The TEST-TUBE METHOD for FLAME TESTING ARTHUR R. CLARK* Franklin and Marshall College, Lancaster, Pennsylvania
This work includes new &me tests for certain metals occurring in elementary qualitative analysis, obtained by use of the test-tube method rather than the regular platinum-wire method. It was found that some metals give ckuracteristicj4ame colors, residues, and residue reactions. A canparison of this method with that of the platinumwire method is included, as well as the action with different a n i m . Many interesting results were obtained when this procedure was used as a means of shortening the work in the u s d qwalitative schemes found in all analytical chemistry books.
A
S FAR as can be ascertained from the literature,
the test-tube method for flame testing was introduced by Meissnerl and was only applied to the tin flame test. In using the Marsh test for arsenic, Meissner noted that in certain cases a sky-blue color was produced in the flame. After much work, he found that tin was present each time the Marsh Test produced a flame of sky-blue color. With tin as the producer, it was his desire to find whether the metal was present as the hydride (SnH,) before the color wasformed. TEE TEST-TUBE METHOD FOR FLAME TESTING
In using this method for flame testing, one needs only a pyrex test-tube, water, and a colorless Bunsen flame. The unknown solution may,be placed in a very small beaker. The test-tube is three-fourths filled with water to prevent the glass from becoming superheated and giving a sodium flame test. The lower part of the test-tube is heated slowly to dry the outside wall. The test-tube is dipped into the unknown solution and then heated very slowly. Certain metals give characteristic colors and residues. If a metal gives a good residue but one which may be confused with the residues of other cations, one should apply residue reactions To obtain a residue reaction, one need only add an acid (or other suitable chemical) to the residue and heat. StoneVescribed a procedure well adapted to the testing of tin residues.
PROCEDURE USED IN FLAME TESTING
In using the test-tube method for detection of cations, one should follow, in general, the following sequence of procedures : (1) Heat and observe flame. (2) Heat to residue and then observe flame. (3) Observe residue. (4) Add HCl to residue and heat; observe flame. (5) Add HCl to residue and heat to residue and observe flame.
At all times use colored filter-glasses. A possible extension of work in this field would be the investigation of various reagents for the residue reactions. In the author's experience HCI produces best results for all metals excepting those of the first group. For these metals, best results were obtained with HNOs. METALS GNING CHARACTERISTIC FLAMES, RESIDUES, AND RESIDUE REACTIONS
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Groupl: Lead-Light red and yellowish blue mixture with few sparks of this color, turning to a spurting type of voluminous flame which dies very quickly. Residue of white screen-like film. Silver-Residue of light gray film (metallic silver).
Group 2: Copper-As nitrate, a spurting emerald green, and as chloride, a blue flame of nonspurting variety; residue of green a m . Bismuth-Enduring light green with a little yellow in flame. (Not spurting in nature as is the barium flame.) AntimonySteel-gray-green which exists only a t the upper edge of the Buusen flame. Tin-(Reported by Meissner1). Brilliant sky-blue color that seems to adhere to bottom and sides of test-tube and persists for a long time. Residue of white mist-like film. (Residue is not * At present a Graduate Assistant in the School of Chemistry heavy and is only visible after the and Physics of the PennsylvaniaState College, Pennsylvania. MeIssNsn, H., "Ein Nachweis Zinn," Z. a d . Chem., 80, process has been repeated several times.) 247-52 (1930). The (white) tin residue appears blue by a STONE, I., "A new test for tin," Chem.-Anelyst, 20, 2 (Mar.. reflected liaht. 1931). 242
