We use cookies to give you the best experience possible. By continuing to use the site, you indicate that you accept cookies. See our cookies policy here.
Better Equipped Practical Teaching Guides
Prepare a sodium carbonate primary-standard solution, use it to standardise hydrochloric acid, and then use the standardised acid to determine the concentration of an unknown sodium hydroxide solution. The guide covers accurate volumetric technique, calculations, indicator choice, concordant titres, uncertainty, troubleshooting, exam preparation and teacher/technician support.
Teacher note: The techniques in this guide align closely with UK A-Level requirements for preparing volumetric solutions and carrying out acid-base titrations. Examination boards may prescribe different reagents or a different titration direction. Centres should adapt the chemical system, quantities, assessment evidence and risk assessment to the current specification and local laboratory procedures they use.
Titration is one of the most widely used quantitative techniques in chemistry. It allows an unknown concentration to be determined from accurately measured volumes and a balanced chemical equation.
Related methods are used in pharmaceutical manufacture, food and drink analysis, environmental monitoring, water treatment and quality control.
The aim is to prepare a primary-standard solution accurately, use it to standardise an acid by titration, and then use the standardised acid to determine the concentration of an unknown alkali.
A titration is a quantitative technique in which a measured amount of one solution is reacted with another solution whose concentration is known. The volume delivered from the burette is used with the balanced equation to determine an unknown amount or concentration.
Equivalence point: the theoretical point at which reactants have been mixed in the exact stoichiometric ratio shown by the balanced equation.
Endpoint: the observed indicator colour change used to estimate the equivalence point.
A suitable indicator changes colour within the steep part of the titration curve, so its endpoint lies close to the equivalence point.
| Term | Meaning |
|---|---|
| Primary-standard solution | Prepared directly from an accurately measured mass of a suitable high-purity solid and an accurately known final volume. |
| Standardised solution | A solution whose concentration has been determined experimentally by titration against a primary standard. |
| Aliquot | A measured portion of solution transferred using a pipette. |
| Titre | The volume delivered from the burette: final reading − initial reading. |
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
Stoichiometric ratio: 1 mol Na2CO3 : 2 mol HCl.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Stoichiometric ratio: 1 mol HCl : 1 mol NaOH.
Prepare 250.0 cm3 of an approximately 0.1000 mol dm−3 solution of anhydrous sodium carbonate, Na2CO3. Confirm the exact formula, grade and supplier assay before calculating the concentration.
Do not use the wrong formula: the calculation below uses anhydrous sodium carbonate with M(Na2CO3) = 106.0 g mol−1. Hydrated sodium carbonate or washing soda has a different formula mass and cannot be substituted using the same target mass.
250.0 cm3 ÷ 1000 = 0.2500 dm3
n = cV = 0.1000 × 0.2500 = 0.02500 mol
m = nM = 0.02500 × 106.0 = 2.650 g
Target mass: aim to transfer approximately 2.650 g. Do not waste time trying to obtain exactly 2.650 g; record the mass actually transferred and use that value to calculate the prepared concentration.
Pre-start: confirm that the balance is clean, level and at zero; the weighing container is clean and dry; the volumetric flask is the correct capacity with a matching stopper; the calibration mark is visible; the glassware is undamaged; and the chemical formula, grade and assay have been checked.
Place the closed weighing bottle containing sodium carbonate on the balance, wait for a stable reading and record the mass before transfer, m1.
Transfer approximately 2.650 g into the 250 cm3 beaker. If solid is spilled elsewhere, stop and repeat with a new measurement.
Reweigh the bottle and remaining solid and record m2.
mass transferred = m1 − m2
Good weighing practice: use the same balance for both readings; record every mass exactly as displayed; keep the weighing bottle closed except while transferring; and do not return unused solid to the stock bottle.
Quantitative transfer: all of the dissolved sodium carbonate must reach the volumetric flask - original solution, beaker washings, glass-rod washings and funnel washings. The wash water does not change the amount of sodium carbonate; the final volume is adjusted later to the calibration mark.
