1.2.2(d)-(f) - Glassware, reflux, filtration and indicators

Choose Apparatus For The Job

Practical chemistry often fails before the reaction even starts: the wrong glassware gives the wrong volume, the wrong flask loses vapour, or the wrong filtration setup lets solid through. You need to recognise apparatus by purpose, not just by name.

Use the apparatus according to the measurement you need:

[DIAGRAM: asset_name: Lesson 1.2.2d-f: Volumetric glassware, reflux, filtration and indicators - diagram 01; asset_slug: 01_02_02b_volumetric_glassware_reflux_filtration_and_indicators__diagram_01; recommended_method: drawn_chem; description: NovaLearn-style line diagram comparing accurate volumetric glassware: burette clamped above a conical flask, volumetric pipette with filler delivering a fixed aliquot, and a volumetric flask filled to the calibration mark and inverted to mix.]

The most important practical distinction is between apparatus that measures and apparatus that merely holds. A conical flask can be rinsed with distilled water during a titration because the water does not change the amount of acid or alkali already in the flask. A pipette or burette must not be left with the wrong solution in it, because that would dilute or contaminate the measured reagent.

Worked reasoning: a student needs 25.00 cm³ of sodium carbonate solution in a conical flask before titrating it with hydrochloric acid. A measuring cylinder is quick, but it is not accurate enough for the fixed aliquot. The correct choice is a volumetric pipette with a pipette filler. The conical flask can then be placed under a burette containing hydrochloric acid.

Titration And Standard Solutions

A titration finds the volume of one solution needed to react exactly with a known volume of another solution. The solution in the burette is added until the indicator shows the endpoint. The volume delivered by the burette is the titre.

For reliable titration technique:

1. Rinse the burette with the solution that will go in the burette, then fill it.
2. Remove the funnel before titrating so drops do not fall in during the run.
3. Remove any air bubble from the burette tip.
4. Read the bottom of the meniscus at eye level.
5. Record burette readings to two decimal places, usually ending in 0 or 5.
6. Use a volumetric pipette and filler to transfer the fixed aliquot to the conical flask.
7. Add only a few drops of indicator.
8. Swirl the conical flask and rinse the flask walls with distilled water if needed.
9. Near the endpoint, add solution dropwise until the first permanent colour change.
10. Repeat until concordant titres are obtained.

The conical flask is not rinsed with the solution being pipetted into it, because extra drops would add extra moles of that reagent. It may be wet with distilled water because the amount of reagent already pipetted in stays the same.

To prepare a standard solution from a solid:

1. Weigh the required mass of solid accurately.
2. Dissolve the solid in a beaker using less distilled water than the final required volume.
3. Transfer the solution to the volumetric flask using a funnel or glass rod.
4. Rinse the beaker, stirring rod and funnel into the volumetric flask so all dissolved solute is transferred.
5. Add distilled water until the bottom of the meniscus is on the calibration mark.
6. Stopper the flask and invert it several times to mix thoroughly.

Worked example: a student is asked to prepare 250.0 cm³ of 0.100 mol dm⁻³ sodium carbonate solution from solid sodium carbonate, Na₂CO₃.

The amount needed is:

The relative formula mass of Na₂CO₃ is:

The molar mass is 106.0 g mol⁻¹, so the mass required is:

The practical method must then match the calculation: weigh 2.65 g, dissolve it in a beaker, transfer all solution and washings into a 250.0 cm³ volumetric flask, make to the mark and invert to mix. The calculation alone is not enough if the transfer loses solute.

Rinse the pipette with the solution it will deliver before filling to the mark. Let a standard volumetric pipette drain under gravity with its tip against the flask wall; do not blow out the residual liquid unless that specific pipette is calibrated for blow-out. Use a white tile beneath the conical flask to see the first permanent indicator change. A rough titre locates the endpoint; calculate a mean from concordant accurate runs using the tolerance specified by the method (commonly within 0.10 cm³).

Worked example: does extra water change the titre?

A conical flask already contains a fixed pipetted amount of alkali. Rinsing its walls with distilled water changes the concentration and total volume, but not the amount of alkali. The same amount of acid is therefore needed to reach equivalence.

Now suppose water is left in the burette and dilutes the acid added to it. Each cubic centimetre delivers less acid, so a larger titre is needed for the same alkali amount. This is why the conical flask may be wet with distilled water but the burette is rinsed with its own reagent. Do not apply the first argument to a pipette that must deliver a known concentration.

A primary standard must be sufficiently pure, stable and of known composition; a hygroscopic or reactive solid cannot automatically give a known concentration from its weighed mass. For the sodium carbonate example use anhydrous material of known purity. Allow a solution that warmed during dissolution to return to the flask's calibration temperature before making up to the line, then add the final water dropwise at eye level. Overshooting the mark cannot be fixed by simply pouring out solution: that removes solute too.

Reflux And Distillation Glassware

Heating under reflux and distillation both use a condenser, but they have different purposes.

In a reflux setup, the condenser is vertical above a pear-shaped or round-bottom flask. The water enters at the bottom of the condenser and leaves at the top, so the condenser jacket stays full of cold water. The top is open to the air, so pressure cannot build up. The flask is held securely with a retort stand and clamp, and the heating source is chosen after considering flammability and other hazards.

Distillation is different. It is used to separate a liquid product or purify a liquid by boiling it, condensing the vapour and collecting the distillate. The condenser slopes down towards a receiving vessel. A thermometer, when used, has its bulb near the still-head outlet so it measures the temperature of vapour entering the condenser, not the liquid in the flask.

