Gravimetric analysis

Analytical balance

Gravimetric analysis ( gravimetry , weight analysis ) is a method of quantitative chemical analysis based on accurate measurement of the mass of a substance. Uses the law of conservation of mass of substances during chemical transformations. He played a large role in establishing the law of constancy of the composition of chemical compounds , the law of multiple ratios , the periodic law , etc. It is used to determine the chemical composition of various objects ( rocks and minerals ), the quality of raw materials and finished products, the content of crystallization water in salts, the ash content of fuel, and so on. Further.

The advantages of gravimetric analysis include high accuracy (usually the error is 0.1-0.2%) and the absence of the need for preliminary calibration of measuring instruments. On the other hand, its implementation is often more time-consuming and takes longer than other methods ^[1] .

Content

1 Analysis Methods
- 1.1 Precipitation methods
- 1.2 Stripping Methods
- 1.3 Isolation Methods
2 Deposition analysis
- 2.1 Sample preparation, sampling and dissolution
- 2.2 Precipitation
- 2.3 Filtering and washing the precipitate
- 2.4 Drying and calcination
- 2.5 Weighing and calculation of analysis results
3 Solubility of precipitation and factors influencing it
- 3.1 Excessive ions of the same name
- 3.2 Effect of pH (hydrolysis)
- 3.3 Complexation
- 3.4 Temperature
- 3.5 Salt effect
4 See also
5 notes
6 Literature

Analysis Methods

Precipitation Methods

Precipitation methods are the most common methods of gravimetric analysis ^[2] ^[3] . A sample of the analyte is dissolved in water or another solvent, and the element being determined is precipitated with a reagent as a sparingly soluble compound. The precipitate obtained is filtered off, washed, dried, calcined and weighed. By mass of the precipitate after calcination, the mass fraction of the element to be determined in the sample is calculated.

Since the precipitated substance may not correspond to what is obtained after calcination, the precipitated and gravimetric (weight) forms of the precipitate are distinguished. The precipitated form is a compound precipitated from a solution when interacting with the corresponding reagent during the analysis ^[4] , and the gravimetric form is formed from the precipitated during drying or calcination ^[5] .

For example:

{\ce {CaCl2 + (NH4)2C2O4 -> CaC2O4 v + 2 NH4Cl}}

{\ displaystyle {\ ce {CaCl2 + (NH4) 2C2O4 -> CaC2O4 v + 2 NH4Cl}}}

{\ce {CaCl2 + (NH4)2C2O4 -> CaC2O4 v + 2 NH4Cl}}

{\ce {CaC2O4 ->[t] CaO + CO2 ^ + CO ^}}

{\ displaystyle {\ ce {CaC2O4 -> [t] CaO + CO2 ^ + CO ^}}}

{\ce {CaC2O4 ->[t] CaO + CO2 ^ + CO ^}}

In this case, CaC ₂ O ₄ is a precipitated form, CaO is a gravimetric form.

Inorganic and organic reagents are used as precipitants. For example, sulfur in the form of sulfate ions is precipitated with barium ions ^[6] , iron is precipitated with an ammonia solution ^[7] , and 8-hydroxyquinoline is often used to precipitate aluminum ^[8] .

Stripping Methods

In these methods, the component to be determined is isolated as a volatile compound by the action of an acid or high temperature. Stripping methods are divided into direct and indirect:

direct methods: the determined component is isolated in the form of a volatile compound and absorbed by the absorber (or condensed). The calculation is based on the change in mass of the absorber.
indirect methods: the substance is weighed, the volatile compound is distilled off and again weighed. The calculation is made to reduce the mass of the sample ^[2] .

Selection Methods

There are also methods based on the separation of the analyte from the analyte and its accurate weighing, for example, the recovery of copper ions to metal with subsequent weighing ^[9] .

Precipitation Analysis

Sample preparation, sample collection and dissolution

First, an average sample is taken, the composition of which would reflect the composition of the studied material. The average sample should be composed of as many samples as possible taken from random points in the material. For further preparation, the quarting method is used : the samples are thoroughly crushed, mixed, then laid out in the form of a square, divided by diagonals, after which 2 opposite triangles are discarded, and the others are mixed again, and so on, until about 25 g of substance remains ^[10] .

