Chemical Analysis

Chemical Analysis E. Bottari, Chemistry Department, “La Sapienza” University of Rome Malta, Summer School 2007, 20th August – 9th September

Analytical chemistry- Methods • Qualitative analysis • Quantitative analysis • Traditional analysis • Instrumental methods of analysis • Suitable reactions for analysis

Qualitative analysis • Recognition of chemical species by means of colour, reaction producing a colour, reaction producing a precipitate, reaction involving a change of a physical parameter. • Colored ions are: Cu2+ (blue), Cr3+ (green), CrO4=(yellow), Cr2O7= (orange), MnO4-(viole), MnO4=(green), Ni2+(green), Co2+(pink, or blu), Mn2+(pink), and generally ions of transition metals. • Precipitates (slight soluble compounds): sulphurs of eavy metals (like: As, Sb, Hg, Cu, Pb, Cd, Sn, Bi, Zn, Ni, Co, Mn), BaSO4, Hg2Cl2, AgCl, PbCl2, Ag2CrO4, many hydroxides of eavy metals.

Traditional - Quantitative analysis • Volumetric analysis A + B = C. A solution of B, at known concentration is added to a known volume of A, until the number of equivalent of B is equal to those of A. The added volume of B is measured and the concentration of A can be calculated. • Gravimetric analysis A + B = C. An excess of B, at unknown concentration, is added to A so that A is completely transformed in C, that can be weighed.

Quantitative analysis – Volumetric 1 • Titration (traditional) To perform a volumetric analysis (titration) is necessary to have a solution at known concentration a glass vessel, a burette (calibrated tube able to measure volume, equipped with a tap) and an indicator (chemical species able to change colour when change the composition of the solution. • A titration is a chemical operation which allows to obtain the unknown concentration of a reagent, A, by adding the reagent B and by stopping the addition when the equivalent number of A and B are exactly equal. • The point corresponding to meq* A = meq B is called equivalent point or point of equivalence. *meq = milli equivalents = VACA = VB CB

Quantitative analysis – Volumetric 2 Reaction must be: • Quick • Stoichiometric with known coefficients • Univocal • Collateral reactions must be absent • Complete • In correspondence to the equivalent point, a sharp change of the followed parameter must occur • This change can be put into evidence by the presence of a colorimetric indicator or by means of an instrumental method

Quantitative analysis – Volumetric 3 • Standard It is very hard to perform a volumetric analysis by comparing a solution with another, without a reference point. It is necessary to have solutions at known concentration, prepared by dissolving a known weight in a measured volume. Compounds having such property are called mother substances or primary standards. • A standard must be very pure, stable, not reacting with air or water (the solvent). It must be possible to dry it at 110°C. It must follow all the rules of volumetric analysis and have a high equivalent weight.

Volumetric Analysis Kind of reactions • Solutions of electrolytes • Acid – base • Precipitimetric (formation of a precipitate) • Chelometric (formation of a chelate) • Redox

Electrolytes • Electrolytes are all the compounds dissociable in ions. For ex. NaCl  Na+ + Cl- or Na2SO4  2 Na+ + SO4= • Electrolytes can be strong or weak. • Strong electrolytes are completely dissociated, weak ones are only partially dissociated and generate an equilibrium. • Salts are strong electrolytes, like NaCl. It exists as Na+ and Cl- • Acids and bases can be strong or weak according their properties. Mineral acids are generally strong, organic acids are weak. H3PO4 is a weak acid and many inorganic acids are weak. NaOH and KOH are strong bases, while NH3 is a weak base. • Dissociation of weak acids or bases is regulated by a constant.

What is an acid, what is a base? • An acid is a compound able to yield protons • A base is a compound to catch protons • A compound able to yield or catch protons, depending • on the experimental conditions, is called ampholite or • amphiprotic substance. • Free protons in a condensed matter do not exist, so that • in a solution the proton comming from an acid must be • taken by a base contemporanily present. Often the solvent • can carry out such task. Example: HA1 + B2 = HA2 + B1. There are two conjugated couples acid – base: HA1 = H+ + B1 and B2 + H+ = HA2

What is Water? Water is able to yield or to acquire a protons, according the following relation: H2O + H2O  H3O+ + OH- (1) Eq. (1) is the sum of the two following: H2O  H+ + OH- (acid behaviour of water) H2O + H+ H3O+ (basic behaviour of water) As eq. (1) is an equilibrium reaction, it is regulated by a constant k = [H3O+] [OH-][H2O]-2, but [H2O] is constant in the solvent water and by involving it in the constant k, it can be written: Kw = [H3O+] [OH-] = 10-14 at 20°C. In a neutral solution [H3O+] = [OH-]= Kw½ = 10-7.

