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Analytical Standards

Analytical Standards . Introduction . Standards are materials containing a known concentration of an analyte. They provide a reference to determine unknown concentrations or to calibrate analytical instruments.

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Analytical Standards

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  1. Analytical Standards

  2. Introduction • Standards are materials containing a known concentration of an analyte. They provide a reference to determine unknown concentrations or to calibrate analytical instruments. • The accuracy of an analytical measurement is how close a result comes to the true value. Determining the accuracy of a measurement usually requires calibration of the analytical method with a known standard. This is often done with standards of several concentrations to make a calibration or working curve

  3. Working Curve or Calibration Curve • A working curve is a plot of the analytical signal (the instrument or detector response) as a function of analyte concentration. These working curves are obtained by measuring the signal from a series of standards of known concentration. The working curves are then used to determine the concentration of an unknown sample, or to calibrate the linearity of an analytical instrument.

  4. Primary standard • A primary standard inchemistry is a reliable, readily quantified substance. Features of a primary standard include: • High purity2. Stability (low reactivity)3. Low hygroscopicity and efflorescence4. High solubility (if used in titration)5. modest cost • 6. High equivalent weight

  5. Some examples of primary standards according to the European Pharmacopoeia

  6. Benzoic acidfor standardisation of waterless basic solutions Potassium bromate(KBrO3) for standardisation of sodium thiosulfate solutions Potassium hydrogen phthalate(usually called KHP) for standardisation of andperchloric acid in acetic acid solutions Arsenic trioxide for making sodium arsenite solution for standardisation of sodium periodate solution

  7. Sodium chloridefor standardisation of silver nitrate solutions Sulfanilic acidfor standardisation of sodium nitrite solutions Zincpowder, after being dissolved in sulfuric or hydrochloric acid, for standardisation of EDTA solutions • Sodium carbonatefor standardisation of aqueous acids: hydrochloric, sulfuric acid and nitric acid solutions(but not acetic acid)

  8. Primary Standards • A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight

  9. Some primary standards for titration of acids: • sodium carbonate: Na2CO3, mol wt. = 105.99 g/mol • tris-(hydroxymethyl)aminomethane (TRIS or THAM): (CH2OH)3CNH2, mol wt. = 121.14 g/mol

  10. Some primary standards for titration of bases: • potassium hydrogen phthalate (KHP): KHC8H4O4, mol wt. = 204.23 g/mol • potassium hydrogen iodate: KH(IO3)2, mol wt. = 389.92 g/mol

  11. Some primary standards for redox titrations: • potassium dichromate: K2Cr2O7, mol wt. = 294.19 g/mol

  12. Secondary Standards • A secondary standard is a standard that is prepared in the laboratory for a specific analysis. It is usually standardized against a primary standard

  13. Precipitation titration

  14. Titrations with precipitating agents are useful for determining certain analytes e.g. Cl- can be determined when titrated with AgNO3.

  15. Detection of end point: • Chemical • Precipitation Type - Mohr’s method • Adsorption – Fajan’s method • For silver analyses –Volhard method

  16. Sensors –Potentiometric or amperometric

  17. The chemical types are also classified into: • Indicators reacting with titrant forming specific color. • Adsorption indicators.

  18. Indicators reacting with the titrant: • Two methods will be discussed where this type of indicators are applied; namely: Mohr and Volhard.

  19. I) Mohr method for determining chloride: • Chloride is titrated with AgNO3 solution. A soluble chromate salt is added as the indicator. This produces a yellow color solution. When the precipitation of the chloride is complete, the first excess of Ag+ reacts with the indicator to precipitate red silver chromate:

  20. 2 Ag+(aq) + CrO42–(aq) → Ag2CrO4(s) Yellow red ppt

  21. The Mohr method must be performed at a pH about 8. This method is useful for determining Cl- in neutral or unbuffered solutions such as drinking water.

  22. II) Volhard titration: • This is an indirect titration procedure for the determination of anions that precipitate with silver like Cl-, Br-, I-, SCN-, and it is preferred in acid (HNO3) solution. A measured excess of AgNO3 is added to ppt the anion, and the excess of Ag+ is determined by back titration with standard potassium thiocyanate solution:

  23. Ag+(aq) + Cl–(aq) → AgCl(s) + excess Ag+ excess Ag+(aq) + SCN–(aq) → AgSCN(s)

  24. The end point is detected by adding iron III (Fe3+) as ferric ammonium sulfate which forms a soluble red complex with the first excess of titrant. Fe3+(aq) + SCN–(aq) → [FeSCN]2+(aq)

  25. These indicators must not form a compound with the titrant that is more stable than the precipitate or the color reaction would occur on addition of the first drop of titrant.

