Soda extract

Full experimental details of the various tests referred to in this section will be found under the reactions of the anions. Reference will be made to the soda extract and to the neutralized soda extract (see 20 below) as these are normally used in systematic analysis. A solution of the sodium salts of the various anions may, of course, be used when trying out these procedures. 

It must be emphasized that these reactions take place only in acid media. Add solid potassium peroxodisulphate and dilute sulphuric acid to the neutralized soda extract of the mixed halides contained in a conical flask heat the flask to about 80°, and aspirate a current of air through the solution with the aid of a filter pump until the solution is colourless (Fig. IV.2. T is a drawn out capillary... 

Method B Acidify the neutralized soda extract (see 20 below) of the halides with dilute nitric acid, add excess silver nitrate solution, filter, and wash until the washings no longer give the reactions of silver ions. Shake the precipitate with ammonium carbonate solution, filter, and add a few drops of potassium bromide solution to the filtrate a yellowish-white precipitate of silver bromide is obtained if a chloride is present. 

Chloride in the presence of iodide Add excess silver nitrate solution to the neutralized soda extract (see 20) and filter reject the filtrate. Wash the precipitate with dilute ammonia solution and filter again. Add dilute nitric acid to the washings a white precipitate of silver chloride indicates the presence of chloride. 

Bromide and iodide in the presence of each other and of chloride The presence of a chloride does not interfere with the reactions described below. To the soda extract, strongly acidified with dilute hydrochloric acid add 1-2 drops of chlorine water (the solution obtained by carefully acidifying a dilute solution of sodium hypochlorite with dilute hydrochloric acid may also be used) and 2-3 ml chloroform or carbon tetrachloride shake a violet colour indicates iodide. Continue the addition of chlorine water or of acidified sodium hypochlorite solution drop by drop to oxidize the iodine to iodate and shake after each addition. The violet colour will disappear, and a reddish-brown colouration of the chloroform or carbon tetrachloride, due to dissolved bromine (and/or bromine monochloride, BrCl), will be obtained if a bromide is present. If iodide alone is present, the solution will be colourless after the violet colour has disappeared. 

Chloride, chlorate, and perchlorate in the presence of each other Each anion must be tested for separately. Divide the soda extract into 3 parts. 

Test for iodide in the neutralized soda extract or in a solution of the sodium salts by the addition of a few drops of chlorine water (or acidified sodium hypochlorite solution) and 2-3 ml chloroform or carbon tetrachloride the latter is coloured violet. Add excess silver sulphate solution to another portion of the neutral solution and filter off the silver iodide remove the excess silver sulphate with sodium carbonate solution. Pass sulphur dioxide into the filtrate to reduce iodate to iodide, boil off the excess sulphur dioxide, and add silver nitrate solution and dilute nitric acid. A yellow precipitate of silver iodide confirms the presence of iodate in the original substance. 

Acidify the soda extract with dilute hydrochloric acid, pass in sulphur dioxide to reduce the arsenate to arsenite, boil off the excess sulphur dioxide (test with potassium dichromate paper), and pass hydrogen sulphide into the solution to precipitate the arsenic as arsenic(III) sulphide continue the passage of hydrogen sulphide until no more precipitate forms. Filter, boil off the hydrogen sulphide, and test the filtrate for phosphate by the ammonium molybdate test or with the magnesium nitrate reagent. 

An alternative method for the elimination of arsenate is the following. Acidify the soda extract with dilute hydrochloric acid and then add about one-quarter of the volume of concentrated hydrochloric acid (the total volume should be about 10 ml). Add 0-5 ml of 10 per cent ammonium iodide solution, heat to boiling, and pass hydrogen sulphide into the boiling solution until precipitation is complete (5-10 minutes). Filter off the arsenic(III) sulphide, and boil off the hydrogen sulphide from the filtrate. Add ammonia solution until alkaline, and excess of the magnesium nitrate reagent. A white precipitate indicates the presence of a phosphate. 

A mixture of the sodium salts or soda extract is employed for this separation. 

Acidify the neutralized soda extract of the sodium salts with acetic acid, add excess of a solution of thorium nitrate and a little Gooch asbestos and shake well filter. 

Table IV.5 Separation and detection of organic acids Add CaCl2 solution to the cold neutralized soda extract, rub sides of vessel with a glass rod, allow to stand for 10 minutes with occasional shaking filter.