What is pH? pH is an usual expression suitable to indicate the acidity of a solution. Practically it is the negative decimal logarithms of the free hydrogen ionic concentration, i.e. pH = -log [H3O+] In the above slide, it was shown that in neutral solution, [H3O+] = [OH-]= Kw½ = 10-7 It can be deduced thatpH = -log [H3O+]= -log 10-7 = 7. A neutral solution has pH =7, an acid one has pH<7 and a basic solution has pH > 7. Similarly, it can be defined pOH and pKw and of consequence pKw = pH + pOH

Calculation of pH 1 • Solution of strong acids or bases: free concentration is equal to the analytical one: CH = cH = [H3O+]. COH = cOH = [OH-]. Solution of HCl = 0.1 M has pH =1. Solution of NaOH = 0.1 M has pH =13. • For solution of weak acid, like acetic acid CH3COOH, 0.1 M, the free concentration [H3O+] must be calculated on the basis of the following equilibrium (0.1 M it the total concentration): CH3COOH + H2O  CH3COO- + H3O+ (2) Acid1 Base2 Base1 Acid2

Calculation of pH 2 • The constant of eq. (2) is ka = [CH3COO-][H3O+] [CH3COOH]-1, where [CH3COO-]= [H3O+], then it follows: ka = [H3O+]2 [CH3COOH]-1 = [H3O+]2 (0.1 - [H3O+])-1. If [H3O+] is negligible with respect to 0.1, it can be written: [H3O+] = (ka CHA)½, where CHA represents the generalization of acid. • For a base solution, i.e. acetate ions, CH3COO- 0.1 M, [H3O+] must be calculated by means of the equilibrium: CH3COO- + H2O  CH3COOH + OH- (3), with a constant Kb = [CH3COOH][OH-] [CH3COO-]-1. By combining (2) and (3), it can be obtainedkaKb = Kw and pOH = (kb CA-)½ (approx. formula). In this case Kb is also called hydrolysis constant, indicated by Kh.

Buffer solutions Solutions are called buffer when their pH does not change appreciably, by adding little amounts of strong acid or bases. Buffer solutions can be those having pH < 3 and pH > 11. They can be also formed by a weak acid in the presence of Its conjugated base. In this case in eq. (2) it is not possible To write [CH3COO-]= [H3O+], then it follows: [H3O+] = ka CCH3COOH CCH3COO--1 They can be also formed by a weak base in the presence of its conjugated acid. For example NH3 and NH4+. [OH-] can be calculated as follows: [OH-] = kb CNH3 CNH4+-1

Procedure for acid – base titrations If the concentration H0 of the solution of a reagent HA must be determined, with accuracy of 0.1 %, a measured volume V0 of the HA solution is transferred in a titration vessel and two drops of indicator are added. A solution of NaOH, at exactly known concentration OHT, is gradually added by means of a Burette, till the indicator changes colour for the addition of a drop more. The final point of the titration is reached. The added volume of NaOH is measured by the burette, VT. In this point the number of equivalent of both reagents is equal. The following realtion can be written: H0 V0 = OHT VT. VT is measured, OHT and V0 are known, H0 can be calculated.

Determination of olive oil acidity This analysis allows to classify oil in a particular category, with different commercial value according the acidity content. Olive oil can be classified in the first value category, if its Content of acidity (expressed as oleic acid) is  0.80 %. Analysis is performed similarly to that previously described. V0 measured of oil is transferred in a vessel for the titration. A solution of OHT standard is gradually added till the change of the colour of a suitable indicator is reached. At this point, the volume VT of added titrant OHT, is read on the burette and the initial acidity of oil H0 = VT OHT V0-1 can be calculated.

Reaction involving a slight soluble compound (precipitate) By mixing a solution of AgNO3 with one of NaCl, a precipitate of AgCl takes place, according to the reaction: Ag++Cl- = AgCl, or AgNO3 + NaCl = AgCl + NaNO3. The equilibrium in this case is shifted to right because a slight soluble substance is formed. According to the equilibrium rules, it can be written: k = [Ag+][Cl-] [AgCl]-1, but AgCl is solid in equilibrium with The ions in solution and its free concentration can be assumed as constant, which involved with k, gives: Ks = [Ag+][Cl-]. Ks is called solubility product. The reaction between Ag+ and Cl- frequently used to determine The quantity of silver present in a sample, in a similar way seen For the reaction acid – base.

