1.2.2(a)-(c) - Measurement apparatus and serial dilutions
Practical biology only becomes evidence when measurements are made with apparatus that suits the question. In this lesson you will learn how to choose apparatus for quantitative measurements, how to make those measurements trustworthy, and how instruments such as colorimeters and potometers turn biological changes into usable data. You will also practise serial-dilution logic used in practical and calculation questions.
Choosing Apparatus For Quantitative Measurements
A quantitative measurement is a measurement with a numerical value and a unit, such as 2.46 g, 30 s, 5.0 cm3 or pH 6.8. The first practical decision is not "What number do I write down?" but "Which apparatus gives a valid measurement for the quantity I need?"
Valid Measurement
A measurement is valid if it measures what it is supposed to measure for the investigation being carried out.
For this row, the named measurement types are mass, time, volume, temperature, length and pH. Each one has a typical apparatus choice and a precision habit.
| Quantity | Suitable apparatus | Usual unit habit | Practical detail that protects data quality |
|---|---|---|---|
| Mass | top-pan or analytical balance | g | tare the balance or weigh by difference when transferring material |
| Time | stopwatch, stopclock or data logger | s, sometimes min or h | human reaction time often makes whole seconds or 0.5 s more realistic than many decimal places |
| Volume of liquid | pipette, burette, measuring cylinder, syringe or volumetric flask | cm3 or dm3 | read the meniscus at eye level to reduce parallax error |
| Gas volume | gas syringe, inverted measuring cylinder or inverted burette | cm3 | choose a size that matches the expected gas volume |
| Temperature | thermometer, temperature probe or data logger | degrees C | let the probe equilibrate before recording |
| Length | ruler, calipers or microscope scale where appropriate | m, cm, mm or um | align the zero point and read to the apparatus resolution |
| pH | pH chart, pH meter or pH probe | no unit | calibrate a pH meter with buffer solution and rinse the probe between samples |
The same biological variable can sometimes be measured in different ways. For example, water uptake by a shoot can be followed by bubble movement in a potometer capillary, or by measuring the loss of mass from a mass potometer. The better choice depends on the question, the expected change and the resolution of the apparatus.
When a question asks for apparatus, name the apparatus and link it to the quantity measured. "Use a balance to measure mass in g" is stronger than "measure the amount".
Here is a quick apparatus-choice check using the same logic.
Measurement Quality, Units And Uncertainty
Good apparatus choice is only the start. Questions often test whether you understand how the apparatus affects the quality of the evidence.
Resolution
Resolution is the smallest change in the quantity being measured that an instrument can detect.
Resolution tells you how fine a change the apparatus can show, but it is not the only quality word used in practical evidence.
Accuracy
Accuracy is how close a measurement is to the true value or accepted value.
Accuracy is about closeness to the true value. Precision is different.
Precision
Precision is how close repeated measurements are to each other under the same conditions.
A balance reading to 0.01 g has a smaller resolution than a balance reading to 0.1 g, so it can detect smaller changes in mass. That does not automatically make a whole method valid: a mis-calibrated balance can give precise but inaccurate results, and a poorly controlled experiment can give neat-looking numbers that do not answer the question.
Uncertainty is an estimate of the possible range around a measurement. A practical rule is:
Percentage Uncertainty
If a quantity is measured by difference, such as mass before minus mass after, both readings contribute uncertainty.
Percentage uncertainty from a balance
A piece of tissue has a measured mass of 2.56 g. The balance reads to 0.01 g, so use an absolute uncertainty of 0.01 g for this single mass reading.
To an appropriate number of significant figures, the percentage uncertainty is 0.39%. This is small because the uncertainty is tiny compared with the mass measured.
The same 0.01 g uncertainty becomes much more important for a small mass. If the mass were only 0.12 g, the percentage uncertainty would be:
That is why choosing apparatus with suitable resolution matters. It is also why an answer such as "use a more accurate balance" should be made specific: use a balance with a smaller resolution, such as 0.001 g, if the mass changes are very small.
Use standard unit discipline: g for mass, degrees C for temperature, no unit for pH, cm3 or dm3 for volume, and mol dm-3 or g dm-3 for concentration. Avoid "M" for concentration in this course. Record raw data consistently: the same type of raw measurement should use the same number of decimal places, matching the apparatus used.
A good practical answer links the apparatus to its resolution, units, calibration and the reason the measurement is valid.
Use this calculation habit whenever the question gives a measurement and an absolute uncertainty.
Colorimeters And Potometers As Quantitative Instruments
Instrumentation means apparatus that converts a biological change into a numerical reading. In this row, the named examples are a colorimeter and a potometer.
A colorimeter measures how much light passes through, or is absorbed by, a coloured solution. This makes it useful when colour intensity is linked to concentration, such as a quantitative food test or a calibration-curve method.
For colorimetry, the method must make the reading fair:
- choose an appropriate wavelength or filter for the colour being measured
- zero the colorimeter with a suitable blank, not automatically with distilled water
- use clean, unscratched cuvettes or tubes
- put cuvettes in the correct orientation if only two sides are clear
- remove precipitate from the light path unless turbidity is the thing being measured
- use known concentrations to make a calibration curve before estimating an unknown concentration
Calibration Curve
A calibration curve is a graph made using known concentrations so an unknown concentration can be estimated from its instrument reading.