After receiving the average sample, a sample is taken. As a rule, the larger the sample, the higher the accuracy of determination, however, the resulting large precipitate is difficult to filter, rinse and calcine. It has been experimentally established that crystalline precipitates weighing about 0.5 g and voluminous amorphous precipitates weighing 0.1-0.3 g are most convenient to use. The required amount of sample is selected based on the norms of precipitation and knowing the relative content of the element being determined in the substance ^[11] .

Weighed substance on the watch glass

The technique of taking a sample may be different:

The watch glass or bottle is precisely weighed, approximately the required amount of substance is placed there and weighed again. The mass of the substance is calculated by the difference in the masses of the watch glass or box with and without a sample.
The substance is placed on a watch glass or in a box in an amount sufficient to take several weights, and weighed. Then a portion of the substance is poured into a glass, the watch glass or bottle with the rest of the substance is weighed again and the mass of the sample is found by the difference in the results of two weighings. This method is more convenient in cases where it is necessary to take several samples of the analyte ^[12] .

The thus obtained sample must be dissolved. As a rule, mineral acids are used for dissolution. Sometimes solutions of alkalis or mixtures of acids (for example, aqua regia ) are used. In some cases, fusion with various meshes, often carbonates and nitrates of alkali metals, is used to transfer the analyte to a soluble compound ^[13] ^[14] .

Before analysis, it is necessary to prepare a solution. This implies creating the required pH , masking interfering ions by precipitation or binding into strong complexes , and, if necessary, evaporation of the solution ^[15] .

Precipitation

The next step in the analysis is the deposition of the element being determined. The following requirements are imposed on the precipitated form ^[16] :

Low solubility: the weight loss of the precipitate should not exceed the error of the analytical balance (0.1 mg).
The precipitate must be clean, that is, as little as possible sorb foreign ions . For this reason, crystalline precipitation is more preferable than amorphous precipitation.
The precipitated form should easily transform into gravimetric.

The deposition process differs depending on the nature of the precipitate. Coarse-grained precipitates are obtained cleaner, therefore, to reduce the rate of formation of crystallization centers ^[17], they are obtained by precipitation from hot diluted solutions, slowly (dropwise) adding a precipitator solution with stirring with a glass rod ^[18] . After precipitation, crystalline precipitates are kept for some time (from 30 minutes to 20 hours) under the mother liquor ^[19] . Amorphous precipitates precipitate from concentrated hot solutions. After precipitation, about 100 ml of hot water is added to them, mixed and filtered immediately, not standing under the mother liquor ^[20] .

Filtration

Filtration and washing of sediment

Ashless filters are used for filtering, that is, those whose ash mass can be neglected. Coarse precipitates are washed directly on the filter, fine crystalline and amorphous precipitates are washed by decantation . During decantation, the solution is drained through a filter, trying to leave a precipitate in a glass. Then a new portion of the washing liquid is added to the precipitate, mixed, allowed to settle again and repeat this operation several times. After several cycles, the precipitate is quantitatively transferred to the filter. Distilled water can be used as a washing liquid, however, more often a diluted precipitant solution is used to reduce sludge losses during filtration ^[21] .

Drying and calcination

Calcination is carried out in crucibles.

After filtering and washing, the precipitate is transferred into a gravimetric form, which has the following requirements:

exact compliance with the chemical formula;
stability in air (for example, the gravimetric form should not absorb moisture or CO ₂ );
a sufficiently large molecular weight and a small mass fraction of the element being determined (to reduce the error).

To obtain a gravimetric form, the precipitate is dried and / or calcined, depending on the substance. As a rule, precipitates formed by an organic precipitant are only dried, and inorganic precipitates are calcined. Some substances undergo chemical changes during calcination (for example, iron (III) hydroxide transforms to oxide ), while others (such as barium sulfate BaSO ₄ ) do not ^[22] .

{\ce {2 Fe(OH)3 ->[t] Fe2O3 + 3 H2O}}

{\ displaystyle {\ ce {2 Fe (OH) 3 -> [t] Fe2O3 + 3 H2O}}}

Drying is carried out in an oven at a temperature of 90-105 ° C or in air, if high speed is not required. After drying, the filter is ashed , that is, heated in the presence of oxygen , first carbonizing it, and then oxidizing (oxidation products - CO and CO ₂ - gases), but without open burning. Some substances are reduced by coal, so they work differently. For example, silver chloride is removed from the filter, the filter is ashed, after which the reduced silver in the crucible is oxidized with aqua regia again to silver chloride, and the excess acid is evaporated, after which the bulk of the precipitate is transferred back to the crucible.

