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.
2.
3.
4.
5.
6.
7.
The best grade, the smallest bottle
Replace the top of every container immediately after removal of the reagent.
Hold the stoppers of reagent hollies between your fingers.
Never return any excess reagent to a bottle.
Never insert spatulas, spoons, or knives into a bottle.
Keep the reagent shelf and the laboratory balance clean and neat.
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
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 HNO3, H2SO4,
H2O2, or aqueous Br2, or a combination.
2D Measuring Mass
2D-1 Types of Analytical Balances
Max. capacity
Analytical balance
Macrobalance
Semimicroanalytical balance
Microanalytical balance
1 g to few kg
160 to 200 g
10 to 30 g
1 to 3 g
Precision
at max. capacity
1 part in 105~106
± 0.1 mg
± 0.01 mg
± 0.001 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
buoyancy error : weighing error that develops when the object being weighed has a
significantly different density than the weights.
W1: the corrected mass of the object
W2: the mass of the standard weights
dobj: the density of the object
dwts: the density of the weights
dair: the density of the air displaced
by them; 0.0012 g-cm-3
d
d
W1 = W2 + W2 ( air − air ) (2-1)
d obj d wts
0.00
-0.10
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.
-0.20
-0.30
0
2
4
6
8
10
12
14
16
18
20
Density of object, g/L
Ex. 2-1
A bottle weighed 7.6500 g empty and 9.9700 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 = 9.9700 - 7.6500 = 2.3200 g.
W1 = 2.3200 + 2.3200(
0.0012 0.0012
−
) = 2.3227 g
0.92
8.0
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
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
KC1.
0
10
20
Time after removal from oven, min
2D-5 Auxiliary Balances
For weighings that do not require great accuracy.
Precision
Max. capacity
Balances
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.
A
2E-2 Desiccators and Desiccants
Oven drying is the most common way.
Desiccator：a device for drying substances or objects.
7
Fig. 2-8
Components of a typical desiccator. The
base contains a chemical drying agent.
Desiccator plate
Ground-glass
surfaces
Desiccant: chemical drying agent, such
as anhydrous calcium chloride, calcium
sulfate (Drierile®), anhydrous
magnesium perchlorate (Anhydrone®
or Dehydrite®), or phosphorus
pentoxide.
Desiccant
2E-3 Manipulating Weighing Bottles
Fig. 2-10
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.
Fig. 2-9
Arrangement
for the drying
of samples.
105 to 110°C
heating
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
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. 2-11
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
Speed
Slow
Rapid
Rapid
Rapid
Convenience
Troublesome, Convenient Convenient Convenient
and ease of
inconvenient
preparation
Max. ignition None
>500
200-500
1100
temp. °C
Chemical
C (reducing Inert
Inert
Inert
reactivity
properties)
Porosity
Many
Several
Several
Several
available
available
available
available
Convenience
Satisfactory Unsuitable; Unsuitable; Unsuitable;
with gelatinous
filter tends filter tends filter tends
precipitate
to clog
to clog
to clog
Cost
Low
Low
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
Al2O3
crucible
Rapid
Convenient
1450
Inert
Several
available
Unsuitable;
filter tends
to clog
High
2F-2 Filtering and Igniting Precipitates
Preparation of Crucibles
Filting and Washing Precipitates
decantation, washing, and transfer
Fig. 2-12 (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. 2-15 Ignition of a
precipitate.
Proper cruciblc
position for
preliminary charring.
Fig. 2-16 Train for vacuum filtration. The trap isolates the
filler flask from the source of vacuum.
2G Measuring Volume
2G-1 Units of Volume
liter (L), milliliter (mL)
10
2G-2 The Effect of Temperature on Volume Measurements
The coefficient of expansion: ≈ 0.025%/°C, standard temp:
20°C
Ex. 2-2 A 40.00-mL sample is taken from an aqueous solution at 5°C. What
volume does it occupy at 20°C?
0.025%
V20o = V5o +
(20 − 5)(40.00) = 40.00 + 0.15 = 40.15mL
100%
2G-3 Apparatus for Precisely Measuring Volume
Tolerances, Class A Transfer Pipets
pipets, buret, and volumetric flask
Capacity, mL
0.5
1
2
5
10
20
25
50
100
Pipets
Volumetric, or transfer, pipet:
(Fig.2-17a) delivers a single, fixed
volume between 0.5 and 200 mL.
Measuring pipets (Fig.2-17b, c): max.
capacity ranging from 0.1 to 25 mL.
Tolerances, mL
± 0.006
± 0.006
± 0.006
± 0.01
± 0.02
± 0.03
± 0.03
± 0.05
± 0.08
Table 2-2 Characteristics of Pipets
Type of
Available
Name
Function
Type of Drainage
Calibration*
Capacity, mL
Volumetric TD
fixed vol.
1-200
Free
Mohr
TD
variable vol.
1-25
To lower calibration line
Serological TD
variable vol.
0.1-10
Blow out last drop
Serological TD
variable vol.
0.1-10
To lower calibration line
Ostwald-Folin TD
fixed vol.
0.5-10
Blow out last drop
Lambda
TC
fixed vol.
0.001-2 Wash out with suitable
(to contain)
solvent
Lambda
TD
fixed vol.
0.001-2 Blow out last drop
Eppendorf
TD
variable or
0.001-1 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
Fig. 2-19
Burets:
(a) glass-bead valve,
(b) Teflon valve
Fig. 2-20 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.
Fig. 2-21 Method for reading a buret.
1
The eye should be level with the
Correct eye meniscus. The reading shown is 34.37
level for
mL. If viewed from position 1, the
reading
reading appears < 34.37 mL, from
2
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
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. 2-23 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 1.000 g of water weighed in air against stainless steel
weights.
T,
°C
10
11
12
13
14
15
Volume, mL
At T
20 °C
1.0013 1.0016
1.0014 1.0016
0.0015 1.0017
1.0016 1.0018
1.0018 1.0019
1.0019 1.0020
T,
°C
16
17
18
19
20
Volume, mL
At T
20 °C
1.0021 1.0022
1.0022 1.0023
1.0024 1.0025
1.0026 1.0026
1.0028 1.0028
T,
°C
21
22
23
24
25
Volume, mL
At T
20 °C
1.0030 1.0030
1.0033 1.0032
1.0035 1.0034
1.0037 1.0036
1.0040 1.0037
T,
°C
26
27
28
29
30
Volume, mL
At T
20 °C
1.0043 1.0041
1.0045 1.0043
1.0048 1.0046
1.0051 1.0048
1.0054 1.0052
Ex 2-3 A 25-mL pipet delivers 24.076 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 = 24.976 g × 1.0040 mL/g = 25.08 mL
At 20 °C: V = 24.976 g × 1.0037 mL/g = 25.07 mL
2I The Laboratory Notebook
13