3.2 Practical skills and techniques

what does weighing accurately approximately mean

mass should be around the mass required but with an accurate reading

what is heating to constant mass

make sure all water or volatile substances are removed

what is the process of heating to constant mass

The sample is heated

The sample is cooled in a desiccator (a sealed container with a drying agent) to prevent water absorption.

The sample is weighed.

Heating, cooling, and weighing is repeated until the mass does not change (constant mass)

what is gravimetric analysis used to determine

used to determine the mass of an element or compound in a substance

what happens in gravimetric analysis

The substance is converted into another substance of known chemical composition, which can be readily isolated and purified, conversion can either be through precipitation or volatilisation

what happens in precipitation conversion

substance undergoes a precipitation reaction. The precipitate is separated from the filtrate and the filtrate tested to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed

what happens in volatilisation conversion

the substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded

suggest an improvement the student could make to reduce uncertainty in the titre value

titrate a larger sample, dilute the solution, use a micro-burette

After the precipitate was filtered, the filtrate was tested with a few drops of the solution. Suggest why the student tested the filtrate in this way

to check the reaction is complete, to check that no more precipitate has formed

why would the value of y for the mass of water be different to the calculations when using the same sample of a hydrated salt

impurities present, reabsorbed water as not cooled in desiccator

describe an experimental procedure that could be used to carry out gravimetric analysis

heating substance, cool and leave in desiccator to prevent reabsorption of water, weigh, repeat the steps to constant mass

For measuring absorbance, the sample solution was diluted to decrease the concentration by half. Explain why the sample solution was diluted

Unknown's Absorbance must not be outwith calibration range

what is a standard solution

a solution of accurately known concentration

what is a primary standard

A substance used to make a standard solution

what must a primary standard be

Very pure, Stable as a solid and in solution, Soluble, Has a reasonably high GFM

what are examples of primary standards

Sodium carbonate, Na2CO3

Silver nitrate, AgNO3

Potassium dichromate, K2Cr2O7\

Potassium iodate, KIO3

Hydrated oxalic acid, H2C2O4·2H2O

Potassium hydrogen phthalate, KH(C8H4O4)

why isn't NaOH a primary standard

GFM is too small, it absorbs moisture as a solid and is unstable in solution

why isn't HCl not a primary standard

it is a gas dissolved in water and its solution concentration cannot be determined by weighing.)

purpose of a burette

Measures variable volumes accurately

purpose of a pipette

Measures a fixed volume accurately

purpose of an indicator

Shows the end point of the titration

how to prepare a standard solution

Weigh the primary standard accurately.

Dissolve in a small amount of distilled water in a beaker.

Transfer to a volumetric flask and rinse the beaker into the flask with distilled water.

Make up to the mark with distilled water.

Stopper and invert to mix.

what are the steps for accurately diluting a concentrated standard solution

A known volume is pipetted into a volumetric flask.

The volume is made up to the mark with distilled water.

The flask is stoppered and inverted

what happens when some solutions (like NaOH) cannot be made accurately because they absorb moisture

they are titrated against a primary standard (like oxalic acid) to find their exact concentration

what is a suitable indicator for an acid-base titration

phenolphthalein or methyl orange based on the steepness of the pH change at the endpoint

what is a suitable indicator for a redox titration

Often no indicator needed because colour changes naturally, for instance using magenta permanganate. Starch solution turns deep blue-black in the presence of iodine and can be used to detect the end point when iodine is the titrant

what is a suitable indicator for a complexometric titration

Use indicators that change colour when metal ions are complexed (e.g., Eriochrome Black T for EDTA titrations)

steps for preparing a titration

A burette is filled with one solution after rinsing with the solution to be used. The rinsings are discarded. Air bubbles are removed from beneath the burette tap.

Using a pipette filler, a pipette is used to measure a fixed volume of the other solution into a conical flask, after rinsing with the solution to be measured. The rinsings are discarded.

A few drops of indicator are added, and the conical flask is placed on a white tile beneath the burette.

A starting reading is taken to 2 decimal places (the last decimal place is 0 if the bottom of the meniscus is on the line, or 5 if it is below).

Titration continues until the colour of the indicator changes (end point).

The final reading is taken and the titre calculated (the volume used = the difference between the starting and final reading).

