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Doc Brown's Chemistry  Qualitative Methods of Analysis Revision Notes

CHEMICAL identification TESTS Part 3 Qualitative tests to test for, and identify, cations (positive ions)

Tests for the following cations - positive ions - mainly metal ions: ammonium ion NH4+, hydrogen ion/oxonium ion/acids H+/H3O+, lithium ion Li+, sodium ion Na+, potassium ion K+, calcium ion Ca2+, strontium ion Sr2+, barium ion Ba2+, copper(II) ion Cu2+, aluminium ion Al3+, magnesium ion Mg2+, iron(II) ion Fe2+, iron(III) ion Fe3+, zinc ion Zn2+, chromium(III) ion Cr3+, manganese(II) ion Mn2+, lead(II) ion Pb2+, metal carbonates - zinc carbonate ZnCO3and copper(II) carbonate CuCO3

 Part 1 Introduction

 Part 2 Qualitative tests to identify organic molecule functional groups of homologous series

Part 3 Metal cations (positive ions), metal carbonates, ammonium ion and hydrogen ions (acids) (this page)

 Part 4 Gases, water and non-metallic elements

 Part 5 Anions (negative ions) including hydroxide (alkalis)

some associated revising links

GCSE/IGCSE Revision Qualitative Analysis Quiz on chemical test methods for identifying ions, gases and compounds

(c) doc b Quantitative analysis: acid-base, silver nitrate-chloride, EDTA titrations * (c) doc b Quantitative analysis: Redox titrations


Full list of KEYWORDS for inorganic/organic identification methods in alphabetical order e.g. test/reagent for: * acid ==> H+ * acid/acyl chloride RCOCl * alcohols - general ROH/prim RCH2OH/sec R2CHOH/tert R3COH) * aldehyde RCHO * prim aliphatic amine R-NH2 * aliphatic/aromatic carboxylic acids * alkali ==> OH- * alkane/alkene >C=C</alkyne -CC- (saturated versus unsaturated) * aluminium/aluminum ion Al3+ * amide RCONH2 * prim aliphatic amines R-NH2 * ammonia gas NH3 * ammonium ion NH4+ * prim aromatic amine C6H5-NH2 etc. * barium ion Ba2+ * Benedict's solution * Brady's reagent * bromide ion Br- * bromine Br2 * caesium ion Cs+ * calcium ion Ca2+ by flame or hydroxide ppt. * carbonate CO32-/hydrogencarbonate HCO3- with acid or effect of heating metal carbonate e.g. MCO3 * carbon dioxide gas CO2 * carboxylic acid RCOOH * carboxylic acid (aliphatic) salts e.g. RCOO-Na+ * chloride ion Cl- * chlorine gas Cl2 * Chomate(VI) ion CrO42- * copper(II) ion Cu2+ by flame or hydroxide ppt. * 24DNPH (for aldehydes/ketones test) * esters RCOOR * Fehlings test/solution * flame test for metal ions * fluoride ion F- * haloalkanes/halogenoalkanes R-X * hydrogen gas H2 * hydrogen sulphide H2S * hydrogen ion, acids H+ * hydrogen bromide gas/hydrobromic acid HBr * hydrogen chloride gas/hydrochloric acid HCl * hydrogen iodide gas/hydriodic acid HI * hydroxide ion, alkali OH- * hydroxy/alcohol/phenol (organic) * iodide ion I- * iodine I2 * iodoform test - formation of CHI3 * iron(II) ion Fe2+ * iron(III) ion Fe3+ * ketone R2C=O * lead(II) ion Pb2+ * lithium ion Li+ * lime water Ca(OH)2(aq) * magnesium ion Mg2+ * metal carbonates-heating e.g. MCO3 * metal ions via hydroxide precipitate * nitrate or nitrate(V) NO3- * nitrite or nitrate(III) NO2- * nitrogen dioxide or nitrogen(IV) oxide NO2 * oxygen gas O2 * phenols C6H5OH etc. * potassium ion K+ * rubidium ion Rb+ * reducing sugars * saturated/unsaturated * silver nitrate AgNO3 (see chloride, bromide, iodide tests) * sugars (reducing) * sodium ion Na+ * strontium Sr+ * 'sulphate/sulfate' or sulphate(VI) SO42- * sulphide S2- * 'sulphite/sulfite' or sulphate(IV) SO32- * sulphur dioxide gas SO2 * Tollen's Reagent * unsaturated/saturated * water H2O * zinc ion Zn2+ *


Use the alphabetical test list above for identifying anions, cations, gases, molecules etc. to find what you require! for your KS3-KS4 Science-GCSE-IGCSE- Chemistry and GCE-AS-A2-IB-US grades 9-12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic compounds-molecules for qualitative analysis.