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It was found that the stannous ion gave better results than did the stannic ion. Group 3: Nickel-Light green flame (slight) with residue of green film. Mmganese--Faint green flame with brownblack residue. If HCl is added to the residue, one obtains a residue spurting ydow-red flame, reminiscent of sodium with a reddish tinge. Aluminum-Residue of powder white. Residue reaction with-HC~produces a red spurting flame, not as yellow as manganese. Chromium-Volcanic yellow-red flame; residue reaction with HCl produces a flame more yellowish in appearance and containmg more volcanoes; spurting and spattering red-yellow flame (fleeting) and residue becoming lumpy and bubbled. Zinc--Residue of powder white; like aluminum residue, but differs from aluminum in the fact that no color is formed when residue reaction with KC1 is applied. Group 4: Barium--Gray-green flame of large volume. Residue of white film. Strontium-Flame of exceptional volume; red with much yellow (deep red mixed in). Residue of white film. Calcium-Flame with brick-red splashes (heavy-heat on upper fringes of residue). Group 5: Potassium-Violet flame with residue of white powder. Sodium-Yellow flame with residue of white powder. SENSITMTY 0 5p FLAME TESTS E
(g./cc. of cation)
Pb = 1 X lo-' Ag = 1 X lo-' cu = 5 x lo-* Bi = 2.2 X 10-8 So = 1.3 X lo-' Sn = 1 X 10-I (Meissner1reports) Sn = 3.5 X lo-" Ni = 2 X lo-"
.,
(8) Mn = 1 X 10-8 10) 11) (12) 13) 14) 15) (16)
1 X lo-: 2 X lo-' 3 X 10-8 5 X 10-6 Sr 1 X lo-' Ca K = 3 X 10-a Na = 1 X lo-'
Cr Zn Ba
tests. Potassium is the only cation for which the platinum wire method equals the test-tube method. CATIONS WITH DIFFERENT ANIONS
Meissnerl found that tin with HCI produced a skyblue flame, with HBr a green flame, and with HSO, a red-violet. I t is a well-known fact that Cu as nitrate is green and with halogens is blue. It is important that anions other than those definitely known be exeluded in testing. MIXTURES OF CATIONS WITHIN GROUPS
Group 1: ( M i u r e of Ag, Pb, Hg). The Pb is detected if any flame color is produced and Ag if any gray residue remains. Group 2: (Mixture of Hg, Pb, Cu, Bi, Cd, As, Sb, Sn). It is possible to detect Cu, Sb, Sn. ( M i u r e of As, Sb, and Sn.) All three may be detected. If all metals are present, it is possible to detect all but As. As8 can be detected bv the molvbdate method and Sb and Sn b i t h e test-tube flame test method. Sb gives the steel-gray-green at the upper edge of the Bunsen flame, whiie Sn gives the sky-blue flame adhering to the bottom of the tube. The two colors are separated and can be distinguished very easily. If too much Sb is present, it will not permit Sn to give a flame test. In this case one must use Zn HCl and liberate all the Sb and As (Hood!). After the action subsides Sn will give its blue flame.
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Group 3: (Miture of Co, Ni, Mn, Fe, Al, Cr, Zn). Ni is detected by a green flame and Cr by volcanoes. It is also possible to pick out Al, Cr, Zn, and Mn in certain mixtures. To detect Al in the presence of Zn heat to residue and in residue reaction with addition of HCl, Al will give a reddish spurting flame, while Zn will give no flame color. Group 4: (Mixture of Ba, Sr, and Ca). Sr is detected only.
= = = =
Group 5: (Mixture of K and Na). Usual detection of each by means of blue glass.
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COMPARISON OF PLATINUM WIRE AND TEST-TUBE METHOD
Using solutions with cation content of 0.01 g. per cc. it was found that by the platinum wire method Pb, Ag, Sn, Ni, Al, Zn gave no tests, while with the test-tube method very good tests were obtained. With the platinum wire method Cu, Bi, Sb, Mn, Cr, Ba, Sr, Ca, and Na were either very faint or too fleeting for obsenration, while- the test-tube method showed good
MIXTURES OF CATIONS OF COMBINED GROUPS
I t is possible (Na free) to detect several metals in certain combinations with the proper standardization of color and experience. For example, a mixture of Ag, Sn, Sb, Mn was examined. Sn, Sb, Mn were easily identified but the Ag test was ruined by the heavy brown-black residue of Mn. In general, it is not possible to identify without group precipitation. Even with the group mixtures the metals must be separated to obtain sure results. The primary utility of the test-tube flame method is in confirmatory testing.
