Anion Analysis: Acetate and Arsenate Ions

Anion Analysis

• The determination of the anions present in a given material or sample

Acetate Ion (C2H3O2)

• Dilute HCl can liberate Hac
• H2SO4 Conc'd forms volatile Hac with vinegar odor
• BaCl2 Solution causes No Visible Change (NVC)
• AgNO3 Solution causes No Visible Change (NVC)
• C2H5OH w/ con'd H2SO4 forms Ethyl Acetate, CH3COOC2H5 with a fruity odor
• FeCl3 forms a red-brown color or ppt
• The reaction is FeCl3 + 2H2O + Ac -------- Fe(OH)2Ac + 2HCl + Cl-

Arsenate Ion (AsO4)

• Dilute HCl causes No Visible Effect (NVE)
• BaCl2 in NH4OH forms a white precipitate of Barium Arsenate, Ba3(AsO4)2 which is soluble in acids and NH4Cl solution.
• AgNO3 causes No Visible Change (NVC)
• C2H5OH w/ con'd H2SO4 forms a dark-red precipitate of Ag3AsO4 which is soluble in acids and NH4Cl.
• Magnesia Mixture precipitates fine white crystals of MgNH4AsO4
• A similar precipitate is produced by the Phosphate ion
• H2S in HCl solution forms yellow precipitates of mixtures of As2S3 and As2S5
• These sulfides are soluble in alkaline sulfides like Ammonium polysulfides
• Ammonium Molybdate Sol'n precipitates yellow Ammonium Arsenomolybdate, (NH4)3ASO4.12M003 from a HNO3 solution
• A similar precipitate is produced by the Phosphate ion
• FeCl3 in Ammonium Acetate Sol'n precipitates yellowish-white Ferric arsenate, Fe2(HAsO4)3 or FeASO4, depending on the acidity of the solution

Arsenite Ion (AsO3)

• Dilute HCl causes No Visible Effect (NVE)
• BaCl2 in NH4OH forms a white precipitate of Barium Arsenite, Ba3(AsO3)2 from neutral solution.
• Formation may not occur in dilute solution
• AgNO3 precipitates yellow Ag3AsO3 from neutral solution
• The precipitate readily dissolves in acid and NH4OH solutions
• H2S precipitates readily yellow As2S3 in slightly acidic solution
• Ammonium Molybdate Sol'n forms no precipitate at 20°-30°C, but when heated with HNO3 gives the same precipitate as Arsenate
• Magnesia Mixture causes no precipitate
• KI Solution yields No Visible Reaction (NVR) with Arsenite, but reacts with arsenate ion

Borate Ion (BO2)

• Dilute HCl results in No Visible Effect (NVE)
• BaCl2 Solution precipitates white Ba(BO2)2 which is soluble in acid
• AgNO3 Solution in slightly acidic solution forms white AgBO2 precipitate, which is soluble in HNO3 and NH4OH
• C2H5OH w/ con'd H2SO4 produces a green flame when ignited due to the formation of volatile Triethyl Boron
• Turmeric dipped in the ion in HCl solution will turn turmeric paper from yellow to red or orange
• Iodides and Chromates may obscure the turmeric test
• Borax Bead Test consists of preparing a bead of borax (Na2B4O7.10H2O), sodium carbonate, or microcosmic salt (NaNH4HPO4) at the end of a piece of platinum or nichrome wire
• The presence of many metals are detected by fusing the sample in the bead
• The color imparted to the bead by the metaborate, oxide, or orthophosphate formed helps identify the presence of specific metals

Bromide Ion (Br2)

• Dilute HCl causes No Visible Reaction (NVR)
• H2SO4 Conc'd, when warmed, forms reddish-brown vapor of Br2
• BaCl2 Solution causes No Visible Reaction
• AgNO3 Solution precipitates pale yellow Silver Bromide, AgBr, which is insoluble in 0.25M NH4OH and HNO3, but soluble in conc'd NH4OH
• Chlorine Water readily oxidizes Br- to free Br2 which can be concentrated in CCl4 or CS2
• KMnO4 acidified solution will oxidize the ion to free Br2
• Iodide also gives a similar reaction
• Sulfur(S) and CN ions interfere with the test by reacting with the reducing agent
• No oxidizing effect from Fe(NO3)3, but it oxidizes Iodide ion

Carbonate Ion (CO3)

• Dilute HCl liberates CO2 gas which forms a white precipitate of CaCO3 with lime water
• This precipitate dissolves in excess CO2
• Sulfite gives the same reaction, but sulfite may be oxidized to sulfate before the test
• BaCl2 solution precipitates white Barium Carbonate, BaCO3 that is soluble in acids
• AgNO3 Solution precipitates white in neutral solution (Ag2CO3) which, when boiled, turns Brown due to formation of Ag20