A potometer measures water uptake by a leafy shoot. In a moving-bubble potometer, water uptake moves an air bubble along a capillary tube. This is used as an estimate of transpiration rate, but the instrument directly measures water uptake, not water vapour leaving the leaf.
For a potometer, valid readings depend on preventing air leaks and keeping the xylem water-filled:
- cut the shoot under water so air is not drawn into the xylem
- keep leaves dry so stomata are not blocked
- remove air bubbles from the apparatus before starting
- seal joints, for example with petroleum jelly
- measure bubble movement over a known time
- control variables such as light intensity, humidity, air movement, leaf area and time
Potometer rate from bubble movement
A bubble moves 18 mm in 30 min through a capillary tube with diameter 1.0 mm. The radius is 0.5 mm.
Volume moved in 30 min:
30 min is 0.5 h, so:
Rounded suitably, the rate is 28 mm3 h-1.
This calculation has two common traps: using the diameter as if it were the radius, and forgetting that 30 minutes is half an hour.
In potometer calculations, check whether the question gives diameter or radius, and check whether time must be converted into hours.
Now check the core meaning of what the instrument is actually measuring.
Glassware And Serial Dilutions
Laboratory glassware is used when a method depends on measured volumes. Different glassware suits different levels of precision.
| Glassware or volume apparatus | Best use | Common practical caution |
|---|---|---|
| Measuring cylinder | quick approximate volumes | lower precision than a pipette or burette |
| Pipette or micropipette | accurate transfer of fixed small volumes | use the correct technique and change tips where contamination matters |
| Volumetric flask | making a known final volume of solution | make up to the calibration mark after dissolving/mixing |
| Burette | delivering variable measured volumes | remove air bubbles from the tip and read the meniscus carefully |
| Test tube | carrying out small-scale reactions | not usually for precise volume measurement unless marked and suitable |
A serial dilution is a sequence of dilutions where each new dilution is made from the previous diluted sample. It is useful when a very concentrated sample must be reduced step by step to make measurable standards or countable microbial colonies.
Serial Dilution
A serial dilution is a stepwise dilution in which each tube or solution is diluted again to produce a series of progressively lower concentrations.
The key idea is that dilution factors multiply. If 1 cm3 of sample is added to 9 cm3 of diluent, the total volume is 10 cm3, so the sample has been diluted tenfold. The new concentration is one-tenth of the previous concentration.
Four tenfold serial dilutions
A culture is diluted by transferring 1 cm3 into 9 cm3 of distilled water. This is repeated four times.
Each step is a tenfold dilution:
Step 1: 10^-1 of the original concentration
Step 2: 10^-2 of the original concentration
Step 3: 10^-3 of the original concentration
Step 4: 10^-4 of the original concentration
The final tube is therefore 1/10 000 of the original concentration.
Good serial dilutions are not just calculations. The practical technique matters:
- mix each dilution thoroughly before transferring to the next tube
- use clean pipettes or change micropipette tips between transfers
- label tubes clearly with dilution factors or concentrations
- keep the final volume consistent in each tube
- repeat plating or readings where the method estimates a population or concentration
- do not discard data just because it is unexpected; only exclude an anomaly when there is a clear procedural reason
In colorimetry, serial dilutions can make a set of known concentrations for a calibration curve. In microbiology, serial dilutions can reduce a dense culture until colonies on a plate can be counted. The same mathematical logic applies in both contexts.
Exam Practical Reasoning
Practical questions usually reward specific reasoning. The answer "repeat it" is often too vague. Better answers say exactly what is repeated, why it improves the evidence, and how the data will be processed.
For example, in a serial-dilution population estimate, "repeat the experiment" could mean starting again with a different bacterial culture, which may not improve the current estimate. A stronger improvement is: plate at least three samples from the same final dilution and calculate a mean colony count. Another strong improvement is to mix each dilution before transferring the next sample, because cells or solute may not be evenly distributed.
In a colorimeter method, a good improvement is not just "be more accurate". It might be to use a suitable blank to zero the colorimeter, clean the cuvette, use known standards to create a calibration curve, or repeat absorbance readings for each concentration and calculate a mean.
In a potometer method, a good plan does not need a long description of setting up the apparatus if the question asks for data collection. It should state the range of the independent variable, repeats at each value, controlled variables, the rate calculation, and a suitable analysis. If five or more temperature values are compared with transpiration rate, Spearman's rank correlation may be suitable; if only two temperatures are compared, Student's t-test may be suitable if the data meet the assumptions taught elsewhere.
Use "repeatable", "reproducible" or "confidence in the evidence" instead of the vague word "reliable". OCR's measurement guidance treats "reliability" as ambiguous.
Apply that precision to an improvement question, where the reason matters as much as the method change.
Exam Practical Reasoning Continued
If you can explain this lesson in plain language, say: "The apparatus is part of the evidence." A balance, pH meter, colorimeter, potometer, pipette or burette is not just equipment on a bench. It decides what is measured, how much uncertainty the number carries, and whether the conclusion is justified.