Know Your Lab Techniques: Titration
Titration is a rite of passage for chemists. If you haven’t done titrations in your general chemistry course yet, you will eventually go through the same trials as every other chemist in training—fumbling with glassware, anxiously reading fluid volumes, struggling to hit the perfect end point, or repeating the process over and over.
Why? Because titration is a versatile, low-cost technique to precisely answer a variety of questions. You can use an acid-base titration to calculate the exact strength of a hydrochloric acid solution to use for other analyses. You can use a redox titration to identify the amount of vitamin C in your drink. You can use a complexometric titration to determine the amount of metals in drinking water. In all of these scenarios, titration is a recipe for success—but only if you take the time to do it right.
Here is what you need to know to master this technique.
How titrations work and what you need
Every titration uses the same players:
- Analyte (aka titrand): the solution whose concentration you want to determine.
- Titrant (aka titrator or standard): a solution of known concentration that reacts with the analyte.
- Indicator: a substance that changes color when the analyte and titrant have completely reacted.
The purpose of any titration is to determine the amount of analyte present by reacting away every last bit of that substance. You will typically measure a precise mass or volume of analyte, then add the titrant dropwise to the analyte with a burette until no analyte remains. The trick, of course, is adding just the right amount of titrant. Even adding 0.5 mL too much or too little means you don’t have an accurate measurement of the analyte reacted.
Some reactions undergo a sharp visual change as soon as all the analyte is reacted. Otherwise, you’ll add one of three types of indicators.
Types of Indicators
Acid-base
These indicators change color at different pHs. Phenolphthalein changes from colorless to pink around pH 7–8, methyl red turns yellow between pH 4.8–6.0, and thymol blue turns from orange to yellow between pH 1.2–2.8 then yellow to blue between pH 8.0–9.6.
Redox
These indicators change color by undergoing redox reactions. Starch goes from colorless to dark blue in the presence of excess iodine, while potassium dichromate turns green when reduced by ferrous salts, iodides, or sulfates.
Precipitation
These indicators form highly visible precipitates, making the solution appear suddenly cloudy when an excess of a specific titrant is available. Examples include calcein for calcium, Fast Sulphon Black F for copper, and hematoxylin for copper; murexide for calcium, cobalt, nickel, and rare-earth metals.
Once you’ve precisely measured the amount of titrant added, you can use reaction stoichiometry to calculate the amount of analyte you have.
How to do a titration
Here are steps and the purpose behind each one that will help you achieve an accurate result.
Step 1
Use a small amount of titrant to rinse the burette at least three times, draining through the burette tip.
Purpose
Use a small amount of titrant to rinse the burette at least three times, draining through the burette tip.
Step 2
Adjust the top of the burette to roughly eye level.
Purpose
There are two reasons: 1, your measurements will be most accurate at eye level, and 2. it is difficult to safely pour liquids when you are reaching above your head.
Step 3
Use a funnel to fill the burette.
Purpose
A funnel is the easiest way to pour liquids into the burette without spilling. Remember to remove the funnel before you start taking measurements to ensure random drops from the funnel don’t affect your measurements.
Step 4
Drain the burette a little into an empty flask to remove air bubbles from the tip of the burette. You may also need to gently tap or partially tip the burette to remove the air bubble.
Purpose
The volume of the air bubble varies with pressure and is likely to come out during your titration. This will make your measurements less accurate and precise.
Step 5
Close the bottom of the burette fully and always keep a container beneath it.
Purpose
Do this just in case there is a leak or spill.
Step 6
Precisely and accurately measure your analyte. Use a volumetric pipet or a second burette to measure liquids (remember to rinse with your analyte) or an analytical balance to measure solids.
Purpose
You need to know the number of moles of analyte you have as precisely as possible. (If using pipet, be sure to rinse it with the analyte prior to use, just you did with the burette.)
Step 7
If necessary, add solvent.
Purpose
You may need to add solvent to dissolve a solid analyte or increase the volume of a liquid analyte to make changes easier to see. Since the solvent doesn’t affect the number of moles of analyte in the flask or the moles of titrant dispensed from the burette, it does not need to be measured precisely.
Step 8
If necessary, add an indicator. Use the same amount of indicator for each titration.
Purpose
Some indicators can have properties that can impact aspects of the titration (for example acid-base properties), so keeping this amount consistent ensures more precise results.
