Key Concepts in Analytical Chemistry
Gravimetric Analysis
Gravimetric analysis is a technique in which the only measurement performed is mass.
Analyte
The analyte is the species whose composition is required to be determined.
Stages of Gravimetry
Precipitation
If the solid sample is weighed, it is dissolved, and then the precipitating agent is added.
Digestion
The precipitate remains in contact with the mother liquor in order to improve the quality of the crystals.
Filtering
Filtering prevents the loss of the solid. It should be done with cold filtration on smooth filter paper.
Washing
The precipitate is washed using very small amounts of solvent or cold solvent to prevent the loss of the solubility product.
Drying
Drying takes place in an oven at the proper temperature.
Weighing
Weighing depends on the pressure and accuracy. It is necessary to use an analytical balance to determine the mass of the product.
Optimal Precipitating Agent
An optimal precipitating agent reacts completely (quantitatively) and selectively only with the analyte to form one solid product.
pH
pH is a measure of a substance that reflects its degree of acidity or basicity.
Acid-Base Indicator
An acid-base indicator is a colorful substance that changes color at a given pH value.
Indicator Color Changes
- Phenolphthalein: pH 8-10, colorless to pink to fuchsia.
- Bromothymol Blue: pH 6 to 7.6, yellow to green to blue.
- Methyl Red: pH 4.2 to 6.3, red to orange to yellow.
- Methyl Orange: pH 3.1-4.4, red to orange to yellow.
Volume
Volume is the amount of space occupied by a body.
Accuracy (Volumes)
Accuracy in the measurement of volumes refers to the possibility of obtaining the same value repeatedly.
Accuracy (General)
Accuracy is the value near the actual value.
- Class A: High accuracy – low error.
- Class B: Low accuracy – higher error.
- Class AS: High accuracy – low error.
Solution
A solution is a homogeneous mixture of two or more substances evenly distributed between them.
Concentration of a Solution
The concentration of a solution is the amount of solute per unit solvent or solution.
- Dissolution: Direct weight or volume of direct measurement of the solute.
- Dilution: From a more concentrated solution.
Purification of a Compound
Purification of a compound that is solid at room temperature is achieved through recrystallization from an appropriate solvent.
Recrystallization
Recrystallization is based on the difference in solubility of different compounds in a solvent or a mixture thereof.
Solubility
Solubility is the maximum amount of solute that can dissolve in 100 parts of solvent at a given temperature.
Melting Point
The melting point is defined as the temperature at which the solid and liquid phases coexist in equilibrium.
Buffer Solutions (Buffers)
Buffer solutions are solutions of weak acids (or weak bases) with their conjugate base or conjugate acid, in the form of salts.
Reversible Reaction
A reversible reaction is one in which both the direct and reverse reactions can occur in appreciable magnitude.
Catalytic Enzyme
Catalytic enzymes are proteins that increase or accelerate a chemical reaction.
Chemical Equilibrium
Chemical equilibrium is the state where the forward and reverse reactions occur simultaneously at the same rate.
Oxidation
Oxidation is defined as the combination reaction of any element with oxygen to produce some oxide.
Reduction
Reduction refers to the removal of oxygen from an oxide, leaving the pure element, with a significant reduction in mass and/or volume over the original material.
Oxidation-Reduction (REDOX)
Oxidation-reduction reactions occur due to the transfer of electrons from one species that loses electrons (reducing) to one that accepts electrons (oxidant).
Oxidizing Agents
Oxidizing agents are substances that oxidize other substances and, therefore, gain electrons.
Reducing Agents
Reducing agents are substances that reduce other substances, and therefore lose electrons to be oxidized.
Reducers
Reducers are the species that lose (donate) electrons, their oxidation number increases, and they undergo a process of oxidation.
Oxidants
Oxidants are the species that accept electrons, their oxidation number decreases, and they undergo a process of reduction.
Le Chatelier’s Principle
Le Chatelier’s principle allows us to qualitatively predict the response of a system to changing external conditions such as pressure, temperature, and concentration. It states that if an equilibrium system is subjected to a disturbance that modifies any of the factors that determine the steady state, the system will adjust as far as possible to annul the effect of that change.