  26. Adsorption indicators: The indicator reaction takes place on the surface of the precipitate. The indicator, which is a dye, exists in solution as the ionized form, usually an anion.

  27. Principle of adsorption: Consider the titration of Cl- with Ag+. Before the equivalent point, Cl- is in excess and the primary layer is Cl-. This repulses the indicator anions; and the more loosely held the secondary (counter) layer of adsorbed ions is cations, such as AgCl : Cl-

  28. Beyond the equivalent point (end point as well), Ag+ is in excess and the surface of the precipitate becomes positively charged, with the 1 layer being Ag+. This will now attract the indicator anion and adsorb it in the 2 (counter) layer: AgCl : Ag+ : : indicator-

  29. The color of the adsorbed indicator is different from that of the un-adsorbed indicator, and this difference signals the completion of the titration. The degree of adsorption of the indicator can be decreased by increasing the acidity.

  30. The titration of chloride using this kind of indicator is called Fajan’s Method.

  31. Fajan’s method is the most recent and most accurate silverhalide method. It is based on the adsorption of dichlorofluorescein (DCF) on the surface of the positively charged silver chloride particles formed in the precipitation titration when Ag+ ion is in excess.

  32. Precipitation Titration Curve • Example – AgI(s) formation • Ag+ + I-AgI(s) • Reverse of the Ksp reaction for the dissociation of AgI(s) so the K for this reaction equal 1/Ksp[AgI(s)] = 1.2 X 1016 • Large K value means that the equilibrium lies to the right for this reaction • Each aliquot addition of Ag+ titrant reacts virtually entirely to form AgI(s)

  33. Precipitation Titration Curve • Titration Curves typically exhibit 3 distinct regions for a single titrant reacting with a single analyte. • Before the Equivalence Point • At the Equivalence Point • After the Equivalence Point

  34. Precipitation Titration Curve • Before the Equivalence Point • Example of AgI(s) formation • Most of the [Ag+] reacts entirely to give AgI • p[Ag+] = • the amount of Ag+ left in solution • The amount of I- present • From Ksp determine the amount of Ag+ present

  35. Precipitation Titration Curve • At the equivalence point • Stoichiometric amount of Ag+ as I- so all has precipitated out as AgI(s) • Regular Ksp calculations

  36. Precipitation Titration Curve • After Equivalence Point • [Ag+] is determined by the Ag+ present after the equivalence point with the dilution factor taken into account

  37. Precipitation Titration Curve Example • 25.00 mL of 0.1000 M I- was titrated with 0.05000M Ag+. Ag+ + I-AgI(s) The solubility product for AgI is 8.3 x 10-17. Calculate the concentration of Ag+ ion in solution (a) after addition of 10.00 mL of Ag+; (b) after addition of 52.00 mL of Ag+; (c) at the equivalence point.

  38. Precipitation Titration Curve Example • 25.00 mL of 0.04132 M Hg2(NO3)2 was titrated with 0.05789 M KIO3. Hg22+ + 2IO3-Hg2(IO3)2(s) The solubility product for Hg2(IO3)2 is 1.3 x 10-18. Calculate the concentration of Hg22+ ion in solution (a) after addition of 34.00 mL of KIO3; (b) after addition of 36.00 mL of KIO3; (c) at the equivalence point.

  39. Precipitation Titration Curve Example • Consider the titration of 50.00 mL of 0.0246 M Hg(NO3)2 with 0.104 KSCN. Calculate the value of pHg22+ at each of the following points and sketch the titration curve: 0.25VT 0.5VT 0.75VT 1.05VT 1.25VT

  40. Titration Curve Shape • 1:1 Stoichiometry of Reagents • Equivalence point is the steepest point of a curve • Maximum slope • An inflection point • Other Stoichiometries • The curve is not symmetric about the equivalence point • The equivalence point is not at the center of the steepest section of the curve • The less soluble the product, the sharper the curve around the equivalence point

  41. Titration Curve Shape

  42. Titration of a Mixture • The less soluble product forms first • If there is sufficient difference in solubility of products • First precipitation is nearly complete before the second one begins • Separation by precipitation (section 6-5) • Coprecipitation • Alters the expected endpoints

  43. Titration of a Mixture Example

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