The gravimetry it is one of the main branches of analytical chemistry comprised of a series of techniques whose common cornerstone is mass measurement. Masses can be measured in countless ways: directly or indirectly. To achieve such essential measurements the scales; gravimetry is synonymous with mass and scales.
Regardless of the route or procedure selected to obtain the masses, the signals or results must always shed light on the concentration of the analyte or species of interest; otherwise, gravimetry would have no analytical value. This would be equivalent to affirming that a team worked without a detector and was still reliable..
The image above shows an old scale with some apples on its concave plate.
If the mass of the apples were determined with this scale, we would have a total value proportional to the number of apples. Now, if they were weighed individually, each mass value would correspond to the total particles of each apple; its protein, lipid, sugar, water, ash, etc..
At the moment there are no hints of a gravimetric approach. But suppose that the balance could be extremely specific and selective, neglecting the other constituents of the apple while weighing only the one of interest..
Adjusting this idealized scale, weighing the apple could directly determine how much of its mass corresponds to a specific type of protein or fat; how much water it stores, how much all its carbon atoms weigh, etc. In this way it would be determining gravimetrically the nutritional composition of the apple.
Unfortunately there is no scale (at least today) that can do this. However, there are specific techniques that allow the components of the apple to be physically or chemically separated; and then, and finally, weigh them separately and build the composition.
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Described the example of apples, when the concentration of an analyte is determined by measuring a mass we speak of a gravimetric analysis. This analysis is quantitative, since it answers the question 'how much is there?' concerning the analyte; but he does not answer it by measuring volumes or radiation or heat, but masses.
In real life the samples are not just apples but practically any type of matter: gas, liquid or solid. However, whatever the physical state of these samples, it must be possible to extract from them a mass or difference thereof that can be measured; which will be directly proportional to the concentration of the analyte.
When it is said to "extract a mass" from a sample, it means to obtain a precipitate, which consists of a compound that contains the analyte, that is, itself.
Returning to apples, to measure their components and molecules gravimetrically, it is necessary to obtain a precipitate for each of them; one precipitate for water, another for proteins, etc..
Once all are weighed (after a series of analytical and experimental techniques), the same result will be reached as that of the idealized balance..
In gravimetric analysis there are two main ways to determine analyte concentration: directly or indirectly. This classification is global, and from them derive methods and endless specific techniques for each analyte in certain samples..
Direct gravimetric analysis is one in which the analyte is quantified by simple measurement of a mass. For example, if you weigh a precipitate of a compound AB, and knowing the atomic masses of A and B, and the molecular mass of AB, you can calculate the mass of A or B separately..
All analyzes that produce precipitates from whose masses the mass of the analyte is calculated is direct gravimetry. The separation of apple components into different precipitates is another example of this type of analysis..
In indirect gravimetric analyzes, mass differences are determined. Here a subtraction is performed, which quantifies the analyte.
For example, if the apple on the scale is weighed first, and then heated to dryness (but not burned), all the water will vaporize; that is, the apple will lose all its moisture content. The dried apple is weighed again, and the difference in masses will be equal to the mass of water; therefore, the water has been quantified gravimetrically.
If the analysis were straightforward, a hypothetical method would have to be devised by which all the water could be subtracted from the apple and crystallized on a separate scale for weighing. Obviously, the indirect method is the easiest and most practical.
It may seem simple at first to obtain a precipitate, but it really implies certain conditions, processes, use of masking agents and precipitating agents, etc., to be able to separate it from the sample and that it is in perfect condition to be weighed..
The precipitate must meet a series of characteristics. Some of these are:
If it were not pure enough, the masses of the impurities would be assumed as part of the masses of the analyte. Therefore, the precipitates must be purified, either by washing, recrystallization, or by any other technique..
Suppose the precipitate can undergo the following decomposition:
MCO3(s) => MO (s) + COtwo(g)
It happens that it is not known until how much of MCO3 (metallic carbonates) has decomposed into its respective oxide. Therefore, the composition of the precipitate is not known, since it could be an OLS mixture.3MO, or MCO33MO, etc. To solve this, it is necessary to guarantee the complete decomposition of the OLS3 to MO, weighing MO only.
If the precipitate is decomposed by ultraviolet light, heat, or by contact with air, its composition is no longer known; and you are again facing the previous situation.
The higher the molecular mass of the precipitate, the easier it will be to weigh, since smaller quantities will be needed to record a balance reading..
The precipitate must be insoluble enough to be filtered without major complications..