Calcination is carried out in muffle (or crucible) furnaces, in crucibles brought to constant weight. After calcination, the crucibles are placed in a desiccator to prevent the absorption of moisture from the air ^[23] .

Weighing and calculating analysis results

Weighing is carried out on an analytical balance. As a rule, the weighing error is 0.1 mg. From the known mass of the gravimetric form, it is not difficult to calculate the mass of the ion being determined. For example, when determining barium , when the gravimetric form is barium sulfate BaSO ₄ , the mass of barium can be calculated by the formula

m({\text{Ba}})=m({\text{BaSO}}_{4})\cdot {\frac {M({\text{Ba}})}{M({\text{BaSO}}_{4})}}

{\ displaystyle m ({\ text {Ba}}) = m ({\ text {BaSO}} _ {4}) \ cdot {\ frac {M ({\ text {Ba}})} {M ({\ text {BaSO}} _ {4})}}}

The ratio of the molar mass of the analyte (element) to the molar mass of the gravimetric form, taking into account stoichiometric coefficients , is called the conversion factor and is denoted by the letter $F$ ${\ displaystyle F}$ ^[24] .

The mass fraction of the determined ion in the sample (and, accordingly, in the sample) can be calculated by the formula:

w_{\text{B}}={\frac {m_{\text{B}}}{m_{\text{навески}}}}

{\ displaystyle w _ {\ text {B}} = {\ frac {m _ {\ text {B}}} {m _ {\ text {hitch}}}}}

Precipitation solubility and factors affecting it

A constant value characterizing the solubility of a substance is its product of solubility . However, the solubility of the same substance under different conditions, i.e., the concentration of a saturated solution, may differ. In order to achieve complete deposition, but keep the relative supersaturation low ^[25] for the growth of large crystals, it is necessary to take into account factors affecting the solubility of precipitation.

Excessive ions of the same name

For a more complete deposition of the element being determined, an excess of precipitating ions is used. For example, in the deposition of lead as PbSO _4, an excess of sulfate ions is used to precipitate. The solubility product of lead sulfate is constant:

$K_{s}({\text{PbSO4}})=[{\text{Pb}}^{2+}]\cdot [{\text{SO}}_{4}^{2-}]=1{,}6\cdot 10^{-8}{\frac {{\text{mol}}^{2}}{{\text{L}}^{2}}}$ ${\ displaystyle K_ {s} ({\ text {PbSO4}}) = [{\ text {Pb}} ^ {2 +}] \ cdot [{\ text {SO}} _ {4} ^ {2-} ] = 1 {,} 6 \ cdot 10 ^ {- 8} {\ frac {{\ text {mol}} ^ {2}} {{\ text {L}} ^ {2}}}}$

It can be seen from the equation that with an increase in the concentration of sulfate ions, the concentration of lead ions will decrease.

During precipitation, as a rule, do not use too much excess of precipitant, since in some cases, with its increase, an increase in the solubility of the precipitate is possible due to other effects, such as complexation ^[26] .

Effect of pH (hydrolysis)

Precipitation is often a salt of weak acids that can undergo hydrolysis . Consider this effect on the example of calcium oxalate :

{\ce {CaC2O4*H2O <=> Ca^2+ + C2O4^2- + H2O}}

{\ displaystyle {\ ce {CaC2O4 * H2O <=> Ca ^ 2 + + C2O4 ^ 2- + H2O}}}

{\ce {C2O4^2- + H+ <=> HC2O4^-}}

{\ displaystyle {\ ce {C2O4 ^ 2- + H + <=> HC2O4 ^ -}}}

{\ce {HC2O4^- + H+ <=> H2C2O4}}

{\ displaystyle {\ ce {HC2O4 ^ - + H + <=> H2C2O4}}}

In this case, upon acidification of the solution, the concentration of C ₂ O ₄ ^2– ions will decrease, therefore, the solubility of the precipitate will increase ^[27] .

