Aqueous solutions flame test

All the cations of Group I produce a characteristic colour in a flame (lithium, red sodium, yellow potassium, violet rubidium, dark red caesium, blue). The test may be applied quantitatively by atomising an aqueous solution containing Group I cations into a flame and determining the intensities of emission over the visible spectrum with a spectrophotometer Jlame photometry). 

Preparation of Benzoyl Peroxide.1—Hydrogen peroxide (50 c.c. of about 10 per cent aqueous solution) kept well cooled in ice and continually shaken (preferably in a glass-stoppered bottle) is treated alternately with 4 A-sodium hydroxide solution and benzoyl chloride, added each a few drops at a time the solution is maintained faintly alkaline throughout. After about 30 c.c. of alkali and 15 g. of benzoyl chloride have been used up, the hydrogen peroxide has been decomposed and the benzoyl peroxide has separated in crystalline flocks, while the odour of the chloride has almost completely disappeared. The peroxide is filtered with suction, washed with water, and dried. Yield 10-12 g. Crystallised from a little alcohol, with which it should be boiled for a short time only, the substance forms beautiful colourless prisms. Melting point 106°-108° decomp. Heat a small quantity rapidly in a dry test tube over a naked flame. An especially pure product is obtained when a 1 von Pechmann and Vanino, Ber., 1894, 27, 1510. 

The scheme that is shown in Figure 9.9 is very simple. More complex qualitative analyses involve many more steps of isolation and identification, including some steps that are not precipitation reactions. For example, some ions, such as sodium, Na", and potassium, K, cannot he precipitated out of an aqueous solution, because the ionic compounds that contain them are always soluble. Instead, chemists identify these ions using a flame test. In the following ThoughtLab, you will simulate a qualitative analysis that includes a flame test. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

Potassium and its salts can he identified hy flame test. It imparts hlac color to the flame. Potassium ion in aqueous solution can be identified by reaction with sodium tetraphenylborate, NaB(C6H5)4. In weakly acid solution, a white precipitate of the potassium salt KB(C6H5)4 is obtained. The precipitate is filtered, dried, and weighed to measure potassium. The test is quantitative. 

Elemental composition K 39.85%, C 24.48%, H 3.08%, 0 32.60%. Potassium may be identified by flame testing. An aqueous solution can be analyzed for potassium by flame photometry, ICP/AES, or ion selective electrode (see Potassium). Acetate anion may be measured in aqueous solution by ion chromatography under appropriate conditions. 

Elemental composition K 32.85%, Br 67.15%. Potassium can be determined in solid form by flame testing. In aqueous solution, potassium can be measured by flame photometry, ICP/AES or electrode methods. Bromide ion can be analyzed in aqueous solution by ion chromatography. 

The normality of KOH in its aqueous solution can he determined by acid-base titration against a standard solution of HCl, H2SO4, or HNO3 using a color indicator or by a pH meter. Potassium can be identified by flame test or by wet methods or instrumental analysis (see Potassium). 

Another qualitative analysis technique is a flame test. A dissolved ionic compound is placed in a flame. Table 9.4 lists the flame colours associated with several ions. Notice that all the ions are metallic. The flame test is only useful for identifying metallic ions in aqueous solution. 

A. A 1 20 aqueous solution responds to the oxalate test for Calcium and to the flame test for Sodium, Appendix IIIA. 

Sample without Added Substances A 1 20 aqueous solution, tested on a platinum wire, does not impart a pronounced yellow color to a nonluminous flame. 

In the presence of anions which are precipitated by calcium chloride solution, proceed as follows. Precipitate the acetic acid test solution with calcium chloride solution, and collect the precipitate on a filter or in a centrifuge tube. Remove the water from the precipitate either by drying or by washing with alcohol and ether. Mix a small amount of the precipitate with diphenylamine in a dry micro test-tube, add a little concentrated phosphoric acid, and heat gently over a free flame. Calcium phosphate and free oxalic acid are formed, and the latter condenses with the diphenylamine to aniline blue and colours the hot phosphoric acid blue. The colour disappears on cooling. Dissolve the melt in alcohol, when a blue colouration appears. Pour the alcoholic solution into water thus precipitating the excess of diphenylamine, which is coloured light blue by the adsorption of the dyestuff. The dye may be extracted from aqueous solution by ether. 

Interferences can be defined as any physical or chemical agents capable of either increasing or decreasing the degree of absorption usually achieved with the test element in aqueous solution. All interferences in flame emission have one common effect, namely, changing... 

Amino-2,3-dihydro-l,4-phthalazdione (3), known as luminol because oxidation of the substance is attended with a striking chemiluminescence, can be prepared in about 25 min. as follows. A mixture of 1 g. of 3-nitrophthalic acid and 2 ml. of an 8% aqueous solution of hydrazine in a 20 x 150-mm. test tube is heated over a free flame until the solid is dissolved, and 3 ml. of triethylene glycol is added. The solution... 

The most common qualitative analytical technique to detect the presence of sodium in aqueous solutions or in solids (which can be dissolved in water) is a flame test. A wire made of an inert material like platinum or nichrome is dipped in a solution and then held in a flame. Sodium ions emit an intense yellow flame, readily verifying the presence of sodium in the sample. 

Characters and Tests.— Ehombic prisms, soluble in water. Imparts a yellow colour to flame, indicating the presence of sodium. Diluted hydrochloric acid added to its aqueous solution causes eflervescence (due to the expulsion of sulphurous anhydride, recognisable by its odour), and the solution is, at the same time, rendered turbid by the precipitation of sulphur. The latter reaction distinguishes this salt from sodium sulphite. 

Characters and Tests. — Transparent oblique prisms has a saline and bitter taste effloresces on exposure to the air soluble in water. Heated in a porcelain crucible, it loses 55 9 per cent, of water. It imparts a yellow colour to flame, indicating the presence of sodium. With barium chloride its aqueous solution furnishes a white precipitate (barium sulphate) insoluble in nitric acid, showing that the salt is a sulphate. Its aqueous solution mixed with diluted hydrochloric acid gives no precipitate with platinic chloride, proving that it is neither a potassium nor an ammonium salt. 

Characters and Tests.—Sometimes in transparent prismatic crystals, but usually in white opaque masses. Soluble in water. It imparts a yellow colour to flame, showing the presence of sodium. On adding diluted hydrochloric acid to its aqueous solution, sulphurous anhydride (recognisable by its odour) is evolved with effervescence, and the solution remains bright and transparent. 

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