13 - 13 Titrimetric Methods; Precipitation Titrimetry...

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72 Titrimetric method : analytical procedures in which the amount of analyte is determined from the amount of a standard reagent required to react with the analyte completely. Three types of quantitative titrimetry 1. volumetric (the most widely used) 2. gravimetric 3. coulometric 13A Some Terms Used in Volumetric Titrimetry * Standard solution (standard titrant) 1. accurate known conc. : 4 significant figures 2. stable 3. stoichiometric reaction : whole-number ratio 4. rapid and quantitatively complete reaction : 99.9 % *Titration *Direct titration *Back-titration, residual-titration : when the rate of reaction between the analyte and reagent is slow or when the reagent lacks stability. *Equivalence point , theoretical point: the point in a titration when the amount of added standard reagent exactly equivalent to the amount of analyte. *End point : the point in a titration when a physical change occurs that is associated with the condition of chemical equivalence. *Titration error E t : the difference in volume or mass between the equivalence point and the end point. (V ep – V eq ) *Indicator : large changes in the relative conc. of analyte or titrant occur in the equivalence-point region. a. appearance or disappearance of a color. b. change in color c. appearance or disappearance of turbidity. Instruments for detect end point: voltmeters, ammeters, ohmmeters, colorimeters, temperature recorders, refractometers.
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73 Fig. 13-1 The titration process * Primary standards : 1. high purity 2. stability in air 3. absence of hydrate water 4. ready availability at modest cost 5. reasonable solubility in the titration medium 6. reasonably large formula weight * Secondary standard 13B Standard Solutions *Standardization, establishing the conc. of standard soln. 1. Direct method: carefully weighed quantity of a primary standard dissolved diluted to an exactly known volume. 2. Standardized by titrating a. a weighed quantity of a primary standard b. a weighed quantity of a secondary standard c. a measured volume of another standard soln secondary standard solution *Methods for expressing the conc. of standard solutions a. molarity C : no. of moles/L soln b. normality C N : no. of equivalent/L soln. 13C Volumetric calculations ) L mol ( C V(L) (g/mol) A mass molar (g) A mass (mol) A amount A × = = ) mL mmol ( C V(mL) (g/mmol) A mass molar (g) A mass (mmol) A amount A × = =
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74 *Calculation of the Molarity of Standard Solutions (SS) Ex. 13-1. Describe the preparation of 2.000 L of 0.0500 M AgNO 3 (169.87 g/mol) from the primary standard-grade solid. amount AgNO 3 = 0.050 L × 2.000 mol/L = 0.1000 mol mass AgNO 3 = 0.1000 mol × 169.87 g/mol =16.98 g dissolving 16.98 g AgNO 3 in water and diluting to 2.000 L. Ex. 13-2. Describe how 500 mL of standard 0.0100 M Na + solution can be prepared from primary standard Na 2 CO 3 (105.99 g/mol). amount Na 2 CO 3 = 500 mL × 0.0100 mmol/mL × ½ = 2.50 mmol mass Na 2 CO 3 = 2.50 mmol × 0.10599 g/mmol = 0.265 g dissolving 0.265 g Na 2 CO 3 in water and diluting to 500 mL.
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This note was uploaded on 02/02/2012 for the course CHEM 211 taught by Professor Santos,j during the Spring '11 term at Abu Dhabi University.

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13 - 13 Titrimetric Methods; Precipitation Titrimetry...

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