What is a titration curve
Titration - FAQ: definition, titration curve, calculation, chemistry etc.
Titrations can be classified based on the indication principles and the chemical reaction that occurs:
The direct measurement of the galvanic potential of an electrode arrangement is called potentiometry, while carrying out a titration according to this method is called potentiometric titration.
direct measurement of the galvanic potential
The resulting potential U should, if possible, be measured without current using a high-impedance signal amplifier, for the following reasons:
- The potentiometry is based on the Nernst equation, which was derived for sensors in chemical and electrical equilibrium. An excessively strong current flow across the affected phase interfaces would disturb this equilibrium.
- Another reason for using a high-resistance measuring input is the special design of pH and ion-selective electrodes. The measuring circuit contains an ion-selective membrane, the electrical resistance of which can easily be between 100 and 1000 MΩ. If the experimental error caused by the voltage divider effect is to be less than 0.1%, the input impedance of the meter must be at least 1000 times higher. This shows the following equation:
For sensors with a very high resistance, signal amplifiers with an input impedance of 1012 Ω required.
This indication method measures the potential difference between two metal electrodes that are polarized by a weak current. As with potentiometry, the voltametric titration curve shows the ratio of potential to volume.
The following measuring arrangement is required:
The stabilized power supply provides the electricity. The resistance connected in the measuring circuit must be selected so that a current Ipol with a strength of 0.1 to 20 µA can be generated. The potential U arising between the electrodes is measured exactly as in potentiometry. One of the main applications of the voltametric indication is the determination of water according to Karl Fischer.
The basis of photometry is the decrease in intensity of a light beam of a certain wavelength that passes through a solution. The light permeability or transmission is the primary measurand in photometry and amounts to
I.0: Intensity of the incoming light beam
I: intensity of the emerging light beam
If all light is absorbed, then I = 0 and thus T = 0. If no light is absorbed, then is
I = I0 and T = 1 (or% T = 100%).
In photometry, the measurand absorption is often used. The relationship between transmission and absorption is described by the law of Bouguer-Lambert-Beer:
A = - log T = A = ε * b * c
ε: extinction coefficient
c Concentration of the absorbing substance
d: path length of the light through the solution
From the above relationship it can be seen that there is a linear relationship between absorption A and concentration c.
Photoelectric sensors have a number of advantages over potentiometric sensors for titration:
- they are easier to use (no refilling of electrolyte solutions, no clogged diaphragm)
- longer lifespan (they are practically unbreakable)
- They can be used to carry out all classic titrations with a color change (no change to traditional methods and standards).
Photometric indication is possible for many analytical reactions:
- Acid-base titrations (aqueous and non-aqueous)
- Redox titrations
- Precipitation titrations
- Turbidity titrations
For photo titration, a wavelength should be selected at which the measured differences in transmission are greatest before and after the equivalence point. In the visible range, these are mainly wavelengths between 500 and 700 nm.
Application examples: complexometric and turbidimetric reactions.
Conductivity is the ability of a solution to let an electric current pass through it. The conductivity is measured in µS / cm (microsiemens / centimeter) or mS / cm (millisiemens / centimeter). A high reading shows a large number of ions. The amount of current flowing through the solution is proportional to the amount of ions. If we know the conductivity of a solution, we can get information about the total ion content. If the ion content is known, we can even make statements about the ion concentration.
To measure the conductivity, a voltage is applied to two plate electrodes immersed in the solution. The plates are metallic, but graphite pencils can also be used. While the dissolved ions begin to migrate to the plate electrodes, the electrical current flows between the two plates.
The principle of conductometric titration
During titration, one of the ions is replaced by another. These two ions always differ in terms of ionic conductivity, so that the conductivity of the solution constantly fluctuates in the course of the titration. If you add the solution of one particular electrolyte to the solution of another, the resulting conductivity will depend on the occurrence of a reaction. However, if there is no chemical reaction in the electrolyte solutions, conductivity increases. The equivalence point can be determined graphically by showing the change in conductivity as a function of the amount of titrant added as a curve.
Principle of the conductometric titration
The elementary statement that every chemical reaction is accompanied by a change in energy precisely describes the basis of thermometric titration. In the case of endothermic reactions, a drop in temperature can be observed as energy is absorbed. In the case of exothermic reactions, however, energy is released. The equivalence point (EQP) of a titration can be identified by monitoring the temperature change (Fig. 1). In the course of an exothermic titration, the temperature rises until the equivalence point is reached. The temperature then stabilizes and then goes back down again. With an endothermic titration it is exactly the opposite.
Schematic representation of an exothermic and an endothermic titration
As described above, a decrease in temperature can be observed in the course of the endothermic titration reaction. As soon as the equivalence point is reached, the temperature stabilizes. The end point is determined by calculating the second derivative of the curve (segmented evaluation).
Only the following are required for thermometric titration: a chemical reaction with a large change in energy, a precise and fast thermometer and a titrator that enables segmented evaluation of the titration curve.
The method of coulometric titration was originally developed in 1938 by Szebelledy and Somogy . It differs from volumetric titration in that the titrant is generated in situ by electrolysis and then reacts stoichiometrically with the substance to be determined. The amount of substance converted is calculated on the basis of the total electrical charge Q passed through in coulombs and not, as in the case of volumetric titration, on the basis of the volume of the titrant used.
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