Titration is repeated until two (preferably three) concordant titres are obtained, typically agreeing within 0.10-0.20 cm3 depending on centre policy and apparatus.

what does colorimetry measure

how much light a coloured solution absorbs

what can causes of deviation be caused by in absorbance

stray light, polychromatic light, chemical equilibria or high concentration

what are the steps for colorimetry

Prepare standard solutions of known concentrations to give absorbances so that a straight calibration line can be produced

Choose the correct filter: Use a filter that is complementary to the colour of the solution

Use a blank: A sample of pure solvent to set the colorimeter to zero

Measure absorbance of each standard solution using a colorimeter

Plot a calibration graph: Absorbance vs concentration which should form a straight line

Test your unknown solution: Measure its absorbance and read its concentration from the graph. The unknown solution's concentration may need to be adjusted to give an absorbance reading within the straight line section of the calibration graph

what is distillation

separates liquids based on boiling points

what is distillation used for

to purify a compound or identify it by its boiling point

what does heating under reflux allow

Allows heat energy to be applied to a reaction mixture over an extended period of time without volatile substances escaping

what is the setup for heating under reflux

Round-bottom flask with anti-bumping granules.

Fitted with a vertical condenser.

Heat gently (water bath or heating mantle).

what is vaccum filtration

a faster way to separate a solid from a liquid using reduced pressure

what is the equipment used for vaccum filtration

Büchner, Hirsch or sintered glass funnel and flask.

A flat disc of filter paper.

Connected to a vacuum source

what does recrystallisation do

Purifies an impure solid by dissolving it in a hot solvent and letting pure crystals form on cooling

how is the solvent for recrystallisation chosen

so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures

what are the steps in recrystallisation

Dissolve impure solid in minimum hot solvent.

Hot filter under vacuum to remove insoluble impurities.

Cool slowly so that pure crystals form, leaving soluble impurities in the solvent.

Filter under vacuum, wash, and dry crystals.

what are separating funnels used for

to separate immiscible liquids, such as water from organic solvents and are commonly used in solvent extractions

what does the tapered shape of a separating funnel allow

allows for efficient separation of the two layers

how can a solute be separated from an aqueous mixture

using a solvent that does not mix with water if the solute is more soluble in it than in water. If the solvent is volatile, it can be easily removed by evaporation to recover the dry solute. The solvent should also be unreactive with the solute

what are the steps for solvent extraction

Add organic solvent to an aqueous solution in a separating funnel.

Shake to mix the solvents, venting frequently to relieve pressure build-up.

The solute moves into the organic layer with the ratio dissolved in each layer determined by the equilibrium constant

The layers are allowed to separate, then drained off into separate containers.

The extraction is repeated with small volumes for better extraction, rather than one large volume.

The organic phase is dried with an anhydrous salt (MgSO4, CaCl2) before evaporation.

The organic solvent is evaporated to recover the dry solute

what are melting points of compounds used for

to confirm identity and assess purity, using a melting point apparatus

how does a melting point apparatus work

quickly heat samples in a glass capillary tube until near the expected melting point at which the temperature is slowly increased to overcome thermal lag

what melting point range do pure substances have

sharp range

what melting point range do impure substances have

lowered melting point and broadened range due to disruption of the intermolecular bonding in the crystal lattice

what is the stationary phase of thin layer chromatography

silica or alumina layered onto a solid support plate of aluminium, glass or plastic

what is thin layer chromatography used for

separating small quantities of mixtures to check for purity, reaction progress or to identify a compound

what happens if the mobile phase is volatile in thin layer chromatography

the run is usually carried out in a lidded chromatography tank where the atmosphere is pre-saturated with the mobile phase to slow down evaporation off the TLC plate

how does TLC separate components of a mixture

based on polarity. How far the compounds are carried up a TLC plate depends on how soluble they are in the chosen solvent (the mobile phase) and how well they adhere to the stationary phase

steps of TLC

A small volume of sample is spotted on the TLC plate at the baseline, which is positioned higher up the plate than the surface of the solvent reservoir.

The bottom of the plate is placed in the solvent reservoir, so that the solvent moves up the plate, dissolving solutes and carrying them with it.

The solvent front is marked the moment the run is complete.

The spots of compound are visualised using UV light or a developing agent, such as ninhydrin for amino acids, marking the centres of the spots.

how are components identified in TLC

by running with a co-spot of a known substance

how are components identified in TLC in relation to Rf values

by comparing to values obtained with the same solvent, temperature and saturation

how can purity be checked in TLC

a pure substance produces a single spot. The presence of more than one spot indicates impurities are present, although some impurities may not be visible by TLC analysis