Compulsory textbook. Recommended textbooks

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1 Compulsory textbook Fundamentals of Analytical Chemistry, D. A. Skoog, D. M. West, F. J. Holler and S. R. Crouch, Brooks/Cole, 2004, 8th Edition Recommended textbooks Principles of Instrumental Analysis, D. A. Skoog, F. J. Holler and T. A. Nieman, Saunders College Publishing, Quantitative Analytical Chemsitry, J. S. Fritz and G. H. Schenk, Allin and Bacon, Instrumental Methods of Analysis, H. H. Willard et al., Wadsworth Publ. Co., Topics of the first semester 1. General introduction (1) 2. Fundamental concepts in analytical chemsitry (1) 3. Gravimetric methods in the analysis (1) 4. Titrimetry general principles and concepts (2) 5. Precipitation titrations (argentometry) (1) 6. Neutralization titrations (acidi-alkalimetry) (2) 7. Complexometric titrations (chelatometry) (1) 8. Redox titrations (oxidi-reductometry) (3)

2 Analytical chemistry involves separating, identifying and determining i the relative amounts of the components (analytes) of the sample Qualitative analysis what is present? chemical identity of the species in the sample (preceeds quant. anal.) Quantitative analysis how much is present? percentage or mass of the analyte in the sample Separation techniques (chromatographies) different components may interfere one with another Analysis types 1. Complete analysis each constituent is analysed 2. Ultimate (elemental) analysis each element is analysed 3. Partial analysis the amount of selected compounds/atoms/components Examples water analysis blood sample analysis N,S,P,C-content in foodstuffs serial analysis of a pharmaceutical product household gas analysis air analysis etc., etc., etc.

3 Methods of analytical chemistry Classical Instrumental gravimetry volumetric methods or titrations electroanalysis spectrometric analysis magnetic methods thermal methods miscellaneous methods To be considered 1. accuracy & reliability required vs. economics 2. no. of samples to be analysed 3. complexity of the samples

4 representative sampling when the sample truly represents the object to be analysed grinding (homogeneity) drying (deliquescence) homogeneous sample: its constituents can be distinguished d visually or with the aid of a light microscope ( heterogeneous sample)

5 replicate samples: portions of the material of (approximately) the same size that are carried through the analytical procedure weighing (by an analytical balance measurement of mass) pipetting (by a pipette measurement of volume) preparing aqueous solutions solubilization (digestion)

6 interference: species other than the analyte, which interferes with the results of the measurement, i.e., causes errors The measured property, X has to vary in a known and reproducible way with the concenctration of the analyte, c A Ideally c A = k X X thesignal k characteristic to the method, usually unknown (except gravimetry and coulometry) calibration the process of determining k

7 For the calculations 1. experimental data 2. stoichiometry 3. instrumental data are required uncertainties associated with the measurements must be known errors in the chemical analysis

8 Chapter 2. Chemicals, Apparatus and Unit Operations of Analytical Chemsitry -dealt with in practical (compulsory) Chapter 3. Using Spreadsheets in Analytical Chemistry - dealt with in practical (optional) Calculations used in analytical chemistry Atom the smallest particle of an element Molecule - the smallest particle of a compound Compounds are combination of elements molecules are made up of atoms The important thing for an (analytical) chemist is the number of atoms reacting (and not the mass) Atomic mass (A r ): relative masses based on the 12 C isotope Molecular mass (M r ): the sum of the atomic masses of the atoms that make up the molecule The chemical mass unit: the mole (1 mole = atoms of an element or molecules of a compound) grams of material (m) Number of moles (n) = formula mass (A r or M r )

9 Expressing concentration of solutions 1. Molar concentration (molarity) the number of moles of solute present in 1 L of solution c = number of moles of the solute volume of solution unit: mole/litre or mole/dm 3 or M (equal to mmol/ml!!!) Expressing concentration of solutions 2. Molal concentration (molality or Raoult s- concentration) m = number of moles of solute mass of solvent unit: mole/kg Advantage of m over c: m is independent of Advantage of m over c: m is independent of temperature

10 Expressing concentration of solutions 3. Mole fraction X = number of moles of solute the moles of solvent + the moles of solute unit: - Grams per volume the mass of the solute divided by the volume of solution mass of solute volume of solution unit: g/l Expressing concentration of solutions 4. ppm the mass of the solute in mg divided by the volume of the solution in litre mass of solute in mg concentration in ppm = volume of solution in litre The mass of 1 litre of water equals to 1000g Unit: ppm part(s) per million, ppb the mass of the solute in µg divided by the volume of the solution in litre mass of solute in µg concentration in ppb = volume of solution in litre Unit: ppb, part(s) per billion

