Anion Analysis

Carbonate and Borate Ions

Some of the original sample mixture is placed in a test tube and 2 M HCl is added. If the analysis contains the CO32– ion, HCl releases CO2↑­, which is seen as bubbling (mixture is stirred). Turmeric paper is soaked with the solution of the previous reaction (acidic solution) and dried. The paper will turn red when dry, if the analysis contains the H2BO3 ion.

 Sulfide Ion

Some of the original sample mixture is placed in a test tube and conc. HCl is added. Filter paper soaked with Na2[Pb(OH)4] solution is brought to the mouth of the test tube, which is heated in a water bath. If the analysis contains the S2 ion, conc. HCl releases H2S↑­ and the filter paper darkens as PbS↓ is formed. If the paper does not change colour, but a black, red or yellow precipitate remains in the test tube, Devarda’s alloy is added and the paper is brought to the mouth of the test tube once more. Devarda’s alloy reduces the potential metal ions of the sulfides that are insoluble in conc. HCl and H2S­↑ can be released.

 Nitrate Ion

Some of the original sample mixture (which does not contain NH4+ ions) is placed in a test tube and 2 M NaOH is added (solution is made basic) as well as Devarda’s alloy. Wet pH paper is brought about 1 cm above the test tube which is heated in a water bath. If the analysis contains the NO3ion, Devarda’s alloy reduces it to NH3↑­, which turns the pH paper blue. If the original sample mixture contains NH4+ ions, 5 drops of the alkali salt solution is made acidic with 2,5 M H2SO4 and then heated (CO2↑­). Then continuing as before (starting with adding 2 M NaOH). Alternatively, the NO3ion can be determined with iron(II) sulfate from the alkali salt solution.

 Sulfate Ion

3 drops of the alkali salt solution (or original mixture soluble in nitric acid) is made acidic with 2,5 M HNO3 in a test tube and then heated (CO2↑­). 0,5 M of BaClsolution is added (if original mixture is used and analysis includes cation group 1, use barium acetate). If a preciptate is formed, BaClsolution is added as long as precipitation continues, whilst stirring and heating. The potential precipitate is separated from the solution. 2 M HCl is added to the precipitate and then stirred. If white BaSO4↓ remains, the analysis contains the SO42– ion.

Chloride, Iodide and Bromide Ions

3 drops of the alkali salt solution (or original mixture soluble in nitric acid) is made acidic with 2,5 M HNO3 in a test tube and then heated (CO2↑­). AgNO3 solution is added. If no precipitate is formed, the analysis does not contain halide ions. If a precipitate forms, the solution contains halide ions and AgNO3 solution is added while stirring as long as precipitation continues. The precipitate (AgCl↓/AgBr↓/AgI↓) is left waiting the potential detection of the Clion.

3 drops of the alkali salt solution (or original mixture soluble in hydrochloric acid) is made acidic with 2 M HCl in a large test tube and then heated (CO2↑­). 0,5 ml of chloroform is added and then chlorine water. The solution shaken vigorously (tube closed with a bung). If the analysis contains the Iion, Cl2­ oxidizes it into violet I2­, the colour of which can be seen in the chloroform. Chlorine water is added until the violet colour disappears as I2­↑ is oxidized into IO3 ions. If the analysis contains the Brion, Cl2­ oxidizes it into yellow-brown Br2­↑, the colour of which can be seen in the chloroform.

If the analysis contains the Brion, find an appropriate method of determining the Clion from literature. If the analysis does not contain Br ions, the AgCl↓/AgI↓ made with AgNOsolution previously is centrifuged from the supernate. 2 M NH3 is added to the precipitate and any potential AgCl↓ dissolves as [Ag(NH3)2]+ ions. The solution is separated from any potential AgI↓ and made acidic with 2,5 M HNO3, causing AgCl↓ to precipitate again, if the analysis contains the Clion.

Arsenite and Arsenate Ions (These ions are determined alongside cation group 2. so it is not required that they are determined here.)

3 drops of the alkali salt solution is made clearly acidic with 5 M HCl. A few drops of thioacetamide is added and then heated. If yellow As2S3↓ is formed, the analysis contains arsenic (compare orange-red Sb2S3↓).

Phosphate Ion

If original mixture does not have iodide or bromide ions, In 2 drops of the alkali salt solution (or original mixture soluble in nitric acid) is added 4 drops of 2,5 M HNOand 2 drops of (NH4)2MoO4 solution.  The mixture is stirred and heated, causing potential PO43 and AsO43– ions to be precipitated as yellow (NH4)3[PMo12O40] and (NH3)3[AsMo12O40]↓.

If original mixture contains iodide or bromide ions, 2 drops of alkaline salt solution (or original mixture soluble in nitric acid) and 4 drops of conc. HNO3 is added to a boiling tube and heated. Nitric acid oxidizes iodide and bromide ions to form iodine and bromine, which are evaporated by boiling. After this PO43–  is detected with (NH4)2MoO4.

If a yellow precipitate was formed and if arsenic was detected, it is precipitated as As2S3↓, same as above. The precipitate is centrifuged from the supernate. The volume of the supernate is quadrupled with conc. HNO3. H2S↑­(and any potential I2↑­) is boiled away and free sulfur is removed. The PO43–-ion is detected from the solution with (NH4)2MoO4.

 Oxalate Ion

3 drops of the alkali salt solution (or original mixture soluble in acetic acid) is made weakly acidic with 2 M acetic acid and then heated (CO2↑­).  2 drops of 0,1 M Ca(CH3COO)2 solution (if the examined solution contains sulfate ions, saturated CaSO4 solution is used) is added to the solution and then heated. If a pale precipitate is formed, it may contain calcium oxalate. If this is the case, precipitation is continued as long as the precipitate continues to be formed. The precipitate is centrifuged from the supernate, washed with hot water and dissolved with a few drops of 2,5 M H2SO4. From this solution the oxalate ion is indicated  by adding a drop of 0,02 M KMnO4 solution while the solution is being heated. If the oxalate ion is present, the colour of the MnO4 ion disappears as oxalate reduces it into Mn2+ while oxidizing into CO2↑.