Acetate If other organic acids are present, obtain the solution of mixed acids as in (ii) otherwise, use the neutralized soda extract. Boil under reflux with an equal volume of potassium dichromate solution and dilute sulphuric... 

Cyanides, thiocyanates, hexacyanoferrate(II)s, and hexacyanoferrate(III)s also yield ammonia under these experimental conditions. The reaction is somewhat slower for these anions up to 5 minutes may elapse before ammonia can be detected from hexacyanoferrate(II)s and hexacyanoferrate(III)s. If these are present, or are suspected as a result of the preliminary tests, particularly that with concentrated sulphuric acid, they must first be removed as follows. Treat the soda extract with excess of nitrate-free silver sulphate, warm the mixture to about 60°, shake vigorously for 3-4 minutes, and filter from the silver salts of the interfering anions and excess of precipitant. Remove the excess silver ions from the filtrate by adding excess sodium hydroxide solution and filter off the precipitated silver oxide. Evaporate the filtrate to about half bulk and test with zinc, aluminium or Devarda s alloy. If cyanides alone are present, they may be rendered innocuous by the addition of a little mercury(II) chloride solution. 

The preparation of this soda extract can be carried out according to the instructions in Table V.29. 

Table V.29 Preparation of the soda extract for testing for anions...

With this soda extract the following tests should then be carried out ... 

Sulphate test To 2 ml of the soda extract add dilute hydrochloric acid until acid (test with litmus paper) and then add 1-2 ml in excess. Boil for 1-2 minutes to expel carbon dioxide completely, and then add about 1 ml barium chloride solution. A white precipitate indicates sulphate. Confirm by the charcoal test (Section IV.24, reaction 1). 

To 2 ml of the soda extract add 1 ml concentrated hydrochloric acid and 2 ml of the manganese(II) chloride reagent. A brown (or black) colouration indicates the presence of nitrate, nitrite, hexacyanoferrate(III), chlorate, bromate, iodate, chromate or permanganate. A negative test indicates the absence of the above oxidizing agents except small amounts of nitrates and nitrites and of arsenate if reducing anions have been found, this test is inconclusive. 

Tests with silver nitrate solution The separation of a comparatively large number of anions from the soda extract into several groups is possible with this reagent. 

Thiosulphate will be detected in the preliminary test with dilute sulphuric acid if it is found, it should be eliminated by heating the original mixture with dilute sulphuric acid until no more sulphur dioxide is evolved, evaporating the residual mixture just to dryness and then heating with 1-5m sodium carbonate solution, etc. The occurrence of thiosulphates in mixtures is comparatively rare and hence this special treatment prior to making the soda extract is rarely necessary. The interference of the S2Of ion with the silver nitrate reaction may... 

Cyanide. This should have been detected and confirmed in the preliminary test with dilute sulphuric acid (Prussian blue test or as Section IV.8, reaction 1). Sulphite. This anion will have been detected in the preliminary test with dilute sulphuric acid (potassium dichromate paper or fuchsin solution test). Hexacyanoferrate(II) (and Thiocyanate). Acidify 1 ml of the soda extract with dilute hydrochloric acid and add a few drops of iron(III) chloride solution. A deep-blue precipitate indicates hexacyanoferrate(II) present. Now add 0-5-1 ml iron(III) chloride solution, 0-2 g sodium chloride and half a Whatman filtration accelerator, shake the mixture vigorously and filter. A deep-red filtrate indicates thiocyanate present. 

Hexacyanoferrate(III). Acidify 1 ml of the soda extract with dilute hydrochloric acid and add a few drops of freshly prepared iron(II) sulphate solution. A deep-blue precipitate indicates hexacyanoferrate(III) present. This precipitate should not be confused with that of light-grey-blue colour produced by a hexacyano-ferrate(II). 

Sulphide, cyanide, and sulphite. Acidify 10 ml of the soda extract with dilute acetic acid and boil gently for 3-4 minutes make certain that the solution remains acid (e.g. to litmus) throughout. If sulphite is present, it is advisable to heat the solution for 10-15 minutes (maintaining the volume, if necessary) whilst a stream of air is drawn through it (cf. Fig. IV.2) test for complete removal of sulphur dioxide with fuchsin solution. 

Hexacyanoferrate(II) and (III). Employ the original solution from 1 or, if sulphide, cyanide, and sulphite are absent, use 10 ml of the soda extract made acid (to Congo red) with dilute acetic acid. Add 0 5m zinc nitrate solution until precipitation ceases, introduce a Whatman filtration accelerator, stir, and filter the zinc and hexacyanoferrate(II) and (III) in the cold wash with a little O 1m zinc nitrate solution. 

