MIDTERMS

• Wet Chemical Methods: titrations, color-forming reactions,

  precipitations, etc.

  Instrumental Methods: spectrometry, chromatography, etc.
• Analytical Process

  Formulating the Question

  Selecting Analytical Procedures

  Sampling

  Sample preparation

  Analysis

  Report and Interpretation of results

  Drawing Conclusions
• Most precipitations take place when the anions and cations of two ionic compounds change partners.
• For molecular compounds, the molecule must be polar for it to be soluble in water
• For ionic compounds it is governed by the solubility rules
• Soluble Compounds

  Almost all salts of Na+ , K+ , NH4+

  Salts of nitrate, chlorate, perchlorate, acetate

  Almost all salts of Cl- , Br- , I- (Halides of Ag+ , Hg2 2+ , Pb2+)

  Salts containing F- (Fluorides of Mg2+ , Ca2+ , Sr2+ , Ba2+ , Pb2+)

  Salts of sulfate, SO4 2- (Sulfates of Ca2+ , Sr2+, Ba2+ , Pb2+ , Ag+)
• Insoluble compounds

  Most salts of carbonate, phosphate, oxalate, chromate, sulfide

  (Salts of NH4+ and alkali metal cations and BaS)

  Most metal hydroxides and oxides

  (Alkali metal hydroxides and Ba(OH)2 and Sr(OH)2)
• Types of Precipitates

  Crystalline - well defined / regularly shaped crystals

  (relatively pure, ready to filter, heated to achieve larger and purer crystals)

  Gelatinous - flocculated colloids; jelly-like, hydrous masses

  (very difficult to work with, ex metal hydroxides)

  Curdy - aggregates of small porous particles

  (in reasonable size they are filtered easily, ex silver halides)
• Group I cations

  Hg2 2+ , Ag+ , Pb2+

  PbCrO4 (yellow ppt.) presence of lead ion

  HgNH2Cl (white/gray ppt.) presence of mercurous ion

  AgCl (white ppt.) presence of silver ion
• H2S is the precipitating agent for group II cations. under acidic conditions
• copper, bismuth, antimony, and tin are the only group 2 cations that are observed since the rest are toxic heavy metals.
• Group 3 cations are precipitated as insoluble metal sulfides or hydroxides in a basic solution saturated with H2S.
• The addition of stannite solution confirms the presence of bismuth with black precipitate
• the addition of ammonia (NH4+) confirms the presence of copper with blue precipitate
• nickel, iron, aluminum, and chromium for group 3 cations
• iron with the addition of thiocyanate forms a deep red color confirming the presence of it in the solution
• nickel with the addition of dimethylglyoxime forms a scarlet red color confirming the presence of nickel in the solution
• aluminum with the presence of aluminon forms a red color confirming the presence of aluminum in the solution
• Chromium is turned into chromate which then reacts with lead to form lead chromate which gives a yellow precipitate confirming the presence of chromium
• Vigorous evolution of a gas confirms the presence of nitrite

  Vigorous effervescence indicates presence of carbonate
• Addition of Ca(Ac)2 seperates group 1 anions from the solution

  Arsenate (brown solution) , Sulfite (white ppt.) , Oxalate (disappearance of pink)

  
• Addition of Ba(Ac)2 to the centrifugate will seperate Group 2 Anions

  Sulfate and Chromate (white residue)

  
• Addition of Cd(Ac)2 seperates the Group 3 Anions

  Sulfide (brown/black test paper) and Ferrocyanide (dark blue ppt)

  
• Addition of Ag(Ac) and HNO3 seperates Group 4 anions and Chlorate and Borate ions.

  Thiocyanate (red) and Iodide (violet)