Titration how does it work

In the titration of an acid HA with a strong base e. NaOH the following two chemical equilibria occur:. Acid-base reactions are very fast, and the chemical equilibrium is established extremely rapidly. Acid-base reactions in aqueous solutions are thus ideal for titrations. If the solutions used are not too dilute, the shape of the titration curves depends only on the acidity constant K a. The amount-of-substance concentration of a solution of an entity X symbol c X is the amount of substance n divided by the volume V of the solution.

The endpoint mode represents the classical titration procedure: the titrant is added until the end of the reaction is observed, e. With an automatic titrator, the sample is titrated until a predefined value is reached, e. The equivalence point is the point at which the analyte and the reagent are present in exactly the same quantities. In most cases it is virtually identical to the inflection point of the titration curve, e.

The inflection point of the curve is defined by the corresponding pH or potential mV value and titrant consumption mL. The equivalence point is calculated from the consumption of titrant of known concentration. The product of concentration of titrant and the titrant consumption gives the amount of substance which has reacted with the sample. In an autotitrator the measured points are evaluated according to specific mathematical procedures which lead to an evaluated titration curve.

The equivalence point is then calculated from this evaluated curve. In a back titration we use two reagents — one that reacts with the original sample A and a second, that reacts with the first reagents B.

First, a precisely measured excess of reagent A is added to the sample. After the reaction ends, the remaining excess of reagent A is then back titrated with a second reagent B. The difference between the added amount of the first and second reagent then gives the equivalent amount of the analyte. The back titration is used mainly in cases where the titration reaction of the direct titration is too slow or direct indication of the equivalence point is unsatisfactory.

Examples: Acid content in wine, milk. Acid content in ketchup. Content of inorganic acids like sulfuric acid. Examples: Salt content in crisps, ketchup and food; Silver content in coins, Sulfate content in mineral water; Sulfate content in electroplating bath.

Examples: Anionic surfactant content in detergents; Anionic surfactant content in washing powders; Anionic surfactant content in liquid cleanser. Titrations can be classified according to the indication principles and the chemical reaction occurring:. The direct measurement of the galvanic potential developed by an electrode assembly is called potentiometry , while the performance of a titration by use of this method is referred to as a potentiometric titration.

The potential U that develops should be measured, if at all possible, at zero current with a high impedance signal amplifier for the following reasons:. This indication technique involves the measurement of the potential difference between two metal electrodes that are polarized by a small current. As in the case of potentiometry, the voltametric titration curve is a potential-volume curve.

The stabilized power supply source provides the current. The resistance R connected in the circuit must be selected such that a current Ipol can be generated in the range 0. The potential U that develops between the electrodes is measured exactly as in potentiometry. One of the main applications of voltametric indication is the determination of water by the Karl Fischer method. The basis of photometric indication is the decrease in intensity at a particular wavelength of a light beam passing through a solution.

The transmission is the primary measured variable in photometry and is given by. In photometry, work is frequently performed using absorption as the measured variable. The relation between transmission and absorption is described by the Bouguer- Beer-Lambert Law:.

From the above relation it can be seen that there is a linear relation between absorption A and concentration c. In comparison with potentiometric sensors, photoelectric sensors have a number of advantages in titration:. In phototitration a wavelength should be selected which gives the greatest difference in transmission before and after the equivalence point.

In the visible region such wavelengths are usually in the range to nm. Conductivity is the ability of a solution let a current pass through. A high value indicates a high number of ions.

The amount of current flowing in the solution is proportional to the amount of ions. If we know the conductivity of a solution, we can get an idea of the total content of ions. Moreover if the ions are known, even a statement about their concentration can be made. To measure conductivity a voltage is applied across two plates immersed in the solution. The plates are metallic, or graphite poles can be used as well. While the solved ions will start to move towards the plates the electric current will flow in between the plates.

During the titration, one of the ions is replaced by the other and invariably these two ions differ in the ionic conductivity with the result that conductivity of the solution varies during the course of titration. Therefore, if you add a solution of one electrode to another, the final conductance will rely on the occurrence of reaction. But if there is no chemical reaction in the electrolyte solutions, there will be an increase in the level of conductance.

The equivalence point may be located graphically by plotting the change in conductance as a function of the volume of titrant added. The elementary statement, that every chemical reaction is accompanied by a change in energy, is precisely what constitutes the basis of thermometric titration. During endothermic reactions, energy is absorbed and a temperature drop is observed.

The opposite is true for exothermic reactions where energy is released. The equivalence point EQP of a titration can be detected by monitoring the change in temperature Figure 1. In the course of an exothermic titration, the temperature increases until the EQP is reached. After that, the temperature initially stabilizes, followed by a subsequent temperature drop.

The opposite happens for endothermic titration. As described above, a temperature decrease is observed during the course of the endothermic titration reaction.

Once the equivalence point has been reached, the temperature stabilizes. The endpoint is determined by calculating the second derivative of the curve segmented evaluation. The only requirements of a thermometric titration are: a chemical reaction with a large energy change, a precise and fast thermometer and a titrator capable of performing a segmented evaluation of the titration curve.

The technique of coulometric titration was originally developed by Szebelledy and Somogy [1] in The method differs from volumetric titration in that the titrant is generated in situ by electrolysis and then reacts stoichiometrically with the substance being determined. The amount of substance reacted is calculated from the total electrical charge passed, Q, in coulombs, and not, as in volumetric titration, from the volume of the titrant consumed.

The titrant is added in constant volume increments dV. Incremental titrant addition is used in non-aqueous titrations, which sometimes have an unstable signal, and also in redox and in photometric titrations, where the potential jump at the equivalence point occurs suddenly.

Notice that in the steepest region of the curve there are relatively few measured points. A constant pH- or potential change per increment allows the variation of the volume increment between minimum and maximum volume increment. Thus, the analysis can be speeded up by using big increments in the flat regions of the titration curve.

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. Suppose a scientist wants to find out the concentration of acid in a nitric acid solution.

First she would pour 25 mL of the solution into a mL Erlenmeyer flask. Then she adds a 0. She then adds an indicator to the flask before opening the buret to slowly add the NaOH solution into the acid solution. When titration is complete, the solution in the flask turns red.

The scientist measures the volume of standard solution added to the flask. Once the scientist has this data, she performs a series of calculations to figure out the ratio of standard solution to the nitric acid and convert it into moles. The end result of these calculations is the concentration of acid in the acidic solution.

Jack Ori has been a writer since He has worked with clients in the legal, financial and nonprofit industries, as well as contributed self-help articles to various publications. Four Characteristics of a Primary Standard Substance. The Uses of Volumetric Analysis.

Types of Titration. How to Calculate Alkalinity After Titration. How to Calculate Titratable Acidity. What Is "Direct Titration"?