Lab Techniques

The Laboratory Program tells you what to investigate and how to stay safe; it assumes you already know how to read a meniscus, do a titration to the endpoint, and transfer a solid without losing half of it. This page is that missing manual — the basic manipulations every lab in this course depends on, why each one is done the way it is, and what a sloppy version does to your result. Read the relevant section before the lab that needs it.

For the math that turns these measurements into an answer with an uncertainty, see Lab Math and Error Analysis.

Choosing glassware for the job

The single biggest technique decision is which piece of glass to reach for. They are not interchangeable.

Rule of thumb: if the number goes into a calculation you report, it came from a volumetric flask, a volumetric pipette, or a burette. If it was "about 20 mL of solvent," a graduated cylinder was fine.

Measuring mass

The balance

Use the same balance for every weighing in one experiment — balances disagree slightly, and consistency matters more than which one is "right." Let it settle, check it reads 0.0000 with nothing on the pan (press tare/zero if not), keep the draft shield closed on an analytical balance, and never put a chemical directly on the pan — always a weigh boat, weigh paper, or a beaker.

Taring

Put the empty container on, press tare so it reads zero, then add your sample; the display now reads the sample mass directly. This is faster than subtracting, but it hides the container mass, so if you might need it later (you spill, you want to check), record the container mass first instead.

Weighing by difference

The most accurate way to get a known mass of a solid you are going to transfer:

1. Weigh the container with the solid in it. Record.
2. Transfer most of the solid to your reaction vessel.
3. Weigh the container again with whatever is left. Record.
4. The mass transferred is the difference.

You never have to hit a target mass exactly — you just have to know precisely how much left the container. Anything stuck to the container is simply not counted.

Quantitative transfer of a solid

When all of a weighed solid must end up in the flask (preparing a standard solution): tip the solid in, then rinse the weigh boat and the funnel with small squirts from a wash bottle, letting every rinse run into the flask. Three small rinses beat one big one. "Quantitative" means none left behind.

Measuring volume

Reading a meniscus

Liquid in narrow glass curves. Read the bottom of the curve (the meniscus) for water and almost every aqueous solution, with your eye level with the mark — looking down inflates the reading, looking up deflates it (this parallax error is systematic and repeats every time you do it). A strip of dark paper held behind the glass makes the meniscus easier to see. Mercury and a few others curve the other way; read the top.

A graduated cylinder

Pour to roughly the volume you want, set it on the bench, crouch to eye level, and read to one digit past the finest graduation (a 50 mL cylinder graduated every 1 mL is read to 0.1 mL, estimated). It is for approximate volumes.

A volumetric flask

It has one ring etched on the neck and a stated volume and temperature. To fill it to the mark: add liquid until you are a centimetre or two below the ring, then add the last bit dropwise — a wash bottle or a dropper — until the bottom of the meniscus sits exactly on the ring at eye level. Cap and invert to mix at least fifteen times; a solution mixed only by swirling is stratified and will give you a different concentration from the top than the bottom.

A volumetric (transfer) pipette

Delivers one fixed volume (10.00 mL, 25.00 mL) very accurately. Never pipette by mouth — use a bulb or a pipette pump.

1. Rinse the pipette twice with a little of the solution you are about to measure (not water — water left inside dilutes your aliquot).
2. Draw liquid a few centimetres above the mark, then let it fall until the meniscus is exactly on the mark, touching the tip to the vessel wall to take off the last drop.
3. Deliver into the receiving flask with the tip against the wall. Let it drain under gravity, wait a couple of seconds, then touch off. Do not blow out the last bit — the pipette is calibrated "to deliver" with that drop left in the tip.

A burette

For a volume you vary and read precisely (the titrant).

1. Close the stopcock. Rinse twice with a few mL of titrant, draining through the tip so the rinse coats everything the solution will touch.
2. Fill above the 0 mark with a funnel, then open the stopcock briefly to fill the tip and expel any air bubble below the stopcock — a bubble that leaves during the titration reads as titrant you did not actually add.
3. Drain to bring the meniscus onto the scale (it does not have to be 0.00). Record the initial reading to 0.01 mL, eye level.
4. Titrate (below). Record the final reading the same way. Volume added is the difference. The burette reads top-down, so the final number is larger.

Preparing a solution

From a solid (making a standard)

1. Calculate the mass needed: mass = M × V × molar mass.
2. Weigh it (by difference, or into a small beaker) — you do not need the exact target, but you need to know the exact mass you used and use that in your concentration calculation.
3. Dissolve it in a beaker in less water than the final volume, stirring.
4. Transfer to the volumetric flask through a funnel; rinse the beaker, the stirring rod, and the funnel into the flask several times.
5. Add water to near the mark, then dropwise to the mark. Cap and invert to mix.

You cannot just add the final volume of water to the solid — the dissolved solid takes up volume, so the solution would end up more dilute than intended.

By dilution

To make a less concentrated solution from a stock: M₁V₁ = M₂V₂ gives the volume of stock (V₁) to take.