11 Expressing concentration of solutions 5. Mass percent the mass of solute divided by the mass of solution mass percent = mass of solute mass of solution x100 Unit : g/100g or m/m% Volume percent - the volume of solute divided by the volume of solvent vol% = volume of solute volume of solution x100 Unit : ml/100ml or V/V% Expressing concentration of solutions 6. Analytical molarity the total number of moles of solute present in a given volume of solution (it says nothing about the actual state of the solute, whether it ionizes or not, etc.) Symbol: c or c T Equilibrium molarity the concentration of ions or molecules actually present in solution, taking into account the possible dissociation of the solute into ions Symbol: [ ] The analytical concentration is equal to the sum of the equilibrium concentrations of the various forms of the solute Example: HAc H + + Ac - c HAc = [Ac - ] + [HAc] this is a mass balance equation

12 Errors in chemical analysis how certain can we be about the results we obtain? Obtained values (results) for a given quantity from N replicates: x 1, x 2, x 3,, x N Mean (median, arithmetic mean, average), x 1 x = N N x i i= 1 Precision the reproducibility of the measurements, or the closeness of results that have been obtained exactly in the same way; can be obtained by repeating the measurements (MEMO-technique: pre=rep) Accuracy the closeness of our measurements to the true or accepted values; cannot be obtained by repeating the measurements; expressed in terms of the absolute error: E = x true -x i

13 Types of errors Random (indeterminate) error causes data to be scattered symmetrically y around the mean value; associated with the precision (or the reproducibility) of the measurement Systematic (determinate) error (for example instrumental, method or personal error); causes the mean of the data set to differ from the true value; associated with the accuracy of the measurement Gross error they occur occasionally and lead to outliers Characterization of random errors, i.e., the precision of the measurement Gaussian (normal error) curve shows the symmetrical distribution of data around the mean of an infinite set of data

14 Sample standard deviation, s the measure of precision of a measurement s = N ( x xi ) i= 1 N 1 2 d i = x x i the deviation of the i-th result, x i from the mean; N - the total number of the measurements; N-1 number of degrees of freedom s standard deviation s 2 sample variance Significant figures The significant figures in a number are all the certain digits plus the first uncertain digit most important example: reporting a burette reading let us say, that t the burette is of 25 ml capacity smallest division is 0.1 ml ml the first three digits are certain the last digit is estimated, i.e., uncertain if you report 12 ml rounding off error (ca. 1.8 %) if you report ml nobody believes you (rightly so) if you try to read < 0.1 ml on the same burette meaningless result - another very important example: reading an analytical balance 0,9668 g the sample weighed must be at least 100 mg (to keep error at 1% level)

15 Sampling The analytical method of choice depends on the sample size and constituent type sample size type of analysis > 0.1 g macro g semimicro g 0.01 g micro < g ultramicro analyte level l type of constituent 1% - 100% major 0.01%(100 ppm) -1% minor 100 ppm 1 ppb trace < 1 ppb ultratrace Minimizing errors in analytical procedures 1. Choosing the correct blank solution 2. Application of separation techniques elimination of interferences 3. Saturation deliberate addition of large amount of interfering components to all samples and standards (this may degrade sensitivity and detectability) 4. Matrix modification a non-interfering component is added to modify the response, to make it independent of the presence of the interfering species 5. Adding of a masking agent it selectively reacts with the interfering component and makes it invisible 6. Dilution method 7. Matrix matching method for example, synthetic seawater

16 Classical methods of chemical analysis includes gravimetry titrimetry argentometry acidi-alkalimetry complexometry redox titrations Gravimetric methods are quantitative methods that are based on determining the mass of the pure compound to which the analyte is chemically related The mass is always measured on an (accurate) analytical balance Types of gravimetric i methods precipitation gravimetry volatilazation gravimetry electrogravimetry thermogravimetry gravimetric titrimetry Steps of precipitation gravimetry: 1. an excess precipitating reagent added to the sample, thus the analyte converted into sparingly soluble product (precipitate) 2. precipitate is filtered 3. precipitate is washed from impurities 4. precipitate is dried or ignited (to convert it to a product with known composition) 5. precipitate is weighed

17 Precipitation gravimetry A successfull gravimetric determination meets the following criteria 1. The analyite must be completely l (quantitatively) precipitated 2. The precipitating agent reacts selectively or, at least, specifically with the analyte Selective reagent reacts only with a single chemical species (rare) Specific reagent reacts with several, but limited number of chemical species (more common) 3. The precipitate must easily filtered and washed free from contaminants 4. Must be of sufficiently low solubility (to avoid loss of the analyte) 5. Must be unreactive with constituents of the atmosphere 6. Its weighed form must be of known composition (gravimetric factors) Solubility of precipitates Precipitate it is formed from a solution which is supersaturated with respect to the solute; when no more precipitate is ible to form, the remaining solution is called saturated solution Types of electrolyte solutions: 1. Non-saturated (or undersaturated) 2. Saturated 3. Supersaturated 1 2 3

