There is a bottle of white vinegar on the bench. The label makes a quantitative claim about what is in it. Today you check that claim to three significant figures, using nothing but a solution of known concentration and the ability to notice the exact moment a colour changes.
This is the most precise measurement you will make all year. It is also the one where technique matters most, because every shortcut has a direction and most of them push the same way.
What you are trying to find out
The acid in vinegar is acetic acid, and it reacts with sodium hydroxide in a ratio the equation fixes:
If you add sodium hydroxide of known concentration until the acid is exactly consumed, then the volume you added is a measurement of how much acid was there. The whole method rests on being able to see “exactly consumed” — which you cannot, so you use an indicator that changes colour very close to it and you accept the small difference.
Two things you are really testing:
- Can you measure a concentration — as opposed to constructing one, which is what you did last lab — and state a sensible uncertainty for it?
- Does the bottle’s claim hold? Read the label, convert your result into the units the label uses, and compare. Do not look up what vinegar “is” and do not take a number from me. The bottle in front of you is the source.
What you have to work with
- White vinegar, in its original labelled bottle.
- Standardised sodium hydroxide solution, at the concentration written on the container — read it and record it, including how many figures it is given to.
- Phenolphthalein indicator.
- Buret and stand, small funnel, white tile.
- Volumetric pipette and pipette bulb or pump. Volumetric flask for the dilution. Conical flasks. Wash bottle of distilled water.
Your design decisions and calculations, done before the lab:
- The dilution factor for the vinegar, calculated rather than guessed. Undiluted vinegar is far too concentrated for a 50 mL buret: work out roughly what titre a 25.00 mL aliquot of the undiluted acid would need, see the problem for yourself, and then choose a dilution that lands your titre comfortably in the middle of the buret’s range. Aiming for 20 mL to 30 mL is sensible, and you should be able to say why the middle of the range is better than either end.
- How you will perform that dilution, using the technique from Preparing a Standard Solution. Pipette and volumetric flask, not a measuring cylinder, and say why.
- How many titrations you will do, and what agreement you will accept between them before you stop. Decide the number now — 0.10 mL is the usual standard — and hold yourself to it.
- How many drops of indicator. More is not better. Say why.
Sodium hydroxide, glassware above your head, and one absolute rule
- Sodium hydroxide is the reagent that does not warn you. Dilute acid stings, so you notice a splash and wash it off. Sodium hydroxide feels soapy or slippery instead — that sensation is the solution attacking the fats in your skin — and it goes on doing damage while it does not hurt. A base burn of the same strength penetrates deeper than an acid burn. Rinse any suspected contact for at least 15 minutes under running water, whether or not it hurts, and tell me. In an eye, this is an emergency: eyewash at once, hold the lid open, and somebody else comes for me while you stay at the eyewash.
- Fill the buret below eye level. Take it out of the stand, or lower the stand, and use a funnel. Filling a buret above your head means pouring sodium hydroxide over your own face if the funnel slips, and it is entirely avoidable.
- Never pipette by mouth. Bulb or pump, for the vinegar, for the distilled water, for anything. There is no exception and there never was.
- Eye protection on for the whole period, including while you are waiting for a partner and including during cleanup.
- Phenolphthalein is dissolved in ethanol, which is flammable. No flames anywhere in the room. Keep the bottle closed between uses.
- Undiluted vinegar has a strong odour. Waft; do not put your face over the flask and inhale. Nothing is tasted — this vinegar has been in lab glassware and is not food.
- Never return unused sodium hydroxide or vinegar to a stock bottle. Waste to the labelled containers.
- Clean up spills of sodium hydroxide immediately and tell me even if you dealt with it. A drop dried on a bench is still caustic.
Technique, and the reason behind each rule
These are not rituals. Each one exists because leaving it out biases your answer in a known direction.
Rinse the buret with the sodium hydroxide it is going to hold, two small portions, draining through the tap each time. A buret rinsed only with water has a film of water on its walls that dilutes the titrant. Diluted titrant means more volume is needed to reach the endpoint, which means a larger titre, which makes your calculated acid concentration come out too high.
Rinse the pipette with the solution it will hold, for the same reason and with the opposite consequence.
Rinse the conical flask with distilled water only. A wet flask is fine: adding water changes nothing about how many moles of acid are in it. Rinsing the flask with vinegar instead puts extra acid in, and your titre comes out high.
Fill the buret tip. Run titrant through until no air bubble remains below the tap. A bubble that escapes during the titration adds its volume to your reading without any titrant having reached the flask, so the titre reads high.
Read the bottom of the meniscus, with your eye level with it, every single time, on a white background. Both the initial and the final reading, to the nearest 0.05 mL.
