Essential practical techniques in chemistry

Analytical balances are accurate for measuring solids to 3, or sometimes 4, decimal places.

They feature draft shielding, which reduces the impact of air current on the measured value and allows the reading to stabilise.

Top pan balance

Top pan balances are less accurate than analytical balances but can be used for heavier samples. Their accuracy generally ranges from 1 to 3 decimal places.

The lack of draft exclusion makes weighing on a top pan balance more rapid.

Stop-clock

A digital or analogue timer is used to measure time in seconds and minutes.

A digital stop clock or stop watch is often started at the beginning of the experiment in time dependent reactions and rate studies.

Gas syringe

Graduated syringes are used for gas collection experiments.

The syringe is usually in capacity and is lightly clamped to allow free movement of the plunger.

The syringe is connected to the reaction mixture by plastic or rubber tubing.

Gas syringes are faster to set up than water displacement but are more prone to leaks.

Burette (with stand)

Burettes are used where an accurate measurement of a variable volume of liquid is required.

Burettes are most commonly used in titration experiments. The volume is calculated by taking the difference of two readings.

All burette readings should be taken at eye level, from the bottom of the meniscus.

The error of measurement on a burette is applied twice – once for each reading.

SAFETY – always support the shaft whilst turning the tap to avoid snapping the glass.

Beaker

Beakers are low-precision containers for holding solids or liquids. They are relatively wide and therefore the space between graduations is related to larger volume differences.

The wide-lipped mouth of a beaker makes it easy to pour into and from, but it is less effective at containing its contents during swirling or heating. As a result, beakers are more often used for transfer, while conical flasks are used for containing reactions.

Pipette (volumetric)

Volumetric pipettes are used in the accurate delivery of a set volume of solution. The glass is calibrated for high precision and marked with a single graduation ring on the narrow part above the bulb.

and volumetric pipettes are used extensively in volumetric work.

Pipette (graduated)

Graduated pipettes have a fixed shaft width. They have graduated markings along the shaft, allowing for the measurement of liquid at variable volume.

A graduated pipette’s measurement precision is between that of a measuring cylinder and a volumetric pipette. You would generally use a graduated pipette where high precision is required but a volumetric pipette is not available in the required volume.

Pipette fillers

Pipette fillers are required for sucking up liquids into graduated and volumetric pipettes. They are built with functionality to draw in and release the liquid to align to the desired graduation mark.

It is important not to draw liquid into the filler.

SAFETY – When attaching the pipette to the filler, hold the glass close to the top, and do not push it towards your body/arm. The shaft is fragile and is easily snapped.

Teat pipette

Teat pipettes are used for delivering small volumes of liquid in drops. They are used where ‘a bit’ of a liquid is required but the exact amount does not matter.

Typical uses for teat pipettes are qualitative analysis, the addition of liquid catalysts, and the addition of indicators.

Measuring cylinders

Graduated cylinders are widely used to measure variable volumes of liquid. They are usually made from glass with a wide base for stability.

Measuring cylinders can also be inverted to collect gas in water displacement experiments.

Most labs have measuring cylinders of various sizes. Their precision is between that of a beaker and a graduated pipette/burette.

Volumetric flask

Volumetric flasks measure a single volume to a high accuracy. They are frequently used for making up exact quantities of standard solutions.

The flask’s wide bottom allows for a high volume to be contained, whereas the narrow neck, featuring the graduation mark, allows for a precise volume of solvent to be added.

Thermometers

Thermometers are for temperature measurement of liquids or vapour.

Conventional thermometers are narrow glass tubes with a reservoir of liquid, such as mercury or coloured alcohol, which expands and rises up a capillary tube against a scale.

Digital thermometers are also available. A digital thermometer’s reading leaves less potential for systematic error than a conventional thermometer.

Thermometers are also available with a ‘quick-fit’ ground glass cone so they can easily connect to a joint in ‘quick-fit’ glass apparatus, such as the set-up for distillation.

Melting point apparatus

Melting point apparatus contains an electrically heated block of metal, in which the thermometer and sample tube are contained in close proximity.

There is a magnified observation lens, allowing the time at which melting starts, and is complete, to be noted and corresponding temperatures read off on the thermometer.

A pure substance will have a sharp melting point.