If the flask is overfilled: do not remove some solution and continue. Removing liquid removes both solute and solvent and does not restore the intended concentration. Prepare the solution again.
| Measurement | Mass / g |
|---|---|
| Weighing bottle + solid before transfer | 24.817 |
| Weighing bottle + solid after transfer | 22.160 |
| Mass transferred | 2.657 |
m = 24.817 − 22.160 = 2.657 g
n(Na2CO3) = 2.657 ÷ 106.0 = 0.02507 mol
c(Na2CO3) = 0.02507 ÷ 0.2500 = 0.1003 mol dm−3
General equation: c = (m1 − m2) ÷ (M × V), with masses in g and V in dm3.
If a certified assay is supplied and the centre's method requires the correction: effective mass = mass transferred × assay ÷ 100.
Use the sodium carbonate primary-standard solution prepared in Part A to determine the concentration of approximately 0.100 mol dm−3 hydrochloric acid. The balanced equation is:
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
Stoichiometric ratio: 1 mol Na2CO3 reacts with 2 mol HCl.
Sodium carbonate reacts with strong acid in stages. Methyl orange is suitable for detecting the complete neutralisation endpoint used here. Phenolphthalein would change colour at an earlier carbonate-to-hydrogencarbonate stage and is therefore not appropriate for the full reaction represented by the equation above.
Expected observation: carbon dioxide effervescence is normal. Add acid more slowly near the endpoint and never stopper the conical flask during the reaction.
| Item | Rinse with | Why? |
|---|---|---|
| Burette | A small portion of HCl | Prevents residual water diluting the titrant. |
| 10.00 cm3 pipette | A small portion of Na2CO3 solution | Prevents residual water diluting the aliquot. |
| Conical flask | Distilled or deionised water only | Extra water changes concentration in the flask but not the measured moles of Na2CO3. |
Burette-reading convention used in this guide: for a burette divided into 0.1 cm3 intervals, estimate to the nearest 0.05 cm3 and record each reading to two decimal places. A valid individual reading will normally end in 0 or 5.
The rough titre is not included in the mean. It is used to locate the endpoint so that later repeats can be approached quickly and then controlled dropwise.
Concordance convention in this guide: a selected set is treated as concordant when the largest and smallest titres differ by no more than 0.10 cm3. Follow the precise convention required by your examination board or centre.
| Titration | Initial / cm3 | Final / cm3 | Titre / cm3 | Use in mean? |
|---|---|---|---|---|
| Rough | 0.20 | 20.55 | 20.35 | No |
| Repeat 1 | 0.10 | 20.20 | 20.10 | Yes |
| Repeat 2 | 1.35 | 21.40 | 20.05 | Yes |
| Repeat 3 | 0.50 | 20.60 | 20.10 | Yes |
Mean titre
(20.10 + 20.05 + 20.10) ÷ 3 = 20.08 cm3
Calculate HCl concentration
n(Na2CO3) = 0.1003 × 0.01000 = 0.001003 mol
n(HCl) = 2 × 0.001003 = 0.002006 mol
c(HCl) = 0.002006 ÷ 0.02008 = 0.09990 mol dm−3
Use the standardised hydrochloric acid from Part B to determine the concentration of an unknown sodium hydroxide solution.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Stoichiometric ratio: 1 mol HCl reacts with 1 mol NaOH.
The sodium hydroxide solution is colourless before indicator is added. Phenolphthalein makes the alkaline solution pale pink. As HCl is added, the correct endpoint is the first permanent disappearance of the pale pink colour, not a strongly acidic endpoint after excess HCl has been added.
Conical-flask rinse: the flask may be wet with distilled or deionised water because this does not change the moles of NaOH in the measured aliquot. Do not rinse the conical flask with NaOH: that would add unmeasured moles and make the titre too large.
Wash down splashes: if solution splashes onto the sides of the conical flask, use a small amount of distilled or deionised water to wash it back into the reaction mixture. Do not wash the sides with acid or alkali.
