Lecture 7. Titration Pt2

Lecture 7. Titration Pt2 - Standard Solutions Standard Gary...

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Unformatted text preview: Standard Solutions Standard Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. Sons, Examples: Na2CO3, AgNO3, disodium oxalate, disodium potassium hydrogen phthalate, … potassium Standard Solutions Standard NaOH is not a good standard because it readily absorbs water NaOH and carbon dioxide. and => We can use NaOH as titrant, but in that case we have to… …standardize the titrant! Standardizing the Titrant Standardizing Example: An approximate 0.1 M HCl solution is prepared by 120-fold dilution of concentrated HCl. It is standardized by titrating 0.1876 g of dried primary standard sodium carbonate: dried CO32- + 2H+ H2O + CO2 * The titration required 35.86 mL acid. The concentration of the HCl. concentration Calculate the molar Types of Titrations Types 1) 2) 3) 4) … Acid-Base Precipitation Complexometric Reduction-Oxidation Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Finding Analyte Concentration Finding (1:1 Stoichiometry) Example: A 0.4671 g sample containing sodium bicarbonate was dissolved and titrated with standard 0.1067 M hydrochloric acid solution, requiring 40.72 mL. The reaction is: solution, HCO3- + H+ H2O + CO2 Calculate the percent sodium bicarbonate in the sample. Other Stoichiometries aA+tTP Example: A 0.2638 g soda ash sample is analyzed by titrating the Example: sodium carbonate with the standard 0.1288 M hydrochloric acid solution, requiring 38.27 mL. The reaction is: solution, CO32- + 2H+ H2O + CO2 Calculate the percent sodium carbonate in the sample. Back Titrations Back 1) Add a known amount of excess reagent 2) Titrate to find out how much excess reagent there was. analyte + reagent 1 product + excess reagent 1 analyte Unknown Known Unknown •If necessary, this reaction can be done at high If temperature, over a long time period, at high pressure, or any other reaction conditions not suitable for a direct titration. suitable excess reagent 1 + reagent 2 product excess Unknown Known • Under conditions suitable for a quick titration with clear end point under simple reaction conditions. clear Back Titration Back Example: Chromium (III) is slow to react with EDTA and is therefore determined by back-titration. A pharmaceutical preparation containing chromium (III) is analyzed by treating a 2.63 g sample with 5.00 mL of 0.0103 M EDTA. Following reaction, the excess EDTA is back-titrated with 1.32 mL of 0.0122 M zinc solution. What is the percent chromium chloride in the pharmaceutical preparation? pharmaceutical Cr3+ + excess EDTA [Cr-EDTA]- + 4 H+ + surplus EDTA Zn2+ + surplus EDTA [Zn-EDTA]2- + 4 H+ Zn Automated end point detection detection E.g., measurement of the electrical E.g., potential with a silver ion selective electrode (see Harris, Chapter 6.5). electrode How do We Find the Equivalence Point? Equivalence Titration Curves OH- + H+ H2O Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman pH Indicators pH pH = 3.7 pH pH = 4.7 pH pH = 5.7 pH Yellow Green Blue (10:1 HA:A-) (1:1 HA:A-) (1:10 HA:A-) Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman pH Indicators pH Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. pH Indicators pH Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Indicator Errors Indicator The Endpoint is different from the Equivalence Point! 1) Color change does not occur exactly at the equivalence point. 2) Indicators are weak acids and bases. Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. Equivalence Point Determination: Errors Equivalence The end point is marked by a sudden change in a observed/measured The quantity, e.g., absorbance, electrical potential, … quantity, ⇒Equivalence point ≠ end point E.g., permanganate: Why is the end point different from the equivalence E.g., point? point? => How can you make a correction for this difference? http://www.chem.ualberta.ca/~iip/chem211irc/TitrationVideo.html EDTA Calcium Titration with EDTA Calcium n Ca+ + (n+m) EDTA4- + m [MgIn]+ n [CaEDTA]2- + m [MgEDTA]2- + m In- n >> m ? n << m ? n=m? In- (Eriochrome T) Analysis of Mixtures Analysis => Harris, Chapter 6.5 Requirements Requirements Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. Sons, ...
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