Worked reasoning: if a reaction is slow and contains a volatile organic reagent, reflux is the safer method to heat it for longer. Simple boiling in an open beaker would let vapour escape, changing the reacting amounts and increasing exposure to fumes. Distillation would remove volatile material from the reaction mixture, so it is only appropriate when the aim is separation or purification of a liquid.

Common error contrast: a condenser is not automatically a distillation setup. In reflux, the condensed liquid returns to the same flask. In distillation, condensed liquid is carried away and collected.

Add anti-bumping granules before heating, or use suitable controlled stirring, so boiling is smooth; never add granules to an already boiling liquid. Check that joints are connected, glassware is supported without strain and cooling water flows before heating. In distillation, cooling water enters the lower end near the receiver and leaves the upper end near the still head. Keep the receiving side vented to atmosphere and stop before the flask boils dry. A sealed heated apparatus can build dangerous pressure.

[DIAGRAM: asset_name: Connected reflux and distillation glassware, open vapour paths, correct cooling-water directions and thermometer position.; asset_slug: 01_02_02b_volumetric_glassware_reflux_filtration_and_indicators__diagram_02; recommended_method: drawn_chem; description: Connected reflux and distillation glassware, open vapour paths, correct cooling-water directions and thermometer position.]

Filtration And Qualitative Tests

Filtration separates an insoluble solid from a liquid. The best filtration method depends on whether you want the filtrate, the solid residue, or a dry solid product.

In ordinary gravity filtration, a filter funnel holds filter paper. Fluted filter paper has folds that increase surface area and create channels for liquid to pass through more quickly. This is useful when the liquid filtrate is wanted, or when hot filtration needs to avoid crystallisation in the funnel.

Filtration under reduced pressure is used when the solid is the product and needs to be collected quickly. A Büchner or Hirsch funnel is fitted into the neck of a thick-walled side-arm filter flask through a tightly fitting rubber seal/adapter. The apparatus must have no gap between funnel and flask, otherwise suction draws outside air through the gap. The filter paper is moistened so it seals against the funnel plate, and the side arm is connected to suction. Reduced pressure below the paper pulls liquid through, leaving solid on the paper.

Risk management matters. Check glassware for cracks before reduced-pressure work, clamp the side-arm flask if needed, and release the vacuum safely before disconnecting. With hot filtration, handle hot glassware carefully and avoid sealing any heated system.

The practical techniques also include using apparatus for qualitative tests for ions and organic functional groups. The exact tests are taught in later chemistry lessons; here, the practical skill is to run small-scale tests cleanly and record observations precisely.

Good qualitative test technique includes:

• using small samples in clean test tubes
• adding reagents dropwise when the method requires it
• heating with a water bath or suitable heater when needed
• keeping controls or known samples separate from unknowns
• avoiding cross-contamination between dropping pipettes
• recording colour changes, precipitates, gases, dissolving and lack of change

Worked qualitative-test example: a student adds aqueous sodium hydroxide dropwise to a small sample and sees a pale blue precipitate. A useful record is "NaOH(aq) added dropwise: pale blue precipitate forms." A weak record is "it changed" because it omits the reagent, colour and physical observation. Keep this as practical technique here; the later ion-test lesson teaches which ions these observations identify.

Worked reasoning: if a solid organic product is collected after recrystallisation, reduced-pressure filtration is usually better than simple gravity filtration because it collects and partly dries the solid more quickly. If the aim is to remove insoluble impurities from a hot solution and keep the solution, gravity filtration with fluted paper is more appropriate.

Use a fresh portion of the unknown for each test unless a specified sequence requires otherwise. For example, an inorganic ion test may require acidification before adding the precipitating reagent; using an acid that itself introduces the ion being tested can invalidate the result. For a functional-group test, place a small measured sample in a separate clean tube, add the prescribed reagent, use a water bath if warming is needed, and compare with an appropriate known sample or reagent blank. A negative observation is evidence only if the reagent and conditions worked. The specific ion and functional-group reagents are taught in Modules 3, 4 and 6; this sequence is the transferable practical technique.

[DIAGRAM: asset_name: Gravity filtration and sealed reduced-pressure filtration with supported funnel, filter paper and suction side arm.; asset_slug: 01_02_02b_volumetric_glassware_reflux_filtration_and_indicators__diagram_03; recommended_method: drawn_chem; description: Gravity filtration and sealed reduced-pressure filtration with supported funnel, filter paper and suction side arm.]

Use Indicators At The Endpoint

An acid-base indicator changes colour over a pH range. In a titration, the endpoint is the observed colour change. The equivalence point is the chemical point where the acid and alkali have reacted in the exact stoichiometric ratio. Good indicator use makes the endpoint as close as possible to the equivalence point.

Only a few drops of indicator are used because indicators are weak acids or weak bases. Adding too much indicator can slightly change the reacting mixture and make the colour change harder to judge.

For this practical-techniques lesson, use these selection rules:

The words strong and weak describe the extent of ionisation in water, not the concentration. A dilute strong acid is still strong because its acid particles fully ionise; a concentrated weak acid is still weak because only a small proportion of acid particles ionise.

Worked example: an acid is titrated with a strong alkali. The sharp colour-change region from the experimental pH data is about pH 8 to pH 10. Phenolphthalein changes colour in roughly this region, so it is a suitable indicator. Methyl orange changes colour at a much lower pH, so it would change before the equivalence region and give a poorer endpoint.

Quick recap

• A pipette delivers an accurate fixed aliquot; a burette measures variable delivery; a volumetric flask contains one accurate final volume.
• Quantitative transfer and thorough mixing are as important as calculating the solute mass.
• Reflux returns condensate to the reaction; distillation collects it separately. Keep heated apparatus open and supported.
• Choose filtration for the required solid or filtrate, and choose an indicator whose transition lies in the steep pH change.