It should be noted that lowering the pH can promote the protonation of many ligands , the destruction of the complexes formed by them, and lead to the precipitation of masked ions. For example, when acidifying a solution containing chloride ions and silver ions disguised with ammonia, silver chloride will precipitate ^[28] :

{\ce {[Ag(NH3)2]+ + 2H+ <=> Ag+ + 2 NH4+}}

{\ displaystyle {\ ce {[Ag (NH3) 2] + + 2H + <=> Ag + + 2 NH4 +}}}

{\ce {Ag+ + Cl- -> AgCl v}}

{\ displaystyle {\ ce {Ag + + Cl- -> AgCl v}}}

Complexation

Complexation has a significant effect on solubility. Cations of many metals are able to form complex compounds, as a result of which their concentration in free form can be significantly reduced. Sometimes this effect is used in order to mask ions that interfere with the analysis, then this is called masking . In other cases, the solubility of the precipitate is undesirable, and extraneous complexing agents, if present, must be removed (for example, organic acids are oxidized to CO ₂ and water).

As ligands can act as external ions or molecules, and ions that make up the precipitate. As a rule, in such cases, with a slight excess of them, the solubility decreases and then increases, therefore, a 50% excess precipitant is often used for precipitation. However, this does not always happen: for example, even a small excess of precipitant increases the solubility of HgI ₂ ^[29] .

Temperature

The temperature dependence of the solubility product is quantitatively described by the formula

{\operatorname {d} \ln \!K_{s} \over \operatorname {d} \!T}={\frac {\Delta H}{RT^{2}}}

{\ displaystyle {\ operatorname {d} \ ln \! K_ {s} \ over \ operatorname {d} \! T} = {\ frac {\ Delta H} {RT ^ {2}}}}

,

Where:

$K_{s}$ ${\ displaystyle K_ {s}}$ - solubility product ,
$T$ ${\ displaystyle T}$ - temperature,
$\Delta H$ ${\ displaystyle \ Delta H}$ - change in enthalpy (thermal effect) of the dissolution reaction,
$R$ ${\ displaystyle R}$ - universal gas constant .

To dissolve most sparingly soluble compounds, it is necessary to expend energy ( $\Delta H>0$ ${\ displaystyle \ Delta H> 0}$ ), therefore, their solubility increases with temperature ^[30] .

Sometimes the composition of the precipitate also depends on temperature. For example, a precipitate of calcium sulfate at temperatures up to 60 ° C has the composition ${\ce {CaSO4*2 H2O}}$ ${\ displaystyle {\ ce {CaSO4 * 2 H2O}}}$ but at a higher temperature it goes into ${\ce {CaSO4 * 1/2 H2O}}$ ${\ displaystyle {\ ce {CaSO4 * 1/2 H2O}}}$ . Since the dissolution of the latter is exothermic, the solubility of the precipitate reaches a maximum at 60 ° C ^[31] .

Salt Effect

In the calculations, the product of solubility , i.e., the product of the concentrations of the ions that make up the substance, in appropriate degrees, is often considered constant. In fact, it is more accurate to consider as constant the product of not molar concentrations, but activities , i.e., concentrations multiplied by activity coefficients. Activity coefficients depend on the ionic strength of the solution , which is affected by the presence of extraneous electrolytes. For example, the solubility of PbSO ₄ in a 0.1 M KNO ₃ solution is about 3 times higher than in pure water ^[32] .

Notes

↑ Vasiliev, 2004 , p. 309-310.
↑ ¹ ² Alekseev, 2013 , p. 65.
↑ Vasiliev, 2004 , p. 281.
↑ Alekseev, 2013 , p. 66.
↑ Vasiliev, 2004 , p. 290
↑ Alekseev, 2013 , p. 165.
↑ Alekseev, 2013 , p. 172.
↑ Alekseev, 2013 , p. 174.
↑ K. I. Yakovlev, G. M. Alekseeva. Gravimetric (weight) analysis. Guidelines for studying the course of quantitative chemical analysis. (unspecified) . SPCPA (2005). Date of treatment January 10, 2019.
↑ Alekseev, 2013 , p. 43.
↑ Alekseev, 2013 , p. 134-135.
↑ Alekseev, 2013 , p. 136.
↑ Vasiliev, 2004 , p. 19-22.
↑ Alekseev, 2013 , p. 136-138.
↑ Алексеев, 2013 , с. 139.
↑ Васильев, 2004 , с. 282.
↑ Васильев, 2004 , с. 284.
↑ Алексеев, 2013 , с. 102.
↑ Васильев, 2004 , с. 288.
↑ Алексеев, 2013 , с. 106-107.
↑ Васильев, 2004 , с. 288—289.
↑ Васильев, 2004 , с. 290—291.
↑ Алексеев, 2013 , с. 150—153.
↑ Васильев, 2004 , с. 291—292.
↑ относительным пересыщением называется величина ${\frac {Q-S}{S}}$ ${\frac {QS}{S}}$ , где Q — концентрация осаждаемой формы, S — её растворимость
↑ Алексеев, 2013 , с. 73-75.
↑ Васильев, 2004 , с. 261.
↑ Алексеев, 2013 , с. 96—97.
↑ Алексеев, 2013 , с. 94—98.
↑ Vasiliev, 2004 , p. 266.
↑ Alekseev, 2013 , p. 83.
↑ Alekseev, 2013 , p. 76-80.

