The Elements of Blowpipe Analysis Part 2

~Examination on Charcoal with Na_{2}CO_{3}.~--Metallic compounds are often difficult to reduce with the blowpipe flame alone, and hence no bead is obtained. In order to facilitate reduction and the obtaining of a metallic bead, the substance in a finely powdered condition is mixed with four parts of sodium carbonate, Na_{2}CO_{3}, and ignited before the Bp. on Ch. The metallic compound is decomposed, the metal being transformed into the carbonate, which in turn, through the agency of the Ch. and the heat of the flame, is reduced to the free metal. Sometimes the reduction is made easier by adding to the substance about its own bulk of pota.s.sium cyanide, KCN, which takes up oxygen from the compound and is converted into pota.s.sium cyanate, KCNO.

The reactions in reducing copper sulphate, CuSO_{4}, with Na_{2}CO_{3} and with KCN before the blowpipe, are here given:--

CuSO_{4} + Na_{2}CO_{3} = CuCO_{3} + Na_{2}SO_{4} } (1) 2CuCO_{3} + C = 3CO_{2} + 2Cu }

CuSO_{4} + Na_{2}CO_{3} = CuCO_{3} + Na_{2}SO_{4} } CuCO_{3} = CuO + CO_{2} } (2) CuO + KCN = Cu + KCNO }

After obtaining beads, it is well to obtain their coatings, for oftentimes it is only in this way that we can distinguish between the metals.

~Examination in Tube with Na_{2}CO_{3} and Charcoal.~--If the substance in a finely pulverized condition be mixed with twelve parts, Na_{2}CO_{3}, and six parts of charcoal powder and the mixture be placed in an ignition tube and subjected to heat, the acid of the substance combines with the soda and the metal is set free.

If this metal is volatile, a sublimate is formed in the upper end of the tube.

Mercury deposits in minute globules, which may be seen with the magnifying gla.s.s. a.r.s.enic forms a ring, which, when examined with the magnifying gla.s.s, is seen to be made up of minute crystals. Ammonia is recognized by its characteristic odor, and also by its turning a slip of moistened red litmus (held over the mouth of the tube) blue.

~Examination on Platinum Wire.~--Many substances possess the property of imparting to the colorless flame of the Bunsen burner characteristic colors.

The chlorides of these substances exhibit these flame reactions best, and hence before applying the flame tests we dip the wire which serves as a support into hydrochloric acid and then into the substance. When the substance has been taken up on the wire, it is placed in the edge of the long colorless flame of the Bunsen burner near the apex, when instantly the flame becomes tinged with the characteristic color of the substance.

_Ill.u.s.trations._ Sodium compounds color the flame yellow, and a crystal of pota.s.sium dichromate appears colorless in the sodium light.

This sodium reaction is extremely delicate, it being possible to detect with ease a quant.i.ty of a sodium salt less than 1/3000000 of a milligram in weight.

Pota.s.sium colors the flame purplish-violet.

Barium colors the flame apple-green.

Strontium colors the flame crimson.

Calcium colors the flame orange-red, distinguished from strontium, by appearing gray when seen through blue gla.s.s.

Boracic acid colors the flame green when the substance has been moistened with glycerine.

~Examination in Borax Bead.~--Borax, Na_{2}B_{4}O_{7}, and microcosmic salt,

NaNH_{4}H . PO_{4},

possess the property of dissolving many of the metallic oxides at the temperature of the Bunsen flame.

For example, with oxide of cobalt, the following reactions take place with the two fluxes:--

CoO + Na_{2}B_{4}O_{7} = Co(BO_{2})_{2} + 2 NaBO_{2}.

On heating, NaNH_{4}H. PO_{4}, it is decomposed into the metaphosphate of sodium, NaPO_{3},

CoO + NaPO_{3} = CoNaPO_{4}.

Now in such cases of solution the metallic oxides impart a characteristic color to the flux.

The platinum wire is the best support,--it is heated to incandescence in the Bunsen flame, and then is quickly dipped into the borax, when a small globule will adhere,--this is removed to the flame again when the borax melts to a clear gla.s.sy bead. While the bead is still melted, touch it to the finely pulverized substance and replace in the flame.

In a few seconds the small particles of the substance will have dissolved, and the bead will be seen to have a.s.sumed the color characteristic of the substance. Note the color when hot and then when cold; often there is a wide difference. Then, too, the test should be made in both O. F. and R. F.

Some a.n.a.lysts prefer to make a small loop in the end of the wire before taking up the borax to make the bead. Care should be taken to see that the bead is colorless before bringing it in contact with the substance.

As the depth of color produced is largely dependent upon the amount of substance taken, some little caution should be exercised to insure taking up about the same quant.i.ty each time.

_Ill.u.s.trations._ Make several beads, and note the colors characteristic of the following oxides: cobalt, nickel, iron, manganese, chromium, and copper.

The microcosmic salt bead dissolves almost every oxide except silica, SiO_{2}, and this is seen to float about in the melted ma.s.s. This is used as a test for silica.

~Examination with Co(NO_{3})_{2}.~--If after examination on the Ch. _per se_, a white infusible residue remains, it is moistened with a drop of cobalt nitrate Co(NO_{3})_{2} and re-ignited before the Bp., when a change of color will be observed. This change in color is owing to the fact that the heat of the Bp. flame decomposes the cobalt nitrate, nitric acid being driven off, and the remaining CoO forming with the oxide of the residue a colored ma.s.s.

_Ill.u.s.trations._ Ignite before the Bp. on Ch. the following oxides,--allow to cool, add a drop of Co(NO_{3})_{2}, re-ignite, and note color,--aluminum, magnesium, zinc, and calcium.

Care should be taken to thoroughly ignite before adding the cobalt nitrate solution.

With the six methods of examination just given almost every simple substance can be detected, but should any doubt remain, a few simple tests in the "liquid way" will be sufficient to substantiate the blowpipe examination.

CHAPTER III

GENERAL REACTIONS FOR THE DETECTION OF THE METALLIC ELEMENTS IN SIMPLE COMPOUNDS

For the sake of convenience, rather than for scientific reasons, the following compounds have been arranged in alphabetic order. Also the oxides of the elements have been taken, since they exhibit the reactions to best advantage.

The student should work through carefully each one of the tests and satisfy himself as to the characteristic reactions of the various elements, for only in this way can he expect to recognize the substances when presented to him as "unknowns." It is advisable to provide a note-book and rule it as follows:--

----------------------------------------------------------------------------- BEHAVIOR OF SUBSTANCE ---------------------------------------------------------------------------- Before Bp. Before Bp. In ignition In flame In flame After first on Ch. on Ch. with tube with on with borax ignition alone Na_{2}CO_{3} Na_{2}CO_{3} platinum bead with and Ch. wire Co(NO_{3})_{2} ---------------------------------------------------------------------------- ---------------------------------------------------------------------------- _Remarks_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Substance_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ----------------------------------------------------------------------------

1. ~Aluminum, Al_{2}O_{3}.~--Before the Bp. on Ch. Infusible. No change.

Before the Bp. on Ch. with Na_{2}CO_{3}. Forms an infusible compound with slight intumescence.

In ignition tube with Na_{2}CO_{3} and Ch. No change. Moisture driven off.

In flame on platinum wire. No change. Becomes incandescent.

In flame with borax bead. In O. F. dissolves slowly, forming a colorless gla.s.s which remains so on cooling.