Chlorate Ion (CIO3)

• Dilute HCl causes No Visible Reaction (NVR)
• H2SO4 Conc'd causes small EXPLOSIONS plus yellow ClO2 gas liberated, when small portions of the sample is added to this acid, indicating chlorates
• BaCl2 Solution causes No Visible Reaction
• AgNO3 Solution causes No Visible Reaction
• Reducing Agents with mild RDA reduce ClO4 to Chloride ion
• Aniline Sulfate in dilute H2SO4 gives a Blue color with Chromate ion

Chloride Ion (Cl2)

• Dilute HCl causes No Visible Effect (NVE)
• H2SO4 Conc'd liberates colorless HCl gas
• BaCl2 Solution results in No Visible Reaction
• AgNO3 Solution precipitates white AgCl which dissolves in 0.25M NH4OH
• Thiocyanate ion interferes with the chloride test

Chromate Ion (CrO4)

• Dilute HCl changes the color from yellow to orange
• BaCl2 Solution precipitates yellow Barium Chromate; BaCrO4, that is soluble in acids and concentrated NH4OH
• AgNO3 Solution precipitates Scarlet Silver Chromate which is soluble in NaOH
• Pb(NO3)3 Solution precipitates Yellow Lead chromate which is soluble in NaOH
• Reducing Agents turns Chromate ion (yellow) to Cr+2 (Green) in acid solutions by many reducing agents such as Sulfide, sulfite iodide ions, and MnCl2 solutions
• H2O2 with HCl gives a blue coloration in ether

Cyanide Ion (CN-)

• Dilute HCl may produce HCN gas, a DEADLY POISON
• H2SO4 Conc'd forms HCN gas, which when distilled close to a NaOH, will form NaCN
• FeSO4 Solution in a basic medium, Fe(CN6)-4 will form in Deep Blue-green color or ppt.
• BaCl2 Solution results in no precipitate in dilute solution
• AgNO3 Solution precipitates white AgCN which dissolves in excess cyanide
• Ammonium Polysulfide forms thiocyanate ion that produces a Red color with Ferric ion

Ferricyanide Ion [Fe(CN)6^-3]

• Dilute HCl slowly decomposes the ion
• No reaction with BaCl2
• AgNO3 forms Red Solid precipitates of Ag3Fe(CN)6 which may be soluble in saturated NaCl solution, as well as white precipitate of AgCl
• MnCl2 and HCl solution turns pink color to Brown
• CuSO4 solution causes Greenish-yellow precipitates of Cu3[Fe(CN)6]2
• Co(NO3)2 solution causes Red precipitates of Co3[Fe(CN)6]2
• Ferrous ion (Fe+2) IRON Salts give Dark-blue precipitates of Fe3[Fe(CN)6]2
• Ferric ion (Fe+3) IRON Salts gives Brownish to Green solution, but no precipitates
• CADMIUM Salts are insoluble in acidic solution

Ferrocyanide Ion [Fe(CN)6^-4]

• Dilute HCl does not decompose.
• BaCl2 solution causes precipitation of White Ba2Fe(CN)6 (Barium Ferrocyanide)
• AgNO3 solution causes precipitation of Silver Ag4Fe(CN)6 Ferrocyanide.
• Which slowly turns in BLUE
• Ferrous sulfate solution IRON Salts reacts to produce a very Light-blue precipitate of Fe2Fe(CN)6
• Ferric Salts IRON Salts gives Dark-blue Fe2Fe(CN)6 ppt
• CADMIUM Salts are insoluble in acidic solution

Fluoride Ion (F2)

• Dilute HCl results in No Visible Effect (NVE)
• H2SO4 liberates H2F2 gas with concentrated acid.
• CaCl2 solution precipitates white CaF2 is formed.
• AgNO3 solution generates no precipitate.
• Etching Glass Test is based on the property of H2F2 to etch glass by the equation below.
• SiO2 (glass) + 2 H2F2 - SiF4 + 2H2O
• Water Drop Test results in cloudy end of glass rod or tubing due to formation of H4SiO4. with SiF4 production from glass etching.
• Zirconium-Alizarin Lake Test is a colorimetric determination of fluorine in water depending on fading of a zirconium-alizarin lake when fluorine is present
• The pink color fades to yellow with the amount and speed of fading in direct proportion to the presence of fluorine
• Creating zirconium-alizarin indicator requires dissolve 0.17 gm alizarin sodium sulphonate in 100 ml distilled water, and dissolve 0.87 gm zirconium nitrate in 100 ml distilled water while adding it slowly with stirring

