Drop-reaction paper

Spot plates. Drop-reaction paper These are employed chiefly for confirmatory tests (see Section II.6 for a full discussion). Spot plates with a number of circular cavities are marketed, either black or white. The former are employed for white or light-coloured precipitates, and the latter for dark-coloured precipitates. Transparent spot plates (e.g. Jena) and combination black and white spot plates (with line of demarcation between the black and white running exactly through the centres of the depressions) are also available commercially. Spot plates are useful for mixing small quantities of reagents, and also for testing the pH of a solution colorimetrically. 

Confirmatory tests for ions may be carried out either on drop reaction paper or upon a spot plate. The technique of spot tests is fully described in Section II.6. 

Spot reactions upon filter paper are usually performed with Whatman drop reaction paper No. 120, but in some cases Whatman No. 3 is utilized the Schleicher and Schuell (U.S.A.) equivalents are Nos. 601 and 598. These papers possess the desirable property of rapidly absorbing the drops without too much spreading, as is the case with thinner papers. Although impurities have been reduced to minimum values, these papers may contain traces of iron and phosphate spot reactions for these are better made with quantitative filter paper (Whatman No. 42 or, preferably, the hardened variety No. 542). The paper should be cut into strips 6x2 cm or 2 x 2 cm, and stored in petri dishes or in vessels with tightly-fitting stoppers. 

The apparatus illustrated in Fig. 11.52 may be employed when the filter paper (or drop reaction paper) must be heated in steam the filter paper is placed on the side arm support. By charging the flask with hydrogen sulphide solution, ammonia solution, chlorine or bromine water, the apparatus can be used for treating the filter paper with the respective gases or vapours. 

Place a drop of the test solution upon drop-reaction paper, and treat successively with 2 drops 3m sodium hydroxide and 1 drop saturated bromine water. Add 2 drops 1 1 ammonia solution remove the excess ammonia by waving the paper over a small flame. Add 2 drops reagent a blue colour develops. Sensitivity 1 pg Pb. Concentration limit 1 in 50,000. 

In the presence of silver, bismuth, cadmium, or copper, proceed as follows. Transfer a drop of the test solution to a drop-reaction paper and add a drop of m sulphuric acid to fix the lead as lead sulphate. Remove the soluble sulphates of the other metals by washing with about 3 drops of m sulphuric acid, followed by a little 96 per cent ethanol. Dry the paper on a water bath, and then apply the test as detailed above. 

Under similar circumstances mercury(II) ions do not react. The spot test technique is as follows. Place a drop of the faintly acid test solution upon drop-reaction paper and add a drop of 50 per cent potassium nitrite solution. A black (or dark grey) spot is produced. The test is highly selective. Coloured ions yield a brown colouration which may be washed away, leaving the black spot. 

Spot test place a drop of the test solution on a watch glass or on a spot plate, add a drop of ammonium carbonate solution and stir (this renders any mercury(I) or lead ions unreactive by precipitation as the highly insoluble carbonates). Remove one drop of the clear liquid and place it on drop-reaction paper together with a drop of the potassium chromate reagent. A red ring of silver chromate is obtained. 

Test on filter paper Moisten a piece of drop-reaction paper with the reagent and place a drop of the slightly acid test solution upon it. An orange-red spot is obtained. 

Thiourea (10%) intense yellow complex with bismuth(III) ions in the presence of dilute nitric acid. The test may be carried out on drop-reaction paper, on a spot plate, or in a micro test-tube. 

Treat some drop-reaction paper with a drop of the weakly acid test solution and a drop of the reagent, and then hold it over ammonia vapour. A green colouration is obtained. 

Place a drop of the neutral test solution upon drop-reaction paper, expose it to ammonia vapour and add a drop of the reagent. A black or greenish-black spot is produced. 

In the presence of nickel, proceed as follows. Impregnate drop-reaction paper with the reagent and add a drop of the test solution acidified with acetic acid, (2m). Two zones or circles are formed the central olive-green or black ring is due to copper and the outer blue-violet ring to nickel. 

Place a drop of the reagent upon drop-reaction paper, add one drop of the test solution (which should be slightly acidified with acetic acid (2m) containing a little sodium potassium tartrate), and then one drop potassium hydroxide (2m). A bright-pink spot, surrounded by a blue circle, is produced. 

For minute quantities of arsenic, it is convenient to use the apparatus depicted in Fig. 11.56. Mix a drop of the test solution with a few grains of zinc and a few drops of dilute sulphuric acid in the micro test-tube. Insert the funnel with a flat rim, and place a small piece of drop-reaction paper moistened with 20 per cent silver nitrate solution on the flat surface. A grey stain will be obtained. 

Place a drop of the test solution upon drop-reaction paper which has been impregnated with the phosphomolybdic acid reagent and hold the paper in steam. A blue colouration appears within a few minutes. 