Step 9
Write down the starting position of the meniscus in the burette. Make sure to record the measurement to the correct number of significant figures to keep your measurements as precise as possible (see Figure A).
Purpose
The amount of titrant dispensed will be the final volume reading minus the initial volume reading, so it doesn’t really matter whether you start at 0.00 mL.
Step 10
To begin the titration, add a small amount of titrant into the analyte solution, swirling to keep the solution homogeneous. (see Figure B)
Purpose
Swirling ensures all the analyte and titrant have the opportunity to react.
Step 11
As you near the end point, start adding the titrant one drop at a time, then slow down and reduce to half-drops. If necessary, use a little solvent to wash the half-drop off the end of the burette into the analyte solution.
Purpose
The smaller the drop, the more precise your measurement of the end point will be. As mentioned in step 7, adding solvent does not impact the number of moles of analyte or titrant.
Step 12
When you reach the end point (the faintest possible change that persists throughout the homogeneous solution), write down the final volume reading of your titrant. (see Figure C)
Purpose
At the end point, the titrant you have added has completely reacted all of the analyte present. The change you observe is actually from a tiny bit of extra titrant; keeping the extra as small as possible will give you the most accurate results.
Step 13
Repeat the titration at least three times.
Purpose
You want at least three titrations yielding similar results; in all probability, your first titration will not be as accurate as subsequent ones. The closer the results, the more likely it is that they are accurate.
Note: A strong color change (see Figure D) means you have added too much titrant. This can happen in just one or two drops, but it is enough to make your measurements inaccurate. However, if you use the same amount of analyte for each titration, you still know roughly where the end point will be. For your next titration, you can add about 90% of the titrant immediately, then add single and half-drops until you reach a more precise end point.
Tips for success
- Read the meniscus correctly. Burettes and pipets are calibrated to give +/- 0.01 mL readings using the bottom of the meniscus. Make sure your eye is level with the meniscus to prevent parallax error. Hold a white card with a black bar just behind and below the meniscus to make it easier to see.
- Make use of blank paper. Place a blank piece of white paper under your analyte flask to make color changes easier to see.
- Start with a trial run. For your first titration, add titrant about one mL at a time to get a rough idea of where the end point is. For example, if your trial run on 10.00 mL of analyte reaches the end point somewhere between 22.00 mL and 23.00 mL of titrant, you can start your next titration off by adding 21.00 mL of titrant right away before slowing down to single and half-drops.
- Use the same amount of analyte for each titration. Using similar amounts of analyte each time gives you a rough idea of where to look for the end point. Although the amount of analyte does not need to be exactly the same, you do need a precise measurement.
- Be aware of potential side reactions. If you know the composition of your sample analyte solution, you can avoid unexpected side reactions that yield falsely high or falsely low results. For example, when doing an acid-base titration, remember that CO2 from the air can dissolve in your solutions and form carbonic acid. Keep your solutions sealed as much as possible and work quickly to minimize the impact.
Troubleshooting your titration
The solution color changed but then reverted to original color.
That means you didn’t reach the end point. It’s normal for a drop of titrant to change the color, especially as you approach the end point, because the concentration is not equal throughout the solution. This is why swirling and slow drops are important. Swirling helps distribute the titrant evenly, and slow drops make it easier to pause and evaluate. The end point should remain visible for 20–30 seconds.
I missed the end point
It’s technically possible to add more analyte and indicator to a flask to reverse the end point if you carefully track the added volumes. But the easier and faster solution is to simply start over.
My end point occurred immediately
Your titration reaction may have been too rapid due to an excessive concentration of titrant, improper mixing, or the presence of impurities. Try reducing the concentration of the titrant, mix completely, and thoroughly rinse your glassware in the next attempt.
My end point didn’t happen at all
First, add some more indicator; forgetting the indicator is a common mistake, and a few extra drops usually won’t impact your results much. If the indicator isn’t the problem, then you simply haven’t hit the end point. If you find you need to refill your burette to reach the end point, it may be easier to switch to more a concentrated titrant or use less analyte.
Titration calculations
Titration calculations are essentially applied stoichiometry with three steps.
About the Author
Max G. Levy
is a freelance science journalist based in Los Angeles, CA. He has a PhD in chemical engineering and writes stories about public health, the environment, and technology.