Hypochlorite Ion (CLO-)

• Dilute HCl rapidly decomposes:
• BaCl2 solution produces no precipitate
• AgNO3 no immediate precipitate, but decomposes forming white precipitates of AgCl
• MnCl2 in HCl changes pink color to brown in HCl solution
• Pb Acetate solution generates Brownish-yellow lead acetate ppt. on warm Basic solution
• Metallic Mercury Test with NaClO shaken produces yellowish-red Hg2CIO
• Indigo Solution decolorizes.
• Starch - KI Paper Test with Cl2 gas produced from hypochlorite will change the color of the blue paper to brown
• This is a filter paper impregnated with Potassium lodide and starch
• The iodine reacts with the starch to form a distinctive blue-black colored paper.

Iodide Ion (I2)

• Dilute HCl results in No Visible Effect (NVE)
• Conc'd H2SO4, when warmed, decomposes iodides and liberates Iodine vapor, which is VIOLET
• BaCl2 solution results in No Visible Effect -NVE
• AgNO3 solution precipitates Yellow AgI, which is insoluble in ammonia (NH3) solution
• HgCl solution forms Scarlet red precipitates of HgI2, which is soluble in excess Iodide ion due to the formation of HgI4
• Oxidizing Agents oxidize Iodide ions to Iodine which is detected by concentrating it with CCl4 (purple), or by allowing it to react with starch solution, turning it from blue to black (indigo)
• The above reactions are NEGATIVE to Bromides and Chlorides

Nitrate Ion (NO3)-

• Dilute HCl results in No Visible Effect - NVE
• Conc'd H2SO4 produces HNO3 which is colorless in pure form, but some decomposition usually occurs, and the BROWN gas NO2 is given off
• BaCl2 solution causes No Reaction
• AgNO3 solution causes No Reaction
• MnCl2 - HCl solutions changes the Pink color to Brown
• BROWN - RING TEST occurs when nitrate ions mixed with FeSO4 solution combines with conc'd and a BROWN Ring forms at the junction of the two liquids
• The Brown Ring test will also give a positive result to Nitrite, Iodides, Bromides, and Chromates, while other colored ions will interfere with the test
• NESSLER'S Reagent or Potassium Tetraiodomercurate(II) - is an aqueous solution of potassium iodide, mercuric chloride, and potassium hydroxide, used as a test for the presence of ammonia
• Nitrate ion is reduced to NH3 gas
• If the ammonia gas is allowed to react with Nessler's reagent a RED precipitate will form
• Brucine in H2SO4 yields a Deep-Red color in the absence of Chlorates
• An environmental test reveals when a water sample contains Nitrate ion and is treated with Brucine in sulfuric acid, a yellow solution results

Nitrite Ion (NO2)-

• Dilute HCl solution decomposes giving an NO2 Brown gas.
• BaCl2 solution produces no precipitate
• AgNO3 solution yields no precipitate in dilute solution, but it generates a White precipitate of AgNO2 in concentrated solution of the ion
• MnCl2 - HCl solutions changes the pink solution to Brown color
• KI solution is in acidified solution where the iodide is oxidized to lodine
• FeSO4 solution gives, in acidified solution, a Dark-brown color
• Oxidizing Agents easily oxidize Nitrite to Nitrate (NO3)
• Thiourea Test evolves Nitrogen (N2) gas and when Fe(III) ion is added creates a Deep-Red colored solution
• Cobalt Salts of Cobaltous salts with Potassium chloride and Nitrites give a Yellow ppt of K3Co(NO2)6

Oxalate Ion (C2O4)

• Dilute HCl - NVE
• Conc'd H2SO4 decomposes oxalates into H2O, CO2, and CO
• CaCl2 solution forms white precipitate of CaC2O4.
• AgNO3 solution forms white precipitate of Ag2C2O4 which is soluble in acid
• KMnO4 Solution is decolorized with acidified Sulfuric acid if heated to 60°C.
• MnSO4 solution when alkaline with NaOH produces a brown color and oxalate produces a RED color
• Resorcinol and H2SO4 where the oxalate is added and the mixture is heated until fumes of SO3 are emitted generates a RED color

Silicate Ion (SiO4)

• Dilute HCl results in No Visible Effect (NVE)
• CaCl2 solution forms white precipitate of CaSiO3
• Water Drop Test - this test for Fluoride ion may be performed for Silicate ions also
• Benzidine Solution is used from the water drop test, concentrated HNO3 and ammonium molybdate are added followed by Benzidine solution to indicate a BLUE coloration resulting in a positive result

Sulfate Ion (SO4)