Use the filtrate from 2 or, if hexacyanoferrate(H) and/or (III) and other interfering anions are absent, acidify 10 ml of the soda extract cautiously with dilute HNOa (to litmus or other indicator paper). Add one-tenth of the volume of concentrated HN03, stir for 30 seconds and then add AgN03 solution with stirring until precipitation is complete. Heat to the boiling point, allow the ppt. to settle, cool, and filter. Wash the ppt. with 2-3 ml 2m nitric acid. 

Notes to Table V.31 1. The soda extract may also be used for the... 

Test with calcium chloride solution For tests 5-7 a neutralized soda extract is required. This is prepared as follows. Take 10 ml soda extract in a porcelain dish and render it faintly acid with dilute nitric acid (use litmus paper or other equivalent test-paper). Boil for 1-2 minutes to expel carbon dioxide, allow to cool, then add dilute ammonia solution until just alkaline and boil for 1 minute to expel the slight excess of ammonia. Divide the solution into three equal parts for tests 5, 6, and 7. 

Test with iron( HI) chloride solution T reat the second third of the neutralized soda extract drop by drop with iron(III) chloride solution until no further change occurs. The results of this test are summarized in Table V.32. 

As a general rule, thiosulphate will not be found here as it should have been more or less completely decomposed in the preparation of the neutralized soda extract. 

Test for silicate To the remaining third of the neutralized soda extract add ammonium chloride and ammonium carbonate solution. A gelatinous precipitate indicates silicate. 

The test may also be conducted with 2 ml of the soda extract. Carbonate, sulphite, and thiosulphate have no influence upon the reaction. Nitrite interferes, presumably owing to the oxidation of the hydrogen cyanide. In the presence of sulphide, the test is complicated by the precipitation of black iron(II) sulphide when sulphate is added to the alkaline solution. It is best to boil the solution containing the suspended iron(II) sulphide, acidify with hydrochloric acid, and boil again to expel most of the dissolved hydrogen sulphide upon adding a drop of iron(III) chloride solution, a blue precipitate is produced if cyanide is present. 

Test for chromate If the soda extract is colourless, chromate is absent. If yellow, chromate may be present hexacyanoferrate(II) and (III) also impart a yellow colour to the solution. The presence of chromate will have been indicated by the precipitation of green chromium(III) hydroxide in the silver nitrate solution tests (Table V.31) and also in the analysis for cations. 

To confirm chromate, acidify 2 ml of the soda extract with dilute sulphuric acid, boil for one minute to expel C02, etc., filter if necessary, add 1-2 ml amyl alcohol, followed by 1-2 ml of 10-volume hydrogen peroxide and shake gently (compare Section IV.33, reaction 4). A blue colouration of the amyl alcohol layer confirms chromate. 

The presence of iodate in the soda extract can be readily detected as follows. Treat 2 ml of the solution with silver nitrate solution until precipitation ceases, heat to boiling for 2-3 minutes and filter. Render the filtrate strongly acid with hydrochloric acid, add 2 ml of 0-5m iron(II) sulphate solution (or 0 5m sodium sulphite solution) and shake it with 2 ml carbon tetrachloride. A purple colouration of the organic layer indicates iodate. 

This test utilizes the fact that silver iodide, but not silver iodate, is precipitated upon the addition of silver nitrate to the soda extract. 

Acidify 3 ml of the soda extract with concentrated perchloric acid and add 1 ml in excess (note 2 below) add silver nitrate solution, slowly and with stirring, until precipitation is complete. Filter and collect the filtrate in a ground-glass stoppered conical flask or test-tube. Add m sodium chloride solution, 05 ml at a time, to the filtrate until no more precipitate forms. Stopper the flask or test-tube and shake the mixture vigorously after each addition. Filter off the silver chloride (note 3 below), transfer the filtrate to the stoppered vessel, add an equal volume of concentrated hydrochloric acid (if a precipitate of sodium chloride forms, filter), cool, then add 1-2 g solid iron(II) ammonium sulphate and 2 ml carbon tetrachloride. Shake the mixture intermittently for 5 minutes. A purple colour in the carbon tetrachloride indicates periodate present. 

Testing for anions in solution This includes the preparation of a soda extract , followed by systematic tests. These, in turn, can be made conclusive by confirmatory tests. 

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