Record raw burette readings immediately, calculate every titre from final minus initial reading, and keep the rough result visibly separate from controlled repeats.
| Titration | Initial / cm3 | Final / cm3 | Titre / cm3 | Selected? |
|---|---|---|---|---|
| Rough | 0.00 | 24.80 | 24.80 | No |
| Repeat 1 | 0.15 | 24.40 | 24.25 | Yes |
| Repeat 2 | 1.05 | 25.25 | 24.20 | Yes |
| Repeat 3 | 0.50 | 24.75 | 24.25 | Yes |
24.25 − 24.20 = 0.05 cm3. This satisfies the ≤0.10 cm3 convention used in this guide.
(24.25 + 24.20 + 24.25) ÷ 3 = 24.23 cm3.
Concordant titres are evidence of precision and repeatability. They do not prove that the result is accurate because the same systematic error may affect every repeat.
| Titration | Initial burette reading / cm3 | Final burette reading / cm3 | Titre / cm3 | Use in mean? |
|---|---|---|---|---|
| Rough | No | |||
| Repeat 1 | ||||
| Repeat 2 | ||||
| Repeat 3 | ||||
| Repeat 4 if required |
A reliable titration calculation should show the measured mean titre, unit conversion, balanced equation, mole ratio and final concentration. Keep unrounded calculator values through intermediate steps and round only the final result to an appropriate number of significant figures.
Use all justified concordant repeat titres. Exclude the rough titre.
Convert cm3 to dm3 by dividing by 1000.
Use n = cV, with V in dm3.
Use the balanced-equation coefficients explicitly.
Convert titrant moles into moles of the unknown species.
Convert the unknown-solution aliquot from cm3 to dm3.
Use c = n/V.
Include units, working and appropriate significant figures.
General titration relationship
n(titrant) = c(titrant) × V(titrant)
n(unknown) = n(titrant) × [coefficient of unknown ÷ coefficient of titrant]
c(unknown) = n(unknown) ÷ V(unknown)
Avoid an unexplained "mole-ratio factor". Write the balanced equation and state exactly how the coefficients convert moles of one reactant into moles of the other.
Given: mean HCl titre = 24.23 cm3; c(HCl) = 0.09990 mol dm−3; NaOH aliquot = 25.0 cm3.
24.23 cm3 = 0.02423 dm3.
n(HCl) = 0.09990 × 0.02423 = 0.002421 mol.
HCl + NaOH → NaCl + H2O, so n(NaOH) = 0.002421 mol.
25.0 cm3 = 0.0250 dm3.
c(NaOH) = 0.002421 ÷ 0.0250 = 0.0968 mol dm−3 to 3 significant figures.
Acid-base titrations depend on stoichiometric neutralisation. For hydrochloric acid and sodium hydroxide, the molecular equation and common ionic representations are:
Molecular equation
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Net ionic equation
H+(aq) + OH−(aq) → H2O(l)
Hydronium representation
H3O+(aq) + OH−(aq) → 2H2O(l)
The theoretical point at which the reactants have been mixed in the exact stoichiometric ratio shown by the balanced equation.
The observed indicator colour change used to estimate the equivalence point. The two are related but are not necessarily identical.
A suitable indicator changes colour within the steep section of the relevant titration curve, so the observed endpoint lies close to the equivalence point.
| Titration type | Suitable approach | Reason |
|---|---|---|
| Strong acid - strong base | Phenolphthalein or methyl orange can be suitable. | The pH changes very steeply through a broad region near equivalence. |
| Weak acid - strong base | Phenolphthalein is generally preferred. | Its transition range lies within the alkaline-side steep section. |
| Strong acid - weak base | Methyl orange is generally preferred. | Its transition range lies within the acidic-side steep section. |
| Weak acid - weak base | A simple visual indicator is generally unsuitable. | The pH change around equivalence is too gradual for a sharp visual endpoint; use a calibrated pH method where appropriate. |
| Na2CO3 with strong acid | Methyl orange for complete neutralisation. | Phenolphthalein responds at the earlier carbonate-to-hydrogencarbonate stage. |
The indicator has a finite transition range, so the visible endpoint can differ slightly from the stoichiometric equivalence point. This introduces a small systematic indicator error. A strong evaluation therefore links the indicator transition range to the steep part of the titration curve rather than simply naming an indicator.