Although not strictly necessary, the precipitate should be as crystalline as possible; that is, the size of its particles must be as large as possible. The smaller its particles, the more gelatinous and colloidal it becomes, and therefore requires more treatment: drying (removing solvent) and calcination (making its mass constant).
Within gravimetry there are four general methods, which are mentioned below.
Already mentioned throughout the subsections, they consist of quantitatively precipitating the analyte in order to determine it. The sample is physically and chemically treated so that the precipitate is as pure and suitable as possible..
In this method, the precipitate is deposited on the surface of an electrode through which an electric current is passed inside an electrochemical cell..
This method is widely used in the determination of metals, since they are deposited, their salts or oxides and, indirectly, their masses are calculated. The electrodes are first weighed before coming into contact with the solution in which the sample has dissolved; then, it is reweighed once the metal is deposited on its surface.
In gravimetric volatilization methods, the masses of gases are determined. These gases originate from a decomposition or chemical reaction that the sample undergoes, which are directly related to the analyte..
As it is gases, it is necessary to use a trap to collect it. The trap, like the electrodes, is weighed before and after, thus indirectly calculating the mass of gases collected..
This gravimetric method is essentially physical: it is based on mixture separation techniques.
Through the use of filters, sieves or sieves, the solids of a liquid phase are collected, and they are weighed directly to determine their solid composition; for example, the percentage of clay, fecal waste, plastics, sand, insects, etc., in a stream of water.
This method consists, unlike the others, in characterizing the thermal stability of a solid or material through its mass variations as a function of temperature. A hot sample can practically be weighed with a thermobalance, and its mass loss is recorded as the temperature increases..
In general terms, some uses of gravimetry are presented, regardless of the method and analysis:
-Separates different components, soluble and insoluble, from a sample.
-Perform a quantitative analysis at a shorter time when it is not required to construct a calibration curve; the mass is determined and it is known at once how much of the analyte is in the sample.
-It not only separates the analyte, but also purifies it.
-Determine the percentage of ash and solids moisture. Likewise, with a gravimetric analysis its degree of purity can be quantified (as long as the mass of the polluting substances is not less than 1 mg).
-It allows to characterize a solid by means of a thermogram.
-The handling of solids and precipitates is usually simpler than that of volumes, thus facilitating certain quantitative analyzes.
-In teaching laboratories, it is used to evaluate the performance of students in calcination techniques, weighing, and in the use of crucibles..
A sample dissolved in aqueous medium can be determined for its phosphites, PO33-, by the following reaction:
2HgCltwo(ac) + PO33-(aq) + 3HtwoO (l) ⇌ HgtwoCltwo(s) + 2H3OR+(aq) + 2Cl-(ac) + 2PO43-(ac)
Note that HgtwoCltwo precipitates. If Hg is weighedtwoCltwo and its moles are calculated, it can be calculated following the stoichiometry of the reaction how much PO33- had originally. An excess of HgCl is added to the aqueous solution of the sample.two to ensure that all PO33- react to form the precipitate.
If a mineral containing lead is digested in an acid medium, for example, the Pb ionstwo+ can deposit as PbOtwo on a platinum electrode using an electrogravimetric technique. The reaction is:
Pbtwo+(aq) + 4HtwoO (l) ⇌ PbOtwo(s) + Htwo(g) + 2H3OR+(ac)
The platinum electrode is weighed before and after, and thus the mass of PbO is determined.two, of which with a gravimetric factor, the mass of lead is calculated.
Calcium in a sample can be precipitated by adding oxalic acid and ammonia to its aqueous solution. In this way the oxalate anion is generated slowly and produces a better precipitate. The reactions are:
2NH3(ac) + HtwoCtwoOR4(ac) → 2NH4+(ac) + CtwoOR4two-(ac)
ACtwo+(ac) + CtwoOR4two-(ac) → CaCtwoOR4(s)
But calcium oxalate is calcined to produce calcium oxide, a precipitate with a more defined composition:
CaCtwoOR4(s) → CaO (s) + CO (g) + COtwo(g)
And finally, the nickel concentration of a sample can be determined gravimetrically by using dimethylglyoxime (DMG): an organic precipitating agent, with which it forms a chelate that precipitates and has a characteristic reddish color. The DMG is generated on-site:
CH3COCOCH3(aq) + 2NHtwoOH (aq) → DMG (aq) + 2HtwoO (l)
2DMG (ac) + Nitwo+(ac) → Ni (DMG)two(s) + 2H+
El Ni (DMG)two It is weighed and with a stoichiometric calculation it is determined how much nickel the sample contained.
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