1. Pipette V₁ of stock into a volumetric flask that is already partly filled with water (for acids, so the heat of dilution is absorbed — add acid to water).
2. Swirl, then add water to the mark and invert to mix.

Titration

The goal: add titrant from the burette until a colour change (or a meter) shows the reaction is exactly complete — the endpoint.

• Put the measured sample (from a pipette) in a conical (Erlenmeyer) flask, not a beaker — you can swirl it hard without slopping. Add the indicator (a few drops; more indicator does not make a sharper endpoint).
• Put a sheet of white paper under the flask so the colour change is obvious.
• Do a rough run first: add titrant a millilitre at a time, swirling, until the colour flashes and fades slowly, then flashes and stays. Note that volume. It tells you where the endpoint is so the careful runs are fast.
• Careful runs: titrate quickly to about 1 mL before the rough endpoint, then add dropwise, swirling after each drop. Near the end, add half-drops — let a partial drop form on the tip, touch it to the wall, and rinse it in with the wash bottle — until one half-drop turns the whole flask to the endpoint colour and it lasts 30 seconds while swirling.
• Do careful runs until two agree within about 0.1 mL (concordant). Average the concordant runs; discard the rough run and any outlier.

A titre that keeps drifting back to colourless means you overshot on a previous drop or a bubble left the burette tip — start that run over.

Heating

Bunsen burner

Connect the hose, close the air holes, light the match/striker first, then open the gas slowly. A yellow, wavy, soot-depositing flame means too little air; open the collar until you get a quiet blue flame with an inner cone. The hottest point is just above the tip of the inner blue cone, around 1500 °C — hold whatever you are heating there, not in the yellow.

Heating a liquid

• In a test tube: fill no more than a third, point the mouth away from everyone, and move it constantly through the flame — a stationary spot superheats and the contents erupt (bumping).
• In a beaker or flask on a hot plate or with a burner and gauze: add a couple of boiling chips or a stir bar so bubbles have a place to start. Never heat a closed container — pressure builds and it fails.

Heating a solid to constant mass

For the hydrate lab and any "drive off the water" step: heat the crucible and contents, cool in a desiccator or covered on the bench, weigh; repeat the heat–cool–weigh cycle until two consecutive masses agree within your balance's uncertainty. One heating is almost never enough — "constant mass" is the actual requirement, not "heated for five minutes."

Separating

Gravity filtration

For recovering or removing a solid when speed does not matter:

1. Fold the filter paper in quarters, then open one side into a cone (or flute it — repeated accordion folds — for faster flow), and seat it in the funnel. Wet it with a little solvent so it clings.
2. The paper must sit below the rim of the funnel; the funnel stem tip should touch the inside wall of the receiving beaker so filtrate runs down, not splashes.
3. Decant first — pour off the clear liquid down a stirring rod held to the funnel — then transfer the solid last, rinsing it out of the original container with solvent.
4. Wash the solid on the paper with small portions of cold solvent to remove trapped mother liquor.

Vacuum filtration

Faster, and gives a drier solid: a Büchner funnel with a paper disc that just covers the holes, wetted, on a filter flask connected to an aspirator or pump. Pour the slurry in; the vacuum pulls the liquid through in seconds. Break the vacuum (disconnect the hose) before turning off the pump, or liquid sucks back into your filtrate.

Evaporation

To recover a dissolved solid: evaporate the solution in an evaporating dish on a steam bath or a low hot plate, stopping while a little liquid remains and letting the residual heat finish it — boiling a dish dry spatters the product and can crack the dish.

Transferring, mixing, and small habits that matter

• Pour down a stirring rod held against the lip to control the stream and stop drips running down the outside.
• Rinse quantitatively: whenever "all of it" has to move, chase the transfer with two or three small solvent rinses rather than one large one.
• Stir with a rod or a stir bar, not the thermometer — thermometers break, and mercury (in older ones) contaminates everything.
• Mix a made-up solution by inversion, not swirling — 15+ inversions of a capped volumetric flask.
• Add reagents in the order the procedure says. "Acid to water" is the famous one; others matter too (a reagent added out of order can precipitate something you needed dissolved).
• Read the thermometer with the bulb fully immersed but not touching the vessel wall or bottom, at eye level, after the reading has stopped moving (thermal equilibrium).

Cleaning up

Rinse glassware with tap water first, then two or three small rinses with deionized water — a full rinse wastes DI water and does no more than three small ones. Do not dry the inside of volumetric glassware with a paper towel or in an oven (it distorts the calibration and leaves lint); leave it to air-dry inverted, or rinse it with the next solution it will hold. Put chemicals in the labelled waste containers, not the sink — aqueous, halogenated organic, heavy metal, and acid/base neutralization streams are usually separate. Wipe the bench, and check your burner gas is off at the valve.

Common technique errors and what they do to your result

Notice that most of these are systematic — they push the answer the same direction every time — which is exactly why identifying which measurement a technique error affects lets you predict whether your result will come out high or low. That reasoning is the point of [Lab Math and Error Analysis](/chemistry/labmath/).