Literature

Vasiliev V.P. Analytical Chemistry: In 2 books: Prince. 1: Titrimetric and gravimetric methods of analysis: Textbook. for stud. Universities studying chemical engineering. specialist. - 4th ed. - M .: Bustard, 2004 .-- 368 p. - ISBN 5-7107-8745-0 . - ISBN 5-7107-8744-2 .
Alekseev V.N. Quantitative analysis. Edited by Dr. Chem. Sciences P.K. Agasyan. - Edition 5th. - M .: Alliance, 2013 .-- 504 p. - ISBN 978-5-903034-30-7 .

[_c4cc12239f99c546-1] Vasiliev, 2004 , p. 309-310.

[_ec155e246567f040-2] ¹ ² Alekseev, 2013 , p. 65.

[_6fe44e108d253589-3] Vasiliev, 2004 , p. 281.

[_ec155e246567f043-4] Alekseev, 2013 , p. 66.

[_6fe44f108d25375b-5] Vasiliev, 2004 , p. 290

[_9027b8d84f898627-6] Alekseev, 2013 , p. 165.

[_9027b9d84f8987f7-7] Alekseev, 2013 , p. 172.

[_9027b9d84f8987f1-8] Alekseev, 2013 , p. 174.

[9] K. I. Yakovlev, G. M. Alekseeva. Gravimetric (weight) analysis. Guidelines for studying the course of quantitative chemical analysis. (unspecified) . SPCPA (2005). Date of treatment January 10, 2019.

[_ec155c246567ed3c-10] Alekseev, 2013 , p. 43.

[_7732a5d913324132-11] Alekseev, 2013 , p. 134-135.

[_9027b5d84f8980cf-12] Alekseev, 2013 , p. 136.

[_038f8dc0cd5349a2-13] Vasiliev, 2004 , p. 19-22.

[_30b5075c36256ae9-14] Alekseev, 2013 , p. 136-138.

[_9027b5d84f8980c0-15] Алексеев, 2013 , с. 139.

[_6fe44e108d25358a-16] Васильев, 2004 , с. 282.

[_6fe44e108d25358c-17] Васильев, 2004 , с. 284.

[_9027b2d84f897bd2-18] Алексеев, 2013 , с. 102.

[_6fe44e108d253580-19] Васильев, 2004 , с. 288.

[_a0a4871b982e0af2-20] Алексеев, 2013 , с. 106-107.

[_34c799458144edb1-21] Васильев, 2004 , с. 288—289.

[_45ee9925ad10c69d-22] Васильев, 2004 , с. 290—291.

[_e752b643e16a6224-23] Алексеев, 2013 , с. 150—153.

[_a94dc560cd4bfe67-24] Васильев, 2004 , с. 291—292.

[25] относительным пересыщением называется величина ${\frac {Q-S}{S}}$ ${\frac {QS}{S}}$ , где Q — концентрация осаждаемой формы, S — её растворимость

[_963cd03ac40969e5-26] Алексеев, 2013 , с. 73-75.

[_6fe44c108d253263-27] Васильев, 2004 , с. 261.

[_800b0dea109422f0-28] Алексеев, 2013 , с. 96—97.

[_e2ea6df7175ec245-29] Алексеев, 2013 , с. 94—98.

[_6fe44c108d253264-30] Vasiliev, 2004 , p. 266.

[_ec1560246567f3e8-31] Alekseev, 2013 , p. 83.

[_e22df8e079d30272-32] Alekseev, 2013 , p. 76-80.