18 Solubility of precipitates Solubility (or equilibrium solubility,s): the concentration of a saturated solution in molarity at a given temperature; characteristic to the given salt (depends on solvent and temperature) MA M + + A - M x A y xm y+ + ya x- S = [M y+ ]/x = [A x- ]/y Solubility yproduct (L, K sp) : the equilibrium constant for the components of the precipitate in a saturated solution (i.e., in a solution, which contains some precipitate) K sp = [M + ][A - ] K sp = [M y+ ] x [ya x- ] y Solubility and solubility product y+ x x y x y [ M ] [ A ] = ( Sx ) ( Sy K = ) sp S x+ y = x K sp x y y Examples: calculate l solubility for AgCl in water, at 25 o C K sp = Ag 2 CrO 4 in water, at 25 o C K sp = Bi 2 S 3 in water, at 25 o C K sp =

19 Factors influencing the solubility of a precipitate 1. Common ion effect common ion will reduce the concentration of the other ion (and therefore the solubility) of ppt (unless the common ion forms complex compound with the ppt) 2. Effect of ph if the anion gets protonated, decrease of ph increases solubility if the cation hydrolyses, increase of ph increases solubility 3. Effect of complexation complexation always increases solubility 4. Effect of foreign ions foreign ions in small quantities increase, while in large quantities decrease solubility (latter is called salting-out) now back to gravimetry Steps of precipitation gravimetry: 1. Precipitation: an excess precipitating reagent added to the sample, thus the analyte converted into precipitate 2. Filtration: precipitate is separated from the solution via filtration 3. Washing: precipitate is washed from impurities 4. Drying: precipitate is dried or ignited (to convert it to a product with known composition) 5. Weighing: precipitate is weighed

20 1. Precipitation Particle size and filterability/washability the larger the better The factor determining the particle size: relative supersaturation = (Q-S)/S (where Q is the concentration of the supersaturated solution) Nucleation - Particle growth - At large relative supersaturation the rate of nucleation is large large naumber of small particles are formed At small relative supersaturation the particle growth dominates, large particles are formed In practice: elevate temperature to increase solubility, use dilute solution (to decrease Q) and add the precipitating agent slowly and under vigorous stirring Filtration and washing Filtration may happen on paper filter or on glass filter Mother liquor is the liquid from which the precipitate is formed Washing liquids distilled water or water saturated with the precipitate Peptization is a process by which the precipitate returns to it dispersed state (behaves as a solution again) Coprecipitation soluble components other than the analyte are removed from the solution together with the precipitate p surface adsorption mixed crystal formation occlusion and mechanical entrapment Precipitation from homogeneous solution

21 Drying and weighing Drying/ignition is necessary to obtain constant mass for the precipitate Drying/ignition leads to the weighing form the form of the analyte with accurately known composition (or stoichiometry) Drying: t < 200 o C Ignition: t = o C If the filtration is done with filter paper, ignition can be done, if glass filter is used, only drying is allowed Weighing is always done by using an analytical balance The weighed mass must always be larger than 100 mg (to have accuracy better than 1%) Examples: SO 4 2- ions in the form of BaSO 4 Ca 2+ ions in the form of Ca(COO) 2.H 2 O Fundamentals of titrimetry A chemical reaction between the titrant solution and the analyte is suitable for titrimetry if the analyte is suitable for titrimetry, if 1. it takes place according to one kind of known stoichiometry 2. it is quantitative (conversion is > 99.9%, no excess of reactant is needed) 3. it isreasonably fast 4. completion of the reaction can be indicated

22 Terms used in titrimetry Standard solution is a reagent of exactly known concentration that is used in the titrimetric analysis Titration is a process in which the standard solution is added to the analyte until the reaction between the analyte and the reagent is complete Equivalence point the point in the titration, when the amount of reagennt added to thge solution is exactly equivalent to the amount of the analyte (theoretical value) End point the point in the titration, when a physical change occurs that is associated with the chemical equivalence (practical value, this is what we obsrerve) Titration error E t = V ep V eq, where V ep is the actual volume of reagent required to reach the end point and V eq is the theoretical volume to reach the equivalence point Perfect titration - V ep = V eq, Terms used in titrimetry Titration curves plot the reagent volume on the horizontal axis and some function of the analyte on the vertical axis; the equivalence point can be read off the titration curve; it can either be sigmoidal or linear segment curve. Indicators they are added to the analyte solution to produce a visually observable physical change (usually colour change) at or very near to the equivalence point Primary standard is an ultrapure compound that serves as a reference material for titrimetric method of analysis high purity atmospheric stability absence of hydrate water reasonable cost reasonable solubility large molar mass Secondary standard a compound, whose purity has been established by chemical analysis and serves as a reference material for titrimetric method of analysis

23 Terms used in titrimetry Preparation of standard solutions: 1. Direct method includes (i) accurate weighing of a primary standard which is then (ii) dissolved in a suitable solvent and (iii) diluted to exactly known volume in a volumetric flask 2. Indirect method includes preparation of the tatrant solution by approximate weighing and dilution to an approximately known volume, followed by standardization, which means (i) titrating a weighed quantity of a primary or a secondary standard or (ii) titrating a known volume of a standard solution