The traditional method of detecting the equivalence point has been to employ an indicator dye, which is a second acid-base system in which the protonated and deprotonated forms differ in color, and whose pK a is close to the pH expected at the equivalence point. If the acid being titrated is not a strong one, it is important to keep the indicator concentration as low as possible in order to prevent its own consumption of OH — from distorting the titration curve.
The observed color change of an indicator does not take place sharply, but occurs over a range of about 1.
Indicators are therefore only useful in the titration of acids and bases that are sufficiently strong to show a definite break in the titration curve. Some plants contain coloring agents that can act as natural pH indicators. These include cabbage shown , beets, and hydrangea flowers. For a strong acid - strong base titration, almost any indicator can be used, although phenolphthalein is most commonly employed.
For titrations involving weak acids or bases, as in the acid titration of sodium carbonate solution shown here, the indicator should have a pK close to that of the substance being titrated. When titrating a polyprotic acid or base, multiple indicators are required if more than one equivalence point is to be seen. The pK a s of phenolphthalein and methyl orange are 9. A more modern way of finding an equivalence point is to follow the titration by means of a pH meter.
Because it involves measuring the electrical potential difference between two electrodes, this method is known as potentiometry. Until around , pH meters were too expensive for regular use in student laboratories, but this has changed; potentiometry is now the standard tool for determining equivalence points. Plotting the pH after each volume increment of titrant has been added can yield a titration curve as detailed as desired, but there are better ways of locating the equivalence point.
A second-derivative curve locates the inflection point by finding where the rate at which the pH changes is zero. The differential plot , showing rate-of-change of pH against titrant volume, locates the inflection point which is also the equivalence point. In a standard plot of pH-vs-volume of titrant added, the inflection point is located visually as half-way along the steepest part of the curve. The idealized plots shown above are unlikely to be seen in practice.
When the titration is carried out manually, the titrant is added in increments, so even the simple titration curve must be constructed from points subject to uncertainties in volume measurement and pH especially if the latter is visually estimated by color change of an indicator. If this data is then converted to differential form, these uncertainties add a certain amount of "noise" to the data.
A second-derivative plot uses pH readings on both sides of the equivalence point, making it easier to locate in the presence of noise.
Locating the equivalence point depends very strongly on correct reading of only one or two pH readings near the top of the plot. A simple curve, plotted from a small number of pH readings, will not always unambiguously locate the equivalence point.
The "noise" in differential plots can usually be minimized by keeping the titrant and analyte concentrations above 10 —3 M. Monitoring the pH by means of an indicator or by potentiometry as described above are the standard ways of detecting the equivalence point of a titration. However, we have already seen that in certain cases involving polyprotic acids or bases, some of the equivalence points are obscured by their close proximity to others, or by the buffering that occurs near the extremes of the pH range.
Similar problems can arise when the solution to be titrated contains several different acids, as often happens when fluids connected with industrial processes must be monitored. If the acid and base are both strong i. See this Wikipedia page for more on thermometric titrations, including many examples. Note also the video on this topic in the "Videos" section near the end of this page.
A typical thermometric titration curve consists of two branches, beginning with a steep rise in temperature as the titrant being added reacts with the analyte, liberating heat.
Once the equivalence point is reached, the rise quickly diminishes as heat production stops. Then, as the mixture begins to cool, the plot assumes a negative slope. Although a rough indication of the equivalence point can be estimated by extrapolating the linear parts of the curve blue dashed lines , the differential methods described above are generally preferred.
Acids and bases are electrolytes , meaning that their solutions conduct electric current. The conductivity of such solutions depends on the concentrations of the ions, and to a lesser extent, on the nature of the particular ions.
Any chemical reaction in which there is a change in the total quantity of ions in the solution can usually be followed by monitoring the conductance. Acid-base titrations fall into this category. Consider, for example, the titration of hydrochloric acid with sodium hydroxide. This can be described by the equation. Each kind of ion makes its own contribution to the solution conductivity. This reflects the much greater conductivities of these ions owing to their uniquely rapid movement through the solution by hopping across water molecules.
However, because the conductances of individual ions cannot be observed directly, conductance measurements always register the total conductances of all ions in the solution. The change in conductance that is actually observed during the titration of HCl by sodium hydroxide is the sum of the ionic conductances shown above.
For most ordinary acid-base titrations, conductimetry rarely offers any special advantage over regular volumetric analysis using indicators or potentiometry. However, in some special cases such as those illustrated below, conductimetry is the only method capable of yielding useful results.
Chemists typically record the results of an acid titration on a chart with pH on the vertical axis and the volume of the base they are adding on the horizontal axis. This produces a curve that rises gently until, at a certain point, it begins to rise steeply. This point — called the equivalence point — occurs when the acid has been neutralized.
The half-equivalence point is halfway between the equivalence point and the origin. This is the point at which the pH of the solution is equal to the dissociation constant pKa of the acid.
In a typical titration experiment, the researcher adds base to an acid solution while measuring pH in one of several ways. One common method is to use an indicator, such as litmus, that changes color as the pH changes. Other methods include using spectroscopy, a potentiometer or a pH meter. As the concentration of base increases, the pH typically rises slowly until equivalence, when the acid has been neutralized.
Learn more. Why does pKa of a acid-base indicator equal to the pH when the equivalence point is reached? Ask Question. Asked 6 years, 6 months ago. Active 3 months ago. Viewed 43k times. I had this question while reading a Chemguide article, on this topic: I think my confusion is caused by the way I look at pKa of acid-base indicator. Improve this question. Add a comment. Active Oldest Votes. Vocabulary : SB : Strong Base. Improve this answer.
I understand what you said is correct, for other situations, but isn't exactly what I am looking for. Show 4 more comments. Consider titrating an acid with NaOH. MaxW MaxW Karsten Theis Karsten Theis It is also very poorly written.
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