Acid-base titrations can also be used to quantify the purity of chemicals. Alkalimetry, or alkimetry, is the specialized analytic use of acid-base titration to determine the concentration of a basic alkaline substance; acidimetry, or acidometry, is the same concept applied to an acidic substance. Below are some common equivalence point indicators:. When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger; if both are of equal strength, then the equivalence pH will be neutral.
Weak acids are not often titrated against weak bases, however, because the color change is brief and therefore very difficult to observe. You can determine the pH of a weak acid solution being titrated with a strong base solution at various points; these fall into four different categories: 1 initial pH; 2 pH before the equivalence point; 3 pH at the equivalence point; and 4 pH after the equivalence point.
The endpoint color is exhibited when the indicator solution forms a complex with the excess mercuric ions from the titrant. This happens between a pH of 2.
They are used for quantifying analytes, the unknown ion or compound being tested for, in addition to being used to standardize acids and bases.
Another type of titration is a redox reaction, when combining the titrant and titrand causes a gain in electrons. This gain is called a reduction. The calibrated burette is the main piece of equipment required for a titration method.
Calibration is important because it is essential for the burette to be as accurate as possible in order to dispense very precise amounts of liquid into the sample. A burette is a long cylindrical piece of glass with an open top for pouring, or pumping, in the titrant.
At the bottom there is a carefully formed tip for dispensing. Burettes usually have a plastic stopper that can easily be turned to deliver mere fractions of a drop of titrant, if needed. Burettes come in many sizes and are marked in milliliters and fractions of milliliters. Burettes are the most common piece of instrumentation used in titrations, but electronic devices can also be used. Potentiometric titrations can use a calibrated pH meter to determine an endpoint.
The endpoint pH reading is similar to using an indicator solution except the chemist is using an instrument to find the exact potential rather than a color change. Complexation titrations can use an ion selective electrode to determine at what point a complex has been reached.
Spectrometry is another option; it allows the chemist to determine very small color changes in the titrand. Indicator solutions are not always necessary for titrations, but they can make manual titrations with a burette easier.
We can measure the pH in a number of ways. The oldest form of pH indicator is litmus paper and it is still in use today. Dyes have been extracted from a number of different lichens since at least the 14th century and were big business in The Netherlands.
These dyes can be used as a solution to put straight into your experiment. However, this contaminates your reactions with a small amount of lots of different compounds. To get around this the litmus solution can be infused into a paper strip. This allows you to dip the paper in and then remove it easily.
In the Laboratory Confessions podcast researchers talk about their laboratory experiences in the context of A Level practical assessments. In this episode, we look at measuring pH and the use of indicators, the use of laboratory equipment, and the use of volumetric flasks and making up a standard solution.
Acids and bases are not all the same strength. They can be described as weak or strong and you might be asked what that means during your exams. Acids are compounds that split into two ions when dissolved in water; one or more positive hydrogen ions and negative counter ions. The splitting is called dissociation. A weak acid, such as citric acid, only partially dissociates. This means that some of the acid splits into its ions but some of it stays together.
A weak acid exists in equilibrium so some of the whole acid is being formed at the same time some of the acid is splitting into its ions so there should always be the same amount of ions present. This applies to bases as well but in this case the ion of interest is OH-. You can never know by looking at a solution how strong it might be so always work as safe as possible! These links are being provided as a convenience and for informational purposes only; they do not constitute an endorsement or an approval by the University of Birmingham of any of the information contained on external website.
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