Chelometric reactions – Hardness of water 1 It was previously described the “dative” bond, i.e. a covalent bond, where a couple of electron coming from the same ion or compound (for example :NH3) is put in common between the transition ion (for example Cu2+) and the donor (:NH3) to form a complex ion or a molecule. Next to :NH3 which is able to give only a couple of electrons, many compounds exist containing several atoms able to give each a couple of electrons, so that a molecule of such compounds is able to bind a metallic ion with several bonds, forming five membered rings particularly stable. The most used compound having this property is called EDTA. The most important application of EDTA is the water hardness determination, i.e. the calcium(II) + magnesium (II) dosage.

Hardness of water 2 - Chelometry The knowledge of hardness of water is very important in many practical cases: beer industry, metallic tubes for water, formation of limestone in caldrons, water heaters, and in general, in washing machines. Hardness is due to the formation of calcium or magnesium carbonate, slight soluble compound forming limestone. EDTA standard solution is able to titrate solutions containing Calcium and magnesium ions to determine their concentration. Analysis is carried out similarly to those previously described, but with different indicator and at well defined and buffered pH. It is possible to know separately the calcium and magnesium present in the same solution. The same analysis is performed to know the calcium and magnesium concentration in milk, or cheese.

Redox reactions 1 • This kind of reaction takes place involving the electrons transfer from the reducing to oxidant, as follows: Ox1 + n1e = Red1 and Red2 = Ox2 + n2e, if n1 = n2, the complete reaction can be written: Ox1 + Red2= Red1 + Ox2 • Oxidant and reducing compounds have different straight, which can be experimental proved. • A solution of Cu2+ is blue. If you put a piece of Zn or of Pb inside the copper solution, you can observe that after some minutes solution becomes colourless, Zn or Pb are dissolved and a red slight soluble compound is formed. This means that Zn or Pb are oxided by Cu2+ reduced to Cu0 Red. The following reactions occur: Cu2+ + Zn = Cu0 + Zn2+ or Cu2+ + Pb = Cu0 + Pb2+

Redox reactions 2 • If you repeat the same experiment by using Pb2+ and Zn, you • find that Zn is oxidized by Pb2+ to Zn2+ according to the • reaction: Pb2+ + Zn = Pb0 +Zn2+ • If you repeat the same experiment by using Ag+ and Cu, you • find that Cu is oxidized by Ag+ to Cu2+ according to the • reaction: Ag+ + Cu = Ag0 +Cu2+ From the above presented examples, you can deduce the following sequence, as straight of oxidation: Ag+ > Cu2+ > Pb2+ > Zn2+, vice versa the straight as reducing The straight as OX of a couple (i.e. Ag+ + e = Ag0) is represented by the redox potential. The table of standard potentials collects all the couples with their values.

Redox reactions 3 Nernst equation shows the dependence of the redox potential on the reagents (ox and red form) free concentration: Nernst equation for a generic couple: Ox1 + n1e = Red1: E1 = E°1 + RT (n1 F)-1 ln {[Ox1] [Red1]-1}, which at 25°C is: E1 = E°1 + 0.05916 (n1)-1 log {[Ox1] [Red1]-1} • The following Redox couple are frequently used in analysis : • MnO4- + 5 e + 8 H+= Mn2+ + 4 H2O • Cr2O7 + 6 e + 14 H+ = 2 Cr3+ + 7 H2O • I2 + 2 e = 2 I-; IO3- + e + 6 H+ = I- + 3 H2O • 2S2O3= = S4O6= + 2 e; C2O4==CO2 +2 e; S= = S + 2 e; • Sn2+ = Sn4+ + 2 e; Fe2+ = Fe3++ e; • NO3- + 3 e + 4 H+ = NO + 2 H2O; 2HgCl2 + 2 e =Hg2Cl2 +2Cl-

Instrumental Analysis • Electro analytical analysis • Optical analysis (spectroscopy) • Thermal analysis • Chromatography

Electro analytical analysis • Electrolysis (electro gravimetric analysis). • Electromotive Force Measurements (Galvanic elements – Piles) (pH measurements). Direct and Titrations. • Conductometry. • Coulometry (direct and indirect). • Polarography.

Spectroscopy • Emission (flame, voltaic arc, sparkly). • Absoption (Molecular: UV violet, IR). • Absorption (Atomic: flame, furnace). • X Ray or more sophisticated methods.

Chromatography • On column • On paper • TLC (Thin layer chromatography) • GC (gas chromatography) • HPLC (High performance liquid chromat.) Many applications: analysis of fat fraction of many substances, Like: Milk, Different kinds of oil, Butter, Meet, Eggs, Vitamins Cosmetic, Pesticides, Dioxins, Herbicides, etc…

Chemical Analysis