2 Chemicals, Apparatus and Unit Operations of Analytical Chemistry

Transcription

1 2 Chemicals, Apparatus and Unit Operations of Analytical Chemistry 2A Selecting and Handling Reagents and Other Chemicals NIST (The National Institute of Standards and Technology) 2A-1 Classifying Chemicals Reagent Grade Reagent Chemical Committee of the American Chemical Society The maximum limits of impurity allowed by the ACS specifications; The actual assay for the various impurities. Primary-Standard Grade in addition to extraordinary purity and the assay is printed on the container label. reference standards, complex substances that have been exhaustively analyzed. Special-Purpose Reagent Chemicals Solvents for spectrophotometry and high-performance liquid chromatography. 2A-2 Rules for Handling Reagents and Solutions 1. The best grade, the smallest bottle 2. Replace the top of every container immediately after removal of the reagent. 3. Hold the stoppers of reagent hollies between your fingers. 4. Never return any excess reagent to a bottle. 5. Never insert spatulas, spoons, or knives into a bottle. 6. Keep the reagent shelf and the laboratory balance clean and neat. 7. Observe local regulations (the disposal of surplus reagents and solution). 2B Cleaning and Marking Laboratory Ware pencil. baked permanently into the glaze by heating at a high temperature saturated solution of iron(iii) chloride It is seldom necessary to dry the interior surface of glassware before use. benzene or acetone, may be effective in removing grease films 4

2 2C Evaporating Liquids Fig. 2-1 Arrangement for the evaporation of a liquid Bumping: sudden, often violent boiling that tends to spatter solution out of its container. Wet-ashing: Oxidation of the organic constituents of a sample with oxidizing reagents such as HNO 3, H 2 SO 4, H 2 O 2, or aqueous Br 2, or a combination. 2D Measuring Mass 2D-1 Types of Analytical Balances Max. capacity Precision at max. capacity Analytical balance 1 g to few kg 1 part in 10 5 ~10 6 Macrobalance 160 to 200 g ± 0.1 mg Semimicroanalytical balance 10 to 30 g ± 0.01 mg Microanalytical balance 1 to 3 g ± mg (1μg) 2D-2 The Electronic Analytical Balance Tare Taring 2D-3 The Single-Pan Mechanical Analytical Balance Precautions in Using an Analytical Balance 1. Center the load on the pan. 2. Protect the balance from corrosion. 3. Observe special precautions (Section 2E-6) for the weighing of liquids. 4. Consult the instructor if the balance appears to need adjustment. 5. Keep the balance and its case scrupulously clean. 6. Allow an object return to room temperature before weighing it. 7. Use longs or finger pads. 2D-4 Sources of Error in Weighing Correction for Buoyancy 5

3 buoyancy error : weighing error that develops when the object being weighed has a significantly different density than the weights W 1 2 ( dair d = W air 2 + W ) (2-1) d d obj wts W 1 : the corrected mass of the object W 2 : the mass of the standard weights d obj : the density of the object d wts : the density of the weights d air : the density of the air displaced by them; g-cm Density of object, g/l Fig. 2-5 Effect of buoyancy on weighing data (density of weights = 8 g cm -3 ). Plot of relative error as a function of the density of the object weighted. Ex. 2-1 A bottle weighed g empty and g after introduction of an organic liquid with a density of 0.92 g-cm -3. The balance was equipped with stainless steel weights (d = 8.0 g-cm -3 ). Correct the mass of the sample for the effects of buoyancy. mass of the liquid = = g W1 = ( ) = g Temperature Effects 1. a buoyant effect on the pan and object 2. warm air trapped in a closed container weighs less than the same volume at a lower temperature. Both effects cause the apparent mass of the object to be low. Allow heated objects to return to room temperature before being weighed. 6

4 Fig. 2-6 Effect of temperature on weighing data. Absolute error in mass as a function of lime after the object was removed from a 110 C drying oven. A: porcelain filtering crucible. B: weighing bottle containing about 7.5 g of KC Time after removal from oven, min 2D-5 Auxiliary Balances For weighings that do not require great accuracy. Balances Max. capacity Precision at max. capacity sensitive top-loading 150 to 200 g 1 mg top-loading 25,000 g ± 0.05 g 2E Equipment and Manipulations Associated with Weighing Drying or ignition to constant mass is a process in which a solid is cycled through heating, cooling, and weighing steps until its mass becomes constant to within 0.2 to 0.3 mg. 2E-1 Weighing Bottles Fig. 2-7 Typical weighing bottles. 2E-2 Desiccators and Desiccants Oven drying is the most common way. Desiccator:a device for drying substances or objects. 7

5 Fig. 2-8 Components of a typical desiccator. The base contains a chemical drying agent. Ground-glass surfaces Desiccant Desiccator plate Desiccant: chemical drying agent, such as anhydrous calcium chloride, calcium sulfate (Drierile ), anhydrous magnesium perchlorate (Anhydrone or Dehydrite ), or phosphorus pentoxide. 2E-3 Manipulating Weighing Bottles Fig. 2-9 Arrangement for the drying of samples. 105 to 110 C heating Fig Quantitative transfer of solid sample. Note the use of tongs to hold the weighing bottle and a paper strip to hold the cap to avoid contact between glass and skin. 2E-4 Weighing by Difference 2E-5 Weighing Hygroscopic Solids 2E-6 Weighing Liquids 2F Filtration and Ignition of Solids 2F-1 Apparatus 1. simple Crucibles Containers, porcelain, aluminum oxide, silica, and platinum (constant mass) Ni, Fe, Ag and gold for high temp fusion 8

