o dissolve the deposited salts on the wall of the beaker. Now add 10 drops of 6 M NaOH. Boil slowly and test the vapors with a piece of moist red litmus. If the test far NH8 is positive after evaporating to a volume of about one ml., repeat the NaOH 1 Prezentcd before the Division of Chemical Education at the 99rh meeting of the A. C. S.. Cincinnati. Ohio, April 10. 1910. 2 Nours ASD 13w.u. "A svstcm of aualirative analysis for the rarhe~ements.~' The ~ a c m a l a nCo., ~ e York w City, 1927, pp. 24.567. 466-86. a Vossuncn, "Introductoq qualitative analysis." revised ed.. The Macmillan Co.. New Ydrk City, 1938, p. 217. 6 CORNOG, "The Iowa system of qualitative chemical analysis." 4th ed.. The Clio Press, Iowa City, 1937, p. 66.
treatment (10 drops) until no test for NH3 is obtained. With the final volume of approximately one ml. add, with stirring. 6 M CHCOOH, one drop a t a time, until the resulting solution is slightly add. (Some solid may remain undissolved, but this is not objectionable.) Withdraw approximately one ml. of the solution and centrifuge. Draw off 8 drops of clear liquid and add t o this 20 drops of a solution of sodium cobaltinitite. (The NaaCo(N0ds solution is prepared by adding 2 ml. of H 2 0 to one g. of NaCo(NO& and stirring until dissolved.) Stir the solntion vigorously and scratch the walls of the tuhe with a stirring rod. A yellow precipitate of K2NaCo(NO& shows the presence K+.. ~ ~ - of --- -Sodium Test.-If K C was found add 5 draps of 6 M HC10,; if a precipitate appears, centrifuge the mixture. (Filtration may, of course, be substituted for centrifugation.) Now add one drop of HCIO, t o the solution which stands above the precipitate in the centrifuge tube. If no further precipitation occurs remove eight drops of the clear liquid and proceed with the sodium test as indicated below. If, however, more precipitate does form, continue the HCIO, treatment until no further precipitation occurs. This procedure is necessary because Large amounts of KC will interfere with the NaC test. If K+ was not present in the unknown the HC104treatment is unnecessary. Place 8 drops of the clear solution t o he tested in a centrifuge tube and add 2 drops of 6 M CHCOOH. Add t o the resulting solution 20 drops of zinc wanyl acetate (sodium reagent). If no precipitate is formed scratch the inside of the tuhe with a stirring rod. If a quick-settling, greenish yellow precipitate forms and settles to the bottom of the tube the presence of the sodium ion is shown. .. - - -
Macro Scale.-The following are essentially the instructions used by a number of students in a freshman qualitative analysis laboratory in which a modification of Cornog'sk text was used. The and were excluded from the. samples of unknowns. The tests, as described, give ex&llent results on NHk+, K+, and Na+ content as low as five per cent. MACRO
SCALE
Procedure for NH*+, KC, and Na' Preparation of the Test So2ution.-Place
one g. of finely powdered orieinal unknown in a 100-ml. beaker and add 10 ml. of water. I h i l the mixture far one minute wirh stirring. Filrrr and reject any preripitate. nivide rhc solurion inro two equal parts. L'se one portion for the *odium test and the other fur the ammonium and potassium tests. Ammonium Test.-To the solution for the ammonium and potassium tests in a 100-ml. beaker add 2 ml. of 6 M NaOH and 6 ml. of 1.5 M NanCOa. Cover with a watch glass and insert a piece of moist red litmus paper between the rim of the heaker and the watch glass. Heat the solution t o boiling and observe the litmus paper.6 If the litmus paper turns un;farmly blue, ammonium ion is present in the unknown. Filter and reject any precipitate and continue boiling the filtrate until the vapors coming off no longer 6 If a blue color appears only in spots it is probably due t o spattering of NaOH.
show the presence of NHr. Now add 0.5 ml. of 6 M NaOH and boil until no more ammonia comes off. Repeat the addition of NaOH and boiling until no NH8 appears on first boiling the solution. Proceed to the test for potassium ion. Potassium Test.-Acidify the clear solution from the ammonium test by adding, with stirring, 6 M CHCOOH. To one ml. of this solution add 2 ml. of freshly prepared sodium cabaltinitrite made from 0.5 g. of solid sodium cohaltinitrite in 2 ml. of water. An immediate yellow crystalline precipitate of K2JaCo(NOz)r indicates the presence of potassium. Sodium Tcrt.-If the solution resenred for this test is highly colored add 3 ml. of 3 M (NH&C08 and filter off any precipitate. (If the reserved solution is colorless the (NH&COr treatment may be omitted.) Add to the solution one ml. of 6 M HCIO, and one ml. of 6 M HCI. If a precipitate is formed filter and reject it, and if potassium is present add more HClO, (one-ml. portions) until no more precipitate appears. If no potassium was found in the potassium test, the addition of HClO, is unnecessary. Test the solution with litmus paper, and if not acid, acidify with 6 M CHaCOOH. To one ml. of the solution add 3 ml. of sodium reagent (zinc uranyl acetate). The formation of a heavy. quickly settling precipitate indicates the presence of sodium. If no precipitate appears, shake for a moment before concluding that sodium is absent. NOTES ON THE MACRO SCALE PROCEDURE
1. Ammonium ion gives the same test as the potassium ion, and must therefore be removed before testing for potassium. This is accomplished by addition of large quantities of OH- ion, in the form of NaOH, and boiling off the ammonia that is formed. In addition to furnishing the necessary OH- ion, the
NaOH and NazCOs cause the precipitation of many cations as hydroxides or carbonates. Some of these might interfere with the potassium test if not removed. 2. The sodium reagent is intended to give a precipitate with sodium ions only, but if there is much K + in the solution, it will also be precipitated by the sodium reagent. HCIOa will precipitate the greater part of K + ions, so that there is no longer enough to interfere with the sodium test. The purpose of the HCl is to remove any large amount of Hg,+2 or Ag+ that may be present, as these also may interfere. The procedures presented here have several advantages over those generally in use. They are as follows. 1. Less time is required for the inexperienced student analyst to make the necessary tests. 2. The procedure can be used more successfully, as has been shown by a t least a ten per cent margin in its favor. 3. The pedagogical value of the chemistry involved is a t least as great, and is probably greater than that involved in the usual procedure. 4. The use of possibly explosive mixtures, such as alcohol in significant concentrations and perchloric acid, is avoided. The author is indebted to M?. Randall Bortner for his help in running several preliminary tests on the semimicro procedure.