Do a rough titration first, then careful ones. The rough one tells you where the endpoint lives so that you can run the careful ones fast until you are a couple of millilitres short, then go drop by drop. Without the rough run you will overshoot, and an overshoot cannot be undone — it can only be discarded.
Swirl constantly, and wash the walls of the flask down with distilled water near the endpoint. Acid clinging to the glass above the liquid never gets titrated, and that pushes your titre low.
Stop at the first faint pink that persists for about thirty seconds of swirling. A strong pink is past the endpoint.
The prediction you write first
Before you fill the buret:
- The dilution you chose, and the titre you therefore expect, in millilitres, with a range.
- The concentration of acetic acid you expect in the undiluted vinegar, in mol/L, derived from the label — show the conversion.
- How many significant figures your final answer will be entitled to, and which measurement limits it. Work it out now, not afterwards.
- Whether you expect to come out above or below the label, and the mechanism for your guess.
What to collect
| Titration | Initial buret reading (mL) | Final buret reading (mL) | Titre (mL) | Used in the mean? |
|---|---|---|---|---|
| Rough | No | |||
| 1 | ||||
| 2 | ||||
| 3 |
Every reading to the nearest 0.05 mL, because that is what a 50 mL buret’s graduations support. Two decimal places, always — 20.80 mL and 20.8 mL are different claims.
The mean titre uses only the concordant runs: the careful ones that agree within your stated tolerance. If run 2 sits 0.6 mL away from the other two, exclude it, say so, and say what you think happened. Quietly averaging it in is the single most common way an honest set of data becomes a dishonest result.
| Quantity | Value | Unit |
|---|---|---|
| Concentration of sodium hydroxide | mol/L | |
| Volume of aliquot pipetted | mL | |
| Dilution factor applied to the vinegar | — | |
| Mean concordant titre | mL | |
| Range of the concordant titres | mL |
The shape of the calculation, with illustrative numbers
These are not your results. They show the route, and the units carried at every step.
Suppose the concordant titres were 20.85 mL, 20.80 mL and 20.90 mL, against sodium hydroxide at 0.100 mol/L, with 25.00 mL aliquots of vinegar that had been diluted by a factor of 10.
Mean titre: mL, and the three agree within 0.10 mL, so they are concordant.
Moles of base delivered:
The equation is 1 to 1, so the aliquot contained mol of acetic acid.
Concentration of the diluted vinegar:
Undo the dilution — multiply by 10: in the original vinegar.
To compare with a label given in grams per 100 mL, use the molar mass of acetic acid, 60.06 g/mol:
Three significant figures, and no more. The sodium hydroxide concentration was given to three, so three is the ceiling however many digits the buret gave you.
What to bring to the consolidation discussion
- Your titre table, including the rough run and any run you excluded, with the reason for excluding it.
- Your calculated concentration, in mol/L and in the label’s units, to a defensible number of significant figures.
- The comparison with the label, as a percentage difference, and whether that difference is larger than your uncertainty. Those are two separate questions and the second one is the interesting one.
- The class spread. Plot everybody’s result. A scatter centred on one value means random error; a scatter piled up on one side of it means the whole class shares a systematic error, and you should be able to name a technique step that would do that.
What you should not claim
- The endpoint is not the equivalence point. The indicator changes slightly after the acid has been consumed, because it takes a small excess of base to turn it. For a weak acid against a strong base this difference is small with phenolphthalein and it is not zero, and it biases the titre upward — so your concentration is very slightly overstated by the method itself, no matter how well you work.
- Most technique failures push the same way, and it is upward. A buret rinsed only with water, an air bubble that escaped from the tip, an overshoot past the faint pink, a flask rinsed with vinegar instead of water — every one of them increases the titre and therefore increases your reported concentration. If your result is above the label, look here before you conclude that the bottle is wrong.
- One thing pushes downward, and it is worth checking too: acid splashed onto the flask wall above the liquid line, never washed down, never titrated.
- Three concordant titres are not proof of accuracy. They are evidence of precision — that you can repeat yourself. A titrant whose stated concentration is wrong gives beautifully concordant results that are all wrong together, and no amount of repetition detects that.
- Agreeing with the label does not verify the label, and disagreeing does not refute it. Bottles carry stated minimums, contents change on standing, and you titrated one bottle once. Report a measurement and a comparison, not a verdict.
- You measured total acid, not acetic acid. The method counts whatever reacts with hydroxide. If anything else acidic is present, your number includes it — which is a real limitation of titration as a technique and not a flaw in your afternoon.
Curriculum connection
E2.7
determine the concentration of an acid or a base in a solution (e.g., the concentration of acetic acid in vinegar), using the acid–base titration technique [PR, AI]
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E2.6
use stoichiometry to solve problems involving solutions and solubility [AI]
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A1.13
express the results of any calculations involving data accurately and precisely, to the appropriate number of decimal places or significant figures
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