A calibrated pH probe can record a titration curve and allow the equivalence volume to be estimated from the inflection or maximum-gradient region. This reduces subjective colour judgement but introduces different sources of uncertainty, including calibration, probe response, mixing and data treatment.
This extension turns the titration into a controlled investigation. Prepare sodium hydroxide solutions of different initial concentrations by accurate dilution, then titrate the same fixed aliquot volume of each against the same standardised HCl.
Initial concentration of NaOH, prepared accurately by dilution.
Mean concordant volume of standardised HCl required to reach the endpoint.
NaOH aliquot volume, HCl concentration, indicator type and drop count, temperature, apparatus, endpoint criterion and technique.
| NaOH concentration / mol dm−3 | Aliquot / cm3 | Mean HCl titre / cm3 |
|---|---|---|
| 0.040 | 25.0 | |
| 0.060 | 25.0 | |
| 0.080 | 25.0 | |
| 0.100 | 25.0 |
Expected relationship: for a fixed 25.0 cm3 NaOH aliquot and fixed HCl concentration, the mean HCl titre should be directly proportional to the initial NaOH concentration for the 1:1 reaction. Plot initial NaOH concentration on the x-axis and mean HCl titre on the y-axis.
High-quality evaluation identifies the type of error, predicts its direction where possible, quantifies important uncertainty and proposes an improvement that directly addresses the cause.
The quantified interval associated with a measurement or calculated result.
Causes repeated measurements to scatter above and below a central value.
Biases results consistently in one direction.
A one-off procedural error such as using the wrong solution or misreading a scale.
| Issue | Type / immediate effect | Likely effect on calculated NaOH concentration |
|---|---|---|
| Endpoint judged inconsistently between repeats | Random / variable titre | Greater spread and poorer repeatability; no single predictable direction. |
| Endpoint repeatedly overshot | Systematic / titre too large | Calculated NaOH concentration too high. |
| Air bubble initially present in burette jet | Some recorded volume fills the jet rather than entering the flask | Titre appears too large; calculated NaOH concentration too high. |
| Burette not rinsed with HCl | Residual water dilutes the HCl in the burette | A larger titre is required; if the undiluted HCl concentration is used in the calculation, NaOH concentration is overestimated. |
| Pipette not rinsed with NaOH | Residual water dilutes the aliquot | Fewer moles of NaOH are delivered; titre and calculated NaOH concentration are too low. |
| Conical flask rinsed with NaOH | Unmeasured additional moles of NaOH enter the flask | Titre and calculated NaOH concentration are too high. |
| Primary standard prepared at wrong concentration | Systematic bias propagates into HCl standardisation | All later NaOH concentrations may be biased even if Part C titres are highly concordant. |
Using the conventional school treatment in this guide, each reading has an estimated reading uncertainty of ±0.05 cm3. A titre is the difference between two readings, so the worst-case absolute reading uncertainty is ±0.10 cm3.
For a 24.23 cm3 titre:
(0.10 ÷ 24.23) × 100 = 0.41%
Use the tolerance printed on the actual pipette and volumetric flask, or the manufacturer's certificate. Do not mix tolerance values from different glassware classes or capacities.
% uncertainty in pipette volume = pipette tolerance ÷ pipette volume × 100
% uncertainty in flask volume = flask tolerance ÷ flask volume × 100
Two balance readings are subtracted. In a simple worst-case treatment, if each reading has uncertainty ±u, the mass difference has uncertainty ±2u.