EMAIL query?comment?test missing? * Gas PreparationsHazard warning signs/symbols-examples of labelling

   Advanced Chemistry Page Index and Links

 3. INORGANIC Qualitative TESTS Cations and Acids

TEST FOR TEST METHOD OBSERVATIONS TEST CHEMISTRY-comments
Test for the Ammonium ion

NH4+

Add COLD sodium hydroxide solution to the suspected ammonium salt and test any gas above the solution with red litmus. Smelly ammonia released! and red litmus turns blue, gentle warming helps BUT the ammonia should be released at room temperature. Ammonia gas is evolved because alkali frees ammonia from its salts.

 NH4+(aq) + OH-(aq) ==> NH3(g) + H2O(l)

The hydroxide ion removes a proton from the ammonium ion to release the ammonia.

Test for acids i.e. the aqueous hydrogen ion i.e. H+ or H3O+ ion (note: to completely identify acids you need to test for the anion e.g. chloride for HCl hydrochloric or sulfate ion if sulfuric acid etc.) (i) Litmus or universal indicator or pH meter

(ii) Adding a little sodium hydrogen carbonate powder.

(i) Litmus turns red, and, a variety of colours with univ. ind. strong - red, weak - yellow /orange, depending on strength of acid.

(ii) Fizzing with any carbonate and test the gas to see if it is carbon dioxide - test for CO2.

(i) A pH meter reading gives a value of less than 7, the lower the pH number the stronger the acid, the higher the H+ concentration,

(ii) HCO3-(aq) + H+(aq) ==> H2O(l) + CO2(g)

However, some salts can give acid or alkaline solutions but advanced acid-base theory is needed to explain this.

Testing for Positive metal cations with a flame test

(see also below for NaOH(aq) and NH3(aq) tests for metal ion) and heating carbonates too.

This test can be done in a more precise and specific manner using an instrument called a spectroscope and the technique is called emission spectroscopy. Specific emission lines of a characteristic frequency are observed - a fingerprint pattern.

The metal salt or other compound  is mixed with concentrated hydrochloric acid and a sample of the mixture is heated strongly in a bunsen flame on the end of a cleaned nichrome wire (or platinum if you can afford it!) Group 1: lithium Li/Li+ crimson All colours are due to electronic excitations to a higher electronic energy levels. You see the light emitted as the electrons return to lower more stable electronic energy levels. This is the basis of atomic emission and absorption spectroscopy. Aluminium, magnesium, iron and zinc do not produce a useful identifying flame colours.

Other metal flame colors in Group 1:rubidium - red and caesium/cesium - blue

Advanced Chemistry Page Index and Links

Group 1: sodium Na/Na+ yellow (can be slightly orangeish)
Group 1: potassium K/K+ violet/lilac (crimson through cobalt blue glass)
Group 2: calcium Ca/Ca2+ brick (yellowish) red (light green through cobalt blue glass)
Group 2: strontium Sr/Sr2+ crimson
Group 2: barium Ba/Ba2+ yellowish/apple green
Transition Metal: copper(II) Cu/Cu2+ blue (flashes of green too)
Testing for positive metal cations via sodium hydroxide (NaOH) or ammonia (NH3) solutions.

Note:

(1) Both are alkalis, giving hydroxide ions, OH-, in their solutions.

(2) Aluminium, magnesium, iron and zinc do not produce a useful identifying flame colour.

(3) A more advanced test to distinguish iron(II) ions, Fe2+and iron(III) ions, Fe3+

(i) If potassium hexacyanoferrate(III) solution is added to the suspected iron solution, iron(II) ions give a deep blue precipitate of Turnbull's blue.

(ii) If potassium hexacyanoferrate(II) solution is added to the suspected iron solution, iron(III) ions give a deep blue precipitate of Prussian blue.

Note that Turnbull's blue is identical in composition to  Prussian blue. For more chemical details see the transition metals page on iron.

Dilute sodium hydroxide (NaOH) solution is added to a solution containing the suspected ion.

Both the precipitate formed and the effect of excess alkali are important observations.

All precipitates are white, unless otherwise stated, and all tend to be gelatinous in nature.

The test should be repeated with aqueous ammonia solution (NH3, 'ammonium hydroxide').

The observations with ammonia solution are usually similar, but not always, the same and the differences can be important clues as to the identity of the metal ion.

ppt. = precipitate.

More on some of these hydroxide precipitates on the 3-d block Transition Metals Series pages.

aluminium ion: Al3+(aq) + 3OH-(aq) ==> Al(OH)3(s) gives a white precipitate

of aluminum hydroxide, which is not soluble in excess of the weak alkali ammonia, but dissolves in the stronger base/alkali sodium hydroxide (amphoteric) to give a clear colourless solution.