Impregnate drop-reaction paper with a solution of phosphomolybdic acid and then hold it for a short time over ammonia gas to form the yellow, sparingly soluble ammonium salt dry. Place a drop of the test solution on this paper a blue spot appears if tin(II) is present. 

Treat a drop of the faintly acidified test solution with 1 drop of the reagent on a spot plate a red colouration is obtained. Alternatively, treat drop-reaction paper (Whatman No. 3 M.M., 1st quality) which has been impregnated with the reagent and dried, with a drop of the test solution a red or pink spot is produced. 

Soak some quantitative filter (or drop-reaction) paper in a saturated alcoholic solution of alizarin and dry it. Place a drop of the acid test solution on the paper and hold it over ammonia fumes until a violet colour (due to ammonium alizari-nate) appears. In the presence of large amounts of aluminium, the colour is visible almost immediately. It is best to dry the paper at 100°C when the violet colour due to ammonium alizarinate disappears owing to its conversion into ammonia and alizarin the red colour of the alizarin lake is then clearly visible. 

The potassium hexacyanocobaltate(III) (or cobalticyanide) test paper is prepared by soaking drop-reaction paper or quantitative filter paper in a solution containing 4 g potassium hexacyanocobaltate(III) and 1 g potassium chlorate in 100 ml water, and drying at room temperature or at 100°C. The paper is yellow and keeps well. 

In the presence of strontium the reddish-brown stain of barium rhodizonate is treated with 0-5m hydrochloric acid the strontium rhodizonate dissolves, whilst the barium derivative is converted into the brilliant-red acid salt. The reaction is best carried out on drop-reaction paper as above. 

If barium is present, proceed as follows. Impregnate some quantitative filter paper or drop-reaction paper with a saturated solution of potassium chromate, and dry it. Place a drop of the test solution on this paper and, after a minute, place 1 drop of the reagent on the moistened spot. A brownish-red spot or ring is formed. 

Filter or drop-reaction paper should not be used. 

The spot-test technique is as follows. Mix a drop of the test solution with a drop of concentrated sodium hydroxide solution on a watch glass. Transfer a micro drop of the resulting solution or suspension to drop-reaction paper and add a drop of Nessler s reagent. A yellow or orange-red stain or ring is produced. 

Mix 2 drops 5 per cent tannic acid (tannin) solution with 2 drops 20 per cent silver nitrate solution, and place the mixture upon drop-reaction paper or upon a little cotton wool. Hold the paper in the vapour produced by heating an ammonium salt with sodium hydroxide solution. A black stain is formed on the paper or upon the cotton wool. The test is a sensitive one. 

Note that the oxidation number of copper in the solution is +1. The test is best carried out on filter paper or drop-reaction paper and is applicable in the presence of chlorides, bromides, iodides, hexacyanoferrate(II) and (III) ions. 

An alternative procedure is to employ quantitative filter paper or drop-reaction paper which has been impregnated with an ammoniacal solution containing 0-1 g of copper sulphate per 100 ml and dried. Immediately before the test a little hydrogen sulphide is blown on to the paper so that it acquires a uniform brown colour. Place a drop of the test solution upon this paper when a white ring will be obtained. 

Mix a few drops of the test solution with a little dilute sulphuric acid in a micro test-tube and tie (or otherwise fix) a piece of drop-reaction paper which has been moistened with a mixture of equal parts of the copper acetate and benzidine reagents to the top of the tube. A blue colouration is produced. 

Alternatively, place a drop of the test solution (or a few milligrams of the test solid) in the reaction bulb of Fig. 11.56, add 2 thin pieces zinc foil and 2-3 drops dilute sulphuric acid. Place a small circle of quantitative filter paper (or drop-reaction paper) moistened with acetate-benzidine acetate reagent across the funnel. The paper is coloured blue by the hydrocyanic acid carried over with the hydrogen. 

The zirconium-alizarin-S paper is prepared as follows. Immerse quantitative filter paper (or drop-reaction paper) in a 10 per cent solution of zirconium nitrate in 2m hydrochloric acid, drain and place in a 2 per cent aqueous solution of alizarin-S. Wash the paper, which is coloured red-violet by the zirconium lake, until the washings are nearly colourless and then dry in air. 

Place a drop of barium chloride solution upon filter or drop-reaction paper, followed by a drop of a freshly prepared 0-5 per cent aqueous solution of sodium rhodizonate. Treat the reddish-brown spot with a drop of the acid or alkaline test solution. The coloured spot disappears. 

The lead sulphide paper is prepared by soaking drop-reaction paper in a 0 0025m solution of lead acetate, exposing it to a little hydrogen sulphide gas and then drying in a vacuum desiccator. The paper will keep in a stoppered bottle. 

Drop-reaction paper, treated with tannic acid and silver nitrate solution (see Section III.38, reaction 7), may also be used. 

Alternatively, drop-reaction paper treated with 2-3 drops of 5 per cent tannic acid solution and 2-3 drops of 20 per cent silver nitrate solution may be used it is blackened by ammonia. 

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