% uncertainty in mass = (2u ÷ mass transferred) × 100
For multiplication and division, a common A-level treatment is to add the relevant percentage uncertainties. A full uncertainty budget for the final NaOH concentration should include uncertainty in the standardised HCl concentration as well as the Part C titre and pipette volume.
| Concept | Correct interpretation |
|---|---|
| Accuracy | Closeness of a result to a true or accepted value. |
| Precision | Closeness of repeated measurements to one another. |
| Repeatability | Agreement when the same operator uses the same method and equipment over a short period. |
| Concordance | A practical criterion showing that selected titres are sufficiently close; evidence of precision, not proof of accuracy. |
A larger titre reduces the percentage effect of a fixed burette-reading uncertainty. Where the chemistry and safety allow, a titre of roughly 20-30 cm3 is often preferable to a very small titre.
Keep sodium hydroxide stoppered and minimise prolonged exposure to air. Changes in composition during storage can create a systematic bias that repeat titrations will not remove.
| Random or variable | Systematic or biased |
|---|---|
| Small variation in judging the first permanent colour change | Standard solution prepared to the wrong concentration |
| Minor variation in drop size and swirling near the endpoint | Burette consistently read from the wrong eye position |
| Last-digit variation in meniscus estimation | Pipette or burette calibration error |
Top-grade evaluation: identify the most significant uncertainty, explain whether the final result becomes too high, too low or simply more variable, support the evaluation with quantitative evidence where possible, and propose a realistic improvement linked directly to the cause. Repeats reduce random variation but do not correct a systematic bias.
| Problem | Likely cause | Recommended action |
|---|---|---|
| Repeat titres are not concordant | Endpoint overshoot, inconsistent swirling, reading error or incorrect aliquot | Use a fresh aliquot, approach the endpoint dropwise, swirl consistently and read the burette at eye level. |
| One titre is suddenly much larger | Air bubble, leak, wrong solution or extra analyte in the flask | Inspect the burette jet and tap, confirm labels, prepare fresh apparatus and repeat. |
| Burette level changes before titration | Filling funnel left in place, leak or solution draining from wet walls | Remove the funnel, check the tap and jet, allow drainage, then record a fresh initial reading. |
| Endpoint is difficult to see | Too much indicator, poor lighting, dark background or acid added too quickly | Use 2-3 drops, a white tile, consistent lighting and dropwise addition near the endpoint. |
| No pale pink after adding phenolphthalein to NaOH | Indicator omitted/wrong, NaOH too dilute, contaminated or wrong solution | Check labels and method, then prepare a fresh aliquot. |
| Volumetric flask is above the calibration mark | Final water added too quickly | Discard and remake the standard solution; do not remove liquid to try to correct it. |
| Results depend on which aliquot is used | Volumetric solution was not mixed thoroughly | Stopper and invert the flask repeatedly before taking aliquots. |
| Calculated concentration is implausible | Wrong units, formula, mole ratio, selected data or transcription | Recheck cm3-to-dm3 conversion, balanced equation, coefficients and raw readings. |
| Methyl-orange endpoint appears unstable in Part B | HCl added too fast while CO2 is evolving | Slow the addition rate, swirl thoroughly and follow the centre's validated method. |
| Mistake | Immediate consequence | Likely effect on result |
|---|---|---|
| Using target mass instead of actual transferred mass | Prepared concentration is calculated from the wrong amount | Systematic error in every later concentration. |
| Leaving solution in the beaker, rod or funnel | Not all recorded solute enters the volumetric flask | Actual standard concentration is lower than calculated. |
| Making up above the volumetric mark | Final volume too large | Actual standard concentration too low. |
| Failing to invert the volumetric flask | Solution not homogeneous | Aliquots may differ; poor repeatability. |
| Rinsing the conical flask with NaOH | Unmeasured moles added | Titre and calculated [NaOH] too high. |
| Blowing out a transfer pipette | More than the calibrated volume delivered | Aliquot too large; titre too high. |
| Recording 24.23 cm3 as an individual burette reading under the 0.05 cm3 convention | False precision | Technique/recording marks may be lost; reading is not credible. |
| Including the rough titre in the mean | Exploratory value treated as a controlled repeat | Mean may be biased. |
| Using too much indicator | Indicator can affect acidity/basicity and colour intensity | Endpoint may shift and become less reproducible. |
Quick examiner routine: rinse correctly • read at eye level • record valid two-decimal readings • use fresh aliquots • approach the endpoint dropwise • select a justified concordant set • convert cm3 to dm3 • show the balanced equation and ratio.
