Chapter 2 Chemicals, Apparatus, Unit operations of Analytical Chemistry

Transcription

1 Chapter 2 Chemicals, Apparatus, Unit operations of Analytical Chemistry Slide 1 Fig 2-CO, p.20 A. Selecting and Handling Reagents Quality of reagent must be consistent intended use The purity of a reagent determines the accuracy of the measurement Reagent Grade Chemicals Conform to minimum standards set by ACS Quantities of impurities or maximum amounts to expected given Primary-Standard Grade Known exact composition and very high purity Assay results printed on container label National Institute of Standards and Technology (NIST) Special-Purpose Reagent Chemicals Prepared for specific applications Properties pertinent to the intended use are determined Slide 2 1

2 Handling Reagents See lab manual and p. 22 of Skoog et al. Replace top of containers immediately after removal of reagent Hold stoppers between fingers; never set a stopper on any bare surface Never return any excess reagent to bottle Never insert spatulas, spoons, etc into bottles that contain solid chemicals. Use porcelain spoon if you must. Keep reagent shelf (hood) and laboratory balance clean and neat. Dispose of surplus reagents as per the instructions provided or local regulations Slide 3 Slide 4 Fig 2-1, p.23 2

3 Analytical Balances Measuring Mass Determinine mass with a precision of at least 1 part in 10 5 (10 ppm) at maximum capacity Types Macrobalance capacity = g, precision ± 0.1 mg Semimicroanalytical capacity = g, precision ± 0.01 mg Microanalytical Capacity = 1-3 g, precision ± mg Slide 5 Electronic Balance Slide 6 3

4 The Electronic Analytical Balance Principle of operation Use of null detector Use of magnetic field strength as a restoring force An electrical current in the coil induces a magnetic field that supports/levitates the cylinder, the pan, the indicator arm and any mass on the pan. Null is established with empty pan light produces a current that is amplified and used to induced a magnetic field in coil that supports Servo System: small electric signal used to activate a mechanical system to maintain a null position Slide 7 Calibration of Balance and Sources of Error in Weighing All balances in the analytical lab have been calibrated using standard masses (stainless stell, density 8.0 g/cm 3 ) A buoyancy error is introduced if the density of the object weighed is very different from the density of the standard masses Origin: difference in buoyant force exerted by the medium on the object and the standard masses Correction for buoyancy Slide 8 4

5 Correction for buoyancy W 1 = W 2 + W 2 d d air obj d d air wts d air = g / cm 3 W 1 : corrected mass W 2 : measured mass Slide 9 Slide 10 Fig 2-5, p.28 5

6 Temperature Effects Always cool object to ambient temperature before weighing Otherwise, errors are introduced Convection currents exert a buoyant force Warm air is less dense Consequence: apparent mass of object is low Slide 11 A: porcelain filtering crucible B: weighing bottle Slide 12 Fig 2-6, p.29 6

7 Equipment and Manipulations Associated with Weighing Weighing by difference Weighing bottles Slide 13 Dessicator and dessicants Dessicants/ drying agents -Anhydrous calcium chloride (white) -Calcium sulfate + 3% CoCl 2 (Drierite) (blue) Slide 14 Fig 2-8a, p.31 7

8 Drying of Samples Slide 15 Fig 2-9, p.32 Measuring Volume Use of Volumetric Flasks, Volumetric Pipets and Burets Wash with detergent and thoroughly rinse with deionized water If the film on the wall breaks, it is not clean! Check the valve of the buret for leaks Check the tip of the buret for air gaps Do not remove (blow out) the remaining liquid in the tip of the pipet at the end of free drainage Uncertainty (Tolerances) of Class A glassware: -on pages 41 and 42 Slide 16 8

9 Slide 17 Fig 2-19, p.43 Pipetting Quantitative Transfer Slide 18 Fig 2-22a, p.45 9

10 Reading the buret Slide 19 Fig 2-21, p.44 Manual Titration from a Buret Slide 20 Fig 2-23, p.47 10

11 Maintaining a Laboratory Notebook Page 6 in lab manual and page 21 of Skoog et al. Note: do not forget to record observations Slide 21 11