The Tools of the Trade

THE TOOLS OF THE

TRADE Apparatus

Instruments

Glassware

Procedures

WEIGHING

ELECTRONIC BALANCE

VOCABULARY

Pan

Tare

Display

Specifications

Calibration

WEIGHING

Balance Specifications

Maximum Capacity 110 g

Readability 0.1 mg

Repeatability 0.1 mg

Linearity 0.2 mg

Sensitivity 2.5 ppm/C

WEIGHING

The weighing operation consists of:

1. Place a clean receiving vessel on the

balance pan. The mass of the empty

vessel is called the tare.

2. Press the button that resets the tare

reading to zero

3. Add the substance to be weighed to the

vessel and read its new mass

WEIGHING

An Alternate procedure (weighing by difference) have to be used when dealing with hygroscopic reagents.

1. Weight a capped bottle containing the dry reagent

2. Pour some reagent from the weighing bottle into a receiver

3. Cap the weighing bottle and weight it again

WEIGHING

WEIGHING

THE DESSICATOR

Ground-glass rims vaselined

To keep apparatus airtight

Fused Calcium Chloride

Concentrated sulphuric acid

Phosphorus pentoxide

Silica-gel indicator: blue (dry)

WEIGHING

Silica-gel:

Hygrometer

indicator

CoCl2 + 6H2O Co(H2O)62+ + 2Cl-

WEIGHING

CALIBRATION

Balances have to be

calibrated against

standard weights in order

to ensure accurate

measurements

Calibration in electronic

balances is now very

easy

WEIGHING

The balance electronics are adjusted in such a way that the indication of the balance is equal to the mass of the

built-in weight with the built-in weight introduced to the

balance (Rs = ms)

1- as ss

R kgm

1

1- a

s

k

g

WEIGHING

m is the true mass of the weighing object

The conventional mass of an object, is just the reading R of the balance

at 20C

when the air density is exactly 1.2 kg m-3

after adjusting the balance with a reference mass with a density of = 8000 kg m-3, in the same conditions.

1-

1-

a

s

a

m R

MEASURING VOLUMES

BURETTES

A precisely bored glass tube with

graduations that enables us to measure

the volume of liquid delivered.

This is done by reading the level before

and after draining liquid from the burette.

BURETTES

Typical glass burettes have a teflon

stopcock

Sometimes a cap is used to keep dust out

and vapours in

When reading the height of liquid the eyes

have to be at the same level as the top of

liquid to avoid the parallax error

BURETTES Tips & tricks

Check the tip of the burette for an air

bubble and remove it.

BURETTES Tips & tricks

Rinse the tip of the burette with water from a wash bottle and dry it carefully

After a minute, check for solution on the tip to see if your

burette is leaking

The tip should be clean and dry before you take an initial

volume reading.

BURETTES Tips & tricks

Read the bottom of the meniscus

Be sure your eye is at the level of

meniscus, not above or below to avoid the

parallax error.

VOLUMETRIC FLASKS Are used to prepare a solution of a known volume Are calibrated to contain a particular volume of water at

20C when the bottom of the meniscus is adjusted to the

centre of the mark

After the final dilution, remember to mix your solution

thoroughly, by inverting the flask and shaking

PIPETTES AND SYRINGES

Pipettes are used to deliver known volumes of liquid

Transfer pipettes are calibrated to deliver a fixed volume (such as the volumetric flasks) and are the most accurate ones

Touch the tip of the pipette to the side of the beaker (or receiver) and drain the liquid

Do not blow out the last drop from a transfer pipette

PIPETTES AND SYRINGES

Measuring pipettes are calibrated to

deliver a variable volume (graduated)

A pipette pump or rubber bulb should be

used to draw solution into the pipette

PIPETTES AND SYRINGES

To deliver small volumes (microlitres)

micropipettes and syringes may be used

OTHER GLASSWARE

Erlenmeyer flasks and beakers are used

for mixing, transporting, and reacting, but

not for accurate measurements (RSD

about 5%)

OTHER GLASSWARE

Graduated cylinders are useful for measuring liquid volumes to within about 1%

They are for general purpose use, but not for quantitative

analysis

If greater accuracy is needed, use a pipette or volumetric

flask.

FILTRATION

Precipitates for gravimetric analysis are

collected by filtration, washed and then

dried

If the precipitate does not need to be

ignited, it is collected in a fritted-glass filter

crucible

The filter crucible is used with suction by

using a Kitasato

FILTRATION

FILTRATION

Sometimes the precipitate must be isolated, then igniting it at high temperature to convert it to a well defined

product of known composition

When the precipitate have to be ignited, it is collected in

ashless filter paper that leaves little residue when

oxidised

The paper should fit snugly and be seated with a little

liquid water

The correct procedure for filtration is shown in the

following figures

FILTRATION

FILTRATION

Particles adhering to the beaker or rod can be dislodged with a rubber policeman and transferred with a jet of

liquid

The precipitate to be ignited is first collected in the

ashless filter

The filter should be allowed to drain thoroughly

(overnight) with the content protected with a watchglass

Lift the paper out of the funnel, fold it suitably (see figure)

and transfer it to a porcelain crucible

IGNITION

IGNITION

With the filter paper and precipitate inside the crucible, dry the content cautiously with a small flame by

positioning the crucible above the burner with the help of

a clay triangle as indicated in the figure

After it is dry, char the filter by increasing the flame

temperature