The full analytical sequence is substantial. The guide recommends delivering it across three 50-75 minute lessons or two extended practical blocks rather than trying to complete every stage in one ordinary lesson.
| Material | Suggested maximum available | Purpose |
|---|---|---|
| Anhydrous Na2CO3 | Approximately 2.7 g | Prepare 250.0 cm3 primary-standard solution. |
| HCl, approximately 0.100 mol dm−3 | Up to 200 cm3 if rough + three repeats are planned in Parts B and C | Standardise in Part B and use in Part C. |
| Unknown NaOH, approximately 0.100 mol dm−3 | Up to 110 cm3 | 25.0 cm3 aliquots plus pipette rinsing. |
| Methyl orange | Less than 1 cm3 | 2-3 drops per Part B titration. |
| Phenolphthalein | Less than 1 cm3 | 2-3 drops per Part C titration. |
| Distilled/deionised water | At least 500 cm3 | Dissolution, washing and flask rinsing. |
| Lesson | Main activity | Suggested time |
|---|---|---|
| 1 | Introduction, target-mass calculation and safety briefing | 10-15 min |
| 1 | Weighing, dissolution, quantitative transfer and making to the mark | 30-40 min |
| 1 | Mixing, labelling and actual-concentration calculation | 10-15 min |
| 2 | Part B set-up and HCl standardisation | 35-45 min |
| 2 | Concordance, mean titre and HCl calculation | 15-20 min |
| 3 | Part C rough and repeat titrations | 35-45 min |
| 3 | NaOH calculation, uncertainty and evaluation | 20-30 min |
| Observe or review | Evidence |
|---|---|
| Practical planning | Correct mass calculation, reagent formula and apparatus selection. |
| Manipulation | Safe weighing, quantitative transfer, pipette-filler use, burette preparation and swirling. |
| Observation and recording | Valid burette readings, endpoint description and complete tables. |
| Data processing | Concordant-set selection, mean, unit conversions, stoichiometry and significant figures. |
| Evaluation | Quantified uncertainty, direction of errors and realistic improvements. |
This table is a starting point only. The centre must use current supplier safety data sheets, exact products and quantities, student needs and local emergency procedures. A safety data sheet informs a risk assessment but is not a substitute for one. Residual risk should be determined and recorded after controls have been matched to the actual chemicals, concentrations, apparatus, room and learners.
Core controls: wear eye protection; use pipette fillers only; fill burettes below face level; keep glassware secured and away from bench edges; use small working quantities; clean spills promptly; never stopper the carbonate-acid reaction while carbon dioxide is being produced.
