* CHEMICAL TESTS - QUALITATIVE ANALYSIS 3 To identify/test for cations (+ve ions) at Doc Brown's Chemistry

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 Doc Brown's Chemistry  Qualitative Analysis Methods Revision Notes

CHEMICAL identification TESTS Part 3 Qualitative tests to identify cations

 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)

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

Quantitative analysis: acid-base, silver nitrate-chloride and EDTA titrations * Quantitative analysis: Redox titrations


 Part 3 Metal cations (positive ions) , ammonium ion and hydrogen ions (acids)

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 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.

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

   

 3. INORGANIC Qualitative TESTS Cations and Acids

TEST FOR TEST METHOD OBSERVATIONS TEST CHEMISTRY-comments
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. Ammonia gas is evolved:

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

Gentle warming of the mixture may be needed.

Acids 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 etc.) (i) Litmus or universal indicator or pH meter. 

(ii) Add a little sodium hydrogencarbonate powder.

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

(ii) Fizzing with any carbonate - test for CO2 as above.

(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)

Positive metal cations via flame tests.

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

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 (platinum if you can afford it!) Group 1: lithium Li/Li+ crimson All colours are due to electronic excitation to a higher level. You see the light emitted as the electron returns to its lower more stable level. This is the basis of atomic emission and absorption spectroscopy. Aluminium, magnesium, iron and zinc do not produce a useful identifying flame colour.

Other metals in Group 1:rubidium - red and caesium/cesium - blue

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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)
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.

Dilute sodium hydroxide 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 white, unless otherwise stated and all tend to be gelatinous in nature.

The test can be repeated with aqueous ammonia solution ('ammonium hydroxide'). The observations are usually, but not always, similar.

ppt. = precipitate.

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

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

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) grey-green precipitate

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) off-white precipitate

of manganese(II) hydroxide, which is not soluble in excess of NH3 or NaOH and rapidly turns in air due to oxidation to manganese(III) oxide Mn2O3 (and manganese(IV) oxide, MnO2 too?)

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

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)  (with NaOH amphoteric behaviour because zinc hydroxide dissolves in acids too).

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

(i) Lead(II) ion

(i) add potassium iodide solution => yellow precipitate (i) Pb2+(aq) +2I-(aq) ==>PbI2(s) lead(II) iodide ppt.
Metal Carbonates

 

See also carbonates and hydrogencarbonates.

Sometimes heating a metal carbonate strongly to decompose it provides some clues to its identity. Adding acid ==> CO2 and the colour of the resulting solution (e.g. blue Cu2+(aq), may also provide clues. 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)

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

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

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

[white] ==> [yellow hot, white cold] +[colourless gas, test with limewater, white precipitate]top

 

(spanish) Doc Brown Química Métodos cualitativos de análisis Notas de revisión Parte 3 pruebas de identificación cualitativa pruebas químicas para identificar los cationes * (portuguese) Doc Brown Química Métodos de Análise Qualitativa notas de revisão identificação química Parte testes qualitativos três testes para identificar cátions Parte 3 cátions metálicos (íons positivos), carbonatos de metais, íons de amônio e íons de hidrogênio (ácidos) * (indonesian) Kimia Analisis Metode Kualitatif Catatan Revisi KIMIA identifikasi kualitatif UJI Bagian 3 tes untuk mengidentifikasi kation Bagian 3 Logam kation (ion positif), karbonat logam, ion amonium dan ion hidrogen (asam) (halaman ini) * (italian) Doc Brown Metodi di analisi chimica qualitativa Revisione Note Parte 3 prove di identificazione qualitativa analisi chimiche per individuare cationi Parte 3 cationi metallici (ioni positivi), carbonati di metallo, ione ammonio e ioni di idrogeno (acidi) (questa pagina) * (french) chimie qualitative Notes de révision des méthodes d'analyse Les tests d'identification qualitative Partie 3 tests chimiques afin d'identifier les cations Partie 3 cations métalliques (ions positifs), les carbonates métalliques, ion ammonium et des ions hydrogène (acides) * (korean) 브라운 박사는 화학 질적 분석 방법의 수정 사항 화학 식별 실험을 3 부 질적 테스트 cations을 식별하는 3 부 금속 cations (양성 이온), 금속 carbonates, 암모늄 이온과 수소 이온 (지방산) (현재 페이지) * (chinese) 督布朗的化学定性分析方法的修订说明 化学鉴定试验第3部分的定性测试,以确定阳离子 第3部分金属阳离子(正离子)金属碳酸盐,铵离子和氢离子(酸) (此页)第3部分金属阳离子(正离子),铵离子和氢离子(酸) * ChemTestsc (thai) เคมีคุณภาพวิธีการวิเคราะห์ เคมีระบุ Part ทดสอบคุณภาพการทดสอบ ระบุไอออน Part 3 แคตไอออนโลหะ (ไอออนบวก), คาร์บอเนตโลหะไอออนแอมโมเนียมและไอออนไฮโดรเจน (กรด) (หน้านี้) แคตไอออนโลหะ (ไอออนบวก), ไอออนแอมโมเนียมและไอออนไฮโดรเจน (กรด) *

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