An impure substance will melt at a lower temperature and over a wider range of temperatures.

Thermostatic water bath

Water baths can be heated to a desired temperature and can be used for controlling reaction temperatures.

Electric heating mantle

Electric heating mantles are thermostatically controlled and ideal for controlled heating of round-bottomed flasks for reflux or distillation.

Electric heaters are preferred over bunsen burners where flammable liquids are involved as there is no naked flame.

Hot plate

Hot plates are electrically heated plates ideal for warming several flat-bottomed containers at once. They are commonly used for heating solutions prior to recrystallisation.

meter and probe

meters are used where a precise numerical value of is required. The meter must be calibrated by dipping the probe into a set of buffer solutions of known values.

When using a probe, stir the sample before measuring it and take care not to knock the fragile glass tip on the wall of the container.

indicator colour chart

can be measured approximately using an indicator and a colour chart.

A universal indicator can be added to a sample, or indicator paper can be dipped into the solution. The is determined by comparing the colour to a colour chart.

The precision of an indicator-based measurement is very low and should only be used for qualitative measurements.

Titration apparatus

Titrations are set up with a clamped burette and a conical flask, filled using a volumetric pipette.

A white tile is often placed beneath the conical flask to make the colour change more visible and an indicator is added to the flask using a teat pipette.

Distillation apparatus

Distillation is used to separate liquids on the basis of a different boiling point.

In distillation a heated round-bottomed flask is fitted with a water-cooled condenser in a sloping position. A thermometer is attached in line with the mouth of the condenser to read off the vapour temperature. Water enters the condenser from the lower end.

Stands and clamps are used to support the glassware.

In ‘quick-fit’ apparatus, a vented receiving flask is used to collect the distillate. Alternatively, a conical flask can be placed below the mouth of the condenser.

SAFETY – The system cannot be fully sealed, or there can be a dangerous build-up of pressure. This is why the collection flask must either be vented or separated from the apparatus.

Reflux apparatus

Refluxing is used to maintain liquids at their boiling temperature for periods of time without causing material loss.
In reflux, a heated round-bottomed flask is fitted with a water-cooled condenser in a vertical position. Water enters the condenser from the lower end.

SAFETY – The system cannot be fully sealed, or there can be a dangerous build-up of pressure. This is why the condenser does not have a stopper.

Qualitative analysis testing

Qualitative analysis is predominantly carried out on a small scale in test tubes, or on an even smaller scale using dimple trays.

The method varies with the test but in general, a teat pipette is used to add a reagent and an observation is made.

Funnel

Funnels prevent spillage during the transfer of liquids between containers. They are especially necessary when the receiving vessel has a narrow mouth.

Funnels are also used with filter papers for filtration. This technique separates a solid from a liquid by providing a barrier impenetrable to the solid but permeable to the liquid. The solid collects in the paper whilst the liquid passes through and is collected in the receiving vessel.

Vacuum Filtration

Vacuum filtration is filtration under pressure.

A porcelain Buchner funnel with a perforated plate is filled with a disk of filter paper. A vacuum is created in the filter flask using a pump. This causes a pressure difference to the external environment and the solution is pushed through the filter.

Filtration under pressure is an efficient, quick and simple way to filter large quantities of reaction mixture containing fine solid particles. It is used extensively in organic solid preparations.

SAFETY – do not allow the filter flask to fill up close to the air outlet, or liquid may be sucked into the pump.

Separating funnel

Separating funnels are stoppered funnels with taps, used for separating immiscible liquid mixtures.

When using a separating funnel the mixture is shaken and then allowed to settle. Care should be taken in identifying which layer contains the desired product.

SAFETY – the separating funnel is a sealed system; the tap must be opened intermittently to prevent the build-up of pressure.

Thin layer or paper chromatography

Thin layer chromatography (TLC) is an analytical method used to separate and identify mixtures of soluble substances.

Samples are spotted onto a pencil drawn line on a TLC plate using a capillary tube or a bulb pipette. The plate is partially submerged in solvent within a covered beaker.

The solvent must start well below the sample line and the procedure must be stopped before the solvent front reaches the top of the plate.

Mixture components can be coloured and therefore observed on the plate by eye.

Many organic compounds require visualising under UV light or through the addition of a chemical reagent.