Al(OH)3(s) + 3OH-(aq) ==> [Al(OH)6]3-(aq) (amphoteric behaviour because it dissolves in acids too)

calcium ion: Ca2+(aq) + 2OH-(aq) ==> Ca(OH)2(s) gives a white precipitate

of calcium hydroxide with sodium hydroxide IF the concentration of calcium ion is high. It is not soluble in excess of NaOH. No precipitate is formed with ammonia solution.

magnesium ion: Mg2+(aq) + 2OH-(aq) ==> Mg(OH)2(s) gives a white precipitate

of magnesium hydroxide, which is not soluble in excess of either NH3 or NaOH. You could distinguish Mg from Ca with a flame test or ammonia test above.

copper(II) ion: Cu2+(aq) + 2OH-(aq) ==> Cu(OH)2(s) gives a blue/turquoise ppt.

of copper(II) hydroxide, which dissolves in excess ammonia to give a deep blue solution of an ammine complex, but copper(II) hydroxide is NOT soluble in excess NaOH.

Cu(OH)2(s) + 4NH3(aq) ==> [Cu(NH3)4]2+(aq) + 2OH-(aq)

iron(II) ion: Fe2+(aq) + 2OH-(aq) ==> Fe(OH)2(s) gives a dark green precipitate

of iron(II) hydroxide, which is not soluble in excess of NH3 or NaOH. Darkens in air due to oxidation to Fe(OH)3.

iron(III) ion: Fe3+(aq) + 3OH-(aq) ==> Fe(OH)3(s) forms a brown precipitate

of iron(III) hydroxide, which is not soluble in excess of NH3 or NaOH.

Another test for iron(III) ions is to add a few drops of potassium/ammonium thiocyanate solution and a blood-red coloured compound is formed.

zinc ion: Zn2+(aq) + 2OH-(aq) ==> Zn(OH)2(s) a white precipitate formed

of zinc hydroxide, which dissolves in both excess (i) sodium hydroxide or (ii) ammonia to give a clear colourless solution:

(i) Zn(OH)2(s) + 2OH-(aq) ==> [Zn(OH)4]2-(aq)  (amphoteric behaviour because zinc hydroxide dissolves in acids too).

(ii) Zn(OH)2(s) + 4NH3(aq) ==> [Zn(NH3)4]2+(aq)  + 2OH-(aq) (soluble complex ion formation)

chromium (III) ion: Cr3+(aq) + 3OH-(aq) ==> Cr(OH)3(s) a grey-green precipitate forms

of chromium(III) hydroxide, which is soluble in excess of NaOH (amphoteric, dissolves in acids too) but not soluble in excess ammonia NH3. With sodium hydroxide a dark green soluble hexahydroxo-complex ion is formed.

Cr(OH)3(s) + 3NaOH(aq) ==> [Cr(OH)6]3-(aq)

manganese(II) ion: Mn2+(aq) + 2OH-(aq) ==> Mn(OH)2(s) produces an off-white precipitate

of manganese(II) hydroxide, which is NOT soluble in excess of NH3 or NaOH and rapidly turns brown=>black in air due to oxidation to manganese(III) oxide Mn2O3 and then manganese(IV) oxide, MnO2.

lead(II) ion: Pb2+(aq) + 2OH-(aq) ==> Pb(OH)2(s) a white precipitate forms

of lead(II) hydroxide, which dissolves in excess sodium hydroxide (amphoteric) to give a clear colourless solution but does not dissolve in excess ammonia solution.

Pb(OH)2(s) + 2OH-(aq) ==> [Pb(OH)4]2-(aq)  (amphoteric behaviour with NaOH as with zinc hydroxide which also dissolves in acids too)

The barium ion, Ba2+(aq) does not give a hydroxide precipitate because barium hydroxide, Ba(OH)2, is too soluble.
Advanced Chemistry Page Index and Links
MISCELLANEOUS CATION TESTS:

(i) Lead(II) ion

(ii) -

(i) add potassium iodide solution ==> yellow precipitate (i) Pb2+(aq) +2I-(aq) ==>PbI2(s) a yellow precipitate of lead(II) iodide is formed
Metal Carbonates

 

See also carbonates and hydrogencarbonates.

Sometimes heating a metal carbonate strongly to decompose it provides some clues to its identity.

Adding acid to a carbonate ==> CO2 and the colour of the resulting solution e.g. blue copper(II) ion Cu2+(aq), may also provide clues, but no good in most cases because most carbonates you come across are white giving colourless solutions except for some transition metals like copper, nickel and cobalt.

The metal ion solution might also give a flame colour or a hydroxide precipitate with sodium hydroxide e.g. copper.

(i) copper(II) carbonate==> copper(II) oxide + carbon dioxide

CuCO3(s) ==> CuO(s) + CO2(g)

observations [green solid] ==> [black solid residue] + [colourless gas, test with limewater, white precipitate]

(ii) zinc carbonate==> zinc oxide + carbon dioxide

ZnCO3(s) ==> ZnO(s) + CO2(g)

observations [white] ==> [solid residue, pale yellow hot, white cold] + [colourless gas, test with limewater ==> white precipitate]Advanced Chemistry Page Index and Links

 

Tests for NH4+, H+/H3O+, Li+, Na+, K+, Ca2+, Sr2+, Sr2+, Ba2+, Cu2+, Al3+, Mg2+, Fe2+, Fe3+, Zn2+, Cr3+, Mn2+, Pb2+, CuCO3, ZnCO3

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