| Hazard or activity | Potential harm | Control measures | Response / disposal |
|---|---|---|---|
| Anhydrous Na2CO3 solid, approx. 2.7 g | Dust or splashes may irritate eyes; ingestion or skin contact. | Eye protection; avoid dust; use a spatula; keep container closed; wash hands. | Rinse affected area with water and follow local first-aid procedure; clean spill using the approved method. |
| HCl approx. 0.100 mol dm−3, up to 200 cm3 per pair | Eye/skin irritation; splash during filling. | Eye protection; fill burette below face level; use a small working beaker and funnel; clean spills promptly. | Irrigate exposure with water and seek assistance; dispose as directed locally. |
| NaOH approx. 0.100 mol dm−3, up to 110 cm3 per pair | Alkaline solution may irritate eyes/skin; exact classification is supplier-dependent. | Eye protection; pipette filler only; keep stoppered; avoid contact. | Rinse with water and follow local first-aid and spill procedure. |
| Methyl orange / phenolphthalein, less than 1 cm3 each | Formulation may be flammable or irritating if alcohol-based; supplier-dependent. | Use only drops; keep away from ignition; cap promptly; avoid contact. | Follow the exact SDS and collect indicator-containing waste as locally directed. |
| Burette, pipette and volumetric glassware | Breakage, cuts or falling apparatus. | Inspect glassware; secure clamp and stand; keep away from bench edge; never force fittings. | Do not pick up shards by hand; use the designated broken-glass procedure. |
| Pipetting | Ingestion or exposure if mouth pipetting is attempted. | Use a compatible pipette filler only and train students before use. | Stop work and follow the centre's local exposure procedure. |
| CO2 evolution in Part B | Fizzing and splash risk if acid is added too quickly; pressure risk if the vessel is stoppered. | Swirl and add acid more slowly near the endpoint. Never stopper the conical flask during the carbonate-acid reaction. | Set the flask down safely; rinse splashes promptly; report exposure and follow local procedures. |
| Liquid spills | Slip, contamination or chemical contact. | Keep benches clear, use small working quantities and clean or mark spills immediately. | Follow the centre's chemical-spill and disposal procedure for the actual product involved. |
| Question | Model answer |
|---|---|
| Why weigh by difference? | It gives the mass that actually left the weighing bottle and entered the beaker, provided none was spilled elsewhere. |
| Why rinse the beaker, rod and funnel into the flask? | To transfer all dissolved solute quantitatively. |
| Why add the final water dropwise? | To avoid passing the volumetric calibration mark. |
| Why invert the volumetric flask? | To produce a homogeneous solution before an aliquot is removed. |
| Why remove the burette funnel? | Drops from the funnel could enter after the initial reading and invalidate the measured titre. |
| Why is the final liquid in a transfer pipette not blown out? | A transfer pipette is calibrated to deliver its stated volume while retaining that residual liquid. |
| Why may the conical flask be wet with distilled water? | The extra water does not change the measured number of moles in the aliquot. |
| Why exclude the rough titre? | It estimates the endpoint and was not obtained using the controlled dropwise approach used for repeat titres. |
| What are concordant titres? | Repeat titres that agree within the stated criterion; in this guide the selected range is ≤0.10 cm3. |
| How can an equivalence volume be estimated instrumentally? | Record a calibrated pH titration curve and locate the inflection or maximum-gradient region. |
Key conclusion: a single titre cannot demonstrate repeatability. Concordant titres provide evidence of precision, although a systematic error may still affect them all.
The actual transferred mass gives the best estimate of the number of moles that entered the volumetric flask. The target mass is only a preparation aim.
They contain some of the dissolved solute. Transferring them ensures the transfer is quantitative.
Any drop entering from the funnel after the initial reading would add unrecorded titrant and invalidate the titre.
They provide evidence of precision and repeatability under the stated criterion, but they do not by themselves prove accuracy.
Concentration is expressed in mol dm−3, so the volume must be in dm3 for the units to be consistent.
The first permanent disappearance of the pale pink colour after swirling.
A systematic bias - such as a wrongly prepared standard solution - can affect every repeat in the same direction while leaving the titres close to one another.
A fixed burette-reading uncertainty forms a smaller percentage of a larger titre. Where chemistry and safety permit, roughly 20-30 cm3 is often a useful design target.
Explore related equipment, practical support and laboratory-planning resources from Better Equipped.
This guide was written and reviewed by Better Equipped's technical team and further reviewed by former A Level Science Teachers. Our technical team draw on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. They include ex-school laboratory technicians and are here to support schools, colleges and laboratories right across the UK. If you have feedback on this guide, we'd love to hear it so please contact us. Don't forget we will be regularly updating our guides and resources on our Better-Resources Hub. If you would like us to cover a particular subject matter in these guides or have some top tips you'd like to share then again we'd love to hear from you.
Last reviewed and updated: August 2026
Browse burettes, pipettes, volumetric flasks, conical flasks, balances, clamp stands, eye protection and other A-Level Chemistry laboratory equipment.
Call us: 
Email us: 