6 2. Filtering Crucibles Containers + filters Sintered-glass (frittled-glass) crucibles: fine, medium, and coarse porosities (marked f, m & c). (200 C) Gooch crucible: perforated bottom that supports a fibrous mat. Glass mats can tolerate temp in excess of 500 C and are substantially less hygroscopic than asbestos. Fig Adapters for filtering crucibles. 3. Filter Paper Ashless: (9- or 11-cm) leave a residue 0.1 mg Table 2-1 Comparison of Filtering Media for Gravimetric analyses Gooch Glass Porcelain Characteristic Paper crucible, crucible Crucible Glass Mat Al 2 O 3 crucible Speed Slow Rapid Rapid Rapid Rapid Convenience and ease of Troublesome, Convenient Convenient Convenient Convenient inconvenient preparation Max. ignition None > temp. C Chemical C (reducing Inert Inert Inert Inert reactivity properties) Porosity Many available Several available Several available Several available Several available Convenience Satisfactory Unsuitable; Unsuitable; Unsuitable; Unsuitable; with gelatinous precipitate filter tends to clog filter tends to clog filter tends to clog filter tends to clog Cost Low Low High High High 4. Heating Equipment Oven: max. temp ranges from 140 to 260 C; for many ppts: 110 C. Microwave laboratory ovens: shorten drying cycles. 9

7 2F-2 Filtering and Igniting Precipitates Preparation of Crucibles Filting and Washing Precipitates decantation, washing, and transfer Fig (a) Washing by decantation. (b) transferring the precipitate. Decantation: pouring a liquid gently so as to not disturb a solid in the bottom of the container. Creeping: a solid moves up the side of a wetted container or filter paper. Do not permit a gelatinous precipitate to dry until it has been washed completely. 2F-3 Directions for Filtering and Igniting Precipitates Preparation of a Filter Paper Transferring Paper and Precipitate to a Crucible Ashing Filter Paper & Using Filtering Crucibles Fig Ignition of a precipitate. Proper cruciblc position for preliminary charring. 2G Measuring Volume 2G-1 Units of Volume liter (L), milliliter (ml) Fig Train for vacuum filtration. The trap isolates the filler flask from the source of vacuum. 10

8 2G-2 The Effect of Temperature on Volume Measurements The coefficient of expansion: 0.025%/ C, standard temp: 20 C Ex. 2-2 A mL sample is taken from an aqueous solution at 5 C. What volume does it occupy at 20 C? V % o = V o + (20 5)(40.00) = = mL 5 100% 2G-3 Apparatus for Precisely Measuring Volume pipets, buret, and volumetric flask Pipets Tolerances, Class A Transfer Pipets Capacity, ml Tolerances, ml 0.5 ± Volumetric, or transfer, pipet: 1 ± (Fig.2-17a) delivers a single, fixed 2 ± volume between 0.5 and 200 ml. 5 ± ± 0.02 Measuring pipets (Fig.2-17b, c): max. capacity ranging from 0.1 to 25 ml ± 0.03 ± 0.03 ± 0.05 ± 0.08 Table 2-2 Characteristics of Pipets Name Type of Available Function Calibration* Capacity, ml Type of Drainage Volumetric TD fixed vol Free Mohr TD variable vol To lower calibration line Serological TD variable vol Blow out last drop Serological TD variable vol To lower calibration line Ostwald-Folin TD fixed vol Blow out last drop Lambda TC (to contain) fixed vol Wash out with suitable solvent Lambda TD fixed vol Blow out last drop Eppendorf TD variable or Tip emptied by air (to deliver) fixed vol. displacement Burets Burets, like measuring pipets, enable the analyst to deliver any volume Volumetric Flasks: 5 ml to 5 L 1. preparation of standard solutions 2. dilution of samples to a fixed volume 11

9 Fig Burets: (a) glass-bead valve, (b) Teflon valve Fig Typical volumetric flasks. 2G-4 Using Volumetric Equipment Cleaning soaking in a warm detergent solution, tap water, distilled water It is seldom necessary to dry volumetric ware. Avoiding Parallax Parallax is the apparent displacement of a liquid level or of a pointer as an observer changes position. Parallax occurs when an object is viewed from a position that is not at a right angle to the object. A meniscus is the curved surface of a liquid at its interface with the atmosphere. 1 Correct eye level for reading 2 Fig Method for reading a buret. The eye should be level with the meniscus. The reading shown is ml. If viewed from position 1, the reading appears < ml, from position 2, it appears larger. 2G-5 Directions for Using a Pipet Never use your mouth to draw liquid into a pipet because of the possibility of accidentally ingesting the liquid being pipetted. Cleaning Measuring an Aliquot The small volume remaining inside the tip of a volumetric pipet should not be blown or rinsed into the receiving vessel. 12

10 2G-6 Directions for Using a Buret 1. Cleaning 2. Lubrication of a Glass Stopcock 3. Filling 4. Titration Buret readings should be estimated to the nearest 0.01 ml. Fig Recommended method for manipulating a buret stopcock. 2G-7 Directions for Using a Volumetric Flask 2H Calibrating Volumetric Classware Table 2-3 Volume occupied by g of water weighed in air against stainless steel weights. T, Volume, ml T, Volume, ml T, Volume, ml T, Volume, ml C At T 20 C C At T 20 C C At T 20 C C At T 20 C Ex 2-3 A 25-mL pipet delivers g of water weighed against stainless steel mass at 25 C. Calculate the volume delivered by this pipet at 25 C and 20 C. At 25 C: V = g ml/g = ml At 20 C: V = g ml/g = ml 2I The Laboratory Notebook 13