• Dilute HCl results in No Change
• BaCl2 Solution yields an immediate white precipitate of BaSO4, which is insoluble in acids and ammonia, and forms a colloidal dispersion that is hard to filter unless boiled
• NOTE: Sulfite ion is oxidized and will interfere.
• AgNO3 solution produces No precipitate from dilute solutions
• Reduction with Charcoal reduces the sulfate when heated to Sulfide (S2)
• Placement of the sulfide produced on a Silver coin yields black Ag2S will be produced

Sulfite Ion (SO3)

• Dilute HCl results in an immediate evolution of sulfur dioxide gas
• Thiosulfate also produces the same gas
• BaCl2 solution yields a white precipitate of BaSO3, which is easily dissolved in acids with evolution of SO2 gas
• With oxygen from the air; this precipitate becomes insoluble in acid on standing because of the oxidation of SO3 to SO4
• AgNO3 solution gives a white precipitate of Ag2SO3 at 20° - 30°C.
• Boiling this solution will decompose it and precipitate metallic Silver.
• Pb(NO3)2 Solution forms white precipitates of PbSO4 which easily dissolves in dilute HNO3 and are not decomposed upon boiling
• Oxidizing Agents that exhibit Acidified KMnO4 solution, is decolorized by Sulfite like Oxalate
• Ferric lons is also reduced to Ferous ions by Sulfites
• Starch - HIO3 Paper Test results in - the ion that will liberate iodine from HIO3, which is indicated by the blue color
• Ba(OH)2 Solution with SO2 gas from Sulfites forms a white precipitate of BaSO3 which when oxidized to BaSO4
• Red solution of Fuchsin (Magenta) is is decolorized when sulfur dioxide gas is bubbled through it.

Sulfide Ion (S2)

• Dilute HCl produces an immediate evolution of H2S occurs, while this test is also positive to thiosulfates
• No reaction with BaCl2 solution
• AgNO3 solution forms Black precipitates of Ag2S that is insoluble in cold mineral acids, but soluble in hot HNO3
• Sodium Nitroprusside Solution yields a solution of this compound gives a PURPLE color with 2NaS.
• Metallic Ions give most metallic ions give insoluble sulfides, particularly from an alkaline solution
• Pb Acetate test is done by exposing a piece of paper wet with Pb Acetate solution to Hydrogen sulfide gas produced by treating a sulfide with an acid
• The sulfide is confirmed with a BLACK precipitate of PbS

Thiocyanate Ion (SCN)

• Dilute HCl results in No Visible Effect (NVE)
• AgNO3 - white precipitates of AgSCN
• It dissolves in excess ion due to formation of Ag(SCN)3 or Ag(SCN)4 ions
• FeCl3 solution yields Blood red coloration due to Fe(SCN)6 ion formation
• CoCl solution causes the solution to change from pink color to BLUE because of Co(SCN)4 ion formation.

Thiosulfate Ion (S2O3)

• Dilute HCl decomposes readily to a gas forming a variety of products, among which are: H2S, SO2, and S2
• BaCl2 Solution - WHITE precipitates of BaS2O3 which is soluble in acids, with decomposition of the S2O3 ion.
• AgNO3 solution - WHITE precipitates of Ag2S2O3 which turns to BROWN and then to BLACK due to the formation of Ag2S
• AgS2O3 is soluble in the presence of excess amount of the ion due to the formation of Ag(S2O3)2 ions
• Iodine Solution - BROWN solution of iodine in Hac is decolorized
• SbCl3 solution - acidified and heated gives an ORANGE precipitates of Sb2S3
• Sulfide ions - gives the same result

Phosphate Ion (PO4)

• Dilute and Conc'd HCl yields NVR since phosphates are nonvolatile ions
• Conc'd H2SO4 results in NVR
• AgNO3 solution - yellow precipitate of Ag3PO4 forms
• The precipitates are soluble in dil. HNO3 due to the lower concentration yield which is weaker than nitric acid diluting insufficient concentration
• AgNO3 solution precipitates yellow Ag3PO4 forms.
• The precipitates are soluble in ammonia solution, due to the formation of the complex ion [Ag(NH3)2]+
• The precipitate is insoluble in acetic acid
• BaCl2 Solution - White precipitates of, the secondary salt (BaHPO4) from neutral medium, or of the more insoluble tertiary salt, Ba3(PO4)2,from ammoniacal or dilute alkaline solutions
• The precipitates are soluble in dilute acids including acetic acid
• The Magnesia Mixture reagent solution forms white crystalline precipitate with phosphates in neutral or ammoniacal solution
• The precipitate is soluble in acetic acid and in mineral acids.