Gravimetry (chemistry)

The gravimetry is a quantitative analysis method in which the measurement of material amounts to the determination of the mass ( final weight ) is based. A distinction is made between precipitation analysis , electrogravimetry and thermogravimetry .

Procedure

Precipitation Analysis

Here ions or molecules are brought into a form of precipitation . The precipitated compound is filtered off. The filtration can either take place in a porcelain filter crucible or glass filter crucible or in a filter paper (special ashless filter papers are used). The filter cake is then washed and dried. If a paper filter was used for filtering, this has to be incinerated . In some cases, for the precipitation analysis, the precipitation form is converted into a stoichiometric weighing form by annealing in specially provided ovens ( muffle ovens ) , with which the quantitative determination of the ingredients can take place.

Sometimes the form of precipitation and weighing are identical. This is particularly the case when the precipitate has a clear stoichiometry and, for example, no changing amounts of water of crystallization are bound: when determining sulfate ions as barium sulfate, nickel with diacetyldioxime or potassium with sodium tetraphenylborate. An example in which the precipitation form and the weighing form are not identical is the determination of iron as iron (III) oxide listed below.

Electrogravimetry

In electrogravimetry, the substance you are looking for is deposited on an electrode and then weighed.

Thermogravimetry

In thermogravimetric analysis methods , the change in the mass of the substance is examined as a function of the temperature. A simple example of this are the gravimetric methods for determining moisture.

Examples

Determination of the iron content of an Fe (III) salt solution

The iron salt solution is mixed with ammonia water, the precipitated hydroxide (precipitated form) is filtered off and then converted into iron (III) oxide by annealing to constant weight. The mass of the oxide is determined by weighing it on the analytical balance .

{\ displaystyle \ mathrm {NH_ {3} + H_ {2} O \ rightleftharpoons OH ^ {-} + NH_ {4} ^ {+}}}

(Basic reaction of ammonia)

{\ displaystyle \ mathrm {Fe ^ {3 +} + \ 3 \ OH ^ {-} + \ x \ H_ {2} O \ \ longrightarrow \ Fe (OH) _ {3} \ cdot x \ H_ {2} O \ downarrow}}

(Precipitation form)

{\ displaystyle \ mathrm {2Fe (OH) _ {3} \ cdot x \ H_ {2} O \ {\ xrightarrow [{800 ^ {o} C}] {\ Delta}} \ Fe_ {2} O_ {3 } \ + \ (x + 3) \ H_ {2} O \ uparrow}}

(Weighing form)

The desired mass a of the element to be determined (in our example iron) is proportional to the balanced mass A of the weighing form (here ). The proportionality factor ( gravimetric factor ) indicates the proportion of the mass a in the balanced mass A. ${\ displaystyle {\ rm {Fe_ {2} O_ {3}}}}$ ${\ displaystyle \ lambda}$

From the balanced mass , multiplication by a factor results in the mass a of the element to be determined . ${\ displaystyle {\ rm {Fe_ {2} O_ {3}}}}$ ${\ displaystyle \ lambda}$

{\ displaystyle \ lambda = {a \ over A}}

or .

{\ displaystyle A \ cdot \ lambda = a}

${\ displaystyle \ lambda}$ contains on the one hand the ratio of the molecular weights of the element to be determined, , to the molecular weight of the Wägeform, . In addition, however, you must also take into account “how often” a formula unit of the substance you are looking for is ultimately found per formula unit in the weighing form. In this example there are 2 iron atoms per formula unit of the weighing product. This is expressed by the stoichiometric coefficient k, here k = 2: ${\ displaystyle M_ {s}}$ ${\ displaystyle M_ {w}}$ ${\ displaystyle \ lambda}$

{\ displaystyle \ lambda = {{k \ cdot M_ {s}} \ over {M_ {w}}}}

where k is the stoichiometric coefficient , the molecular mass of the substance sought and the molecular mass of the weighing form. ${\ displaystyle M_ {s}}$ ${\ displaystyle M_ {w}}$

In our example, let's assume we have weighed 1.25 g of Fe ₂ O ₃ . How much iron did our Fe (III) salt solution originally contain?

Insertion of

A = m ( ) = 1.25 g

{\ displaystyle {\ rm {Fe_ {2} O_ {3}}}}

k = 2

{\ displaystyle M_ {s}}

= M (Fe) = 55.845 g / mol

{\ displaystyle M_ {w}}

= M ( ) = 159.69 g / mol

{\ displaystyle {\ rm {Fe_ {2} O_ {3}}}}

results in:

a = m (Fe) = 0.874 g.

This means that our Fe (III) salt solution contained 874 mg of iron.

Determination of sulfate

The sulphate-containing sample solution is acidified with hydrochloric acid. A 0.1 M barium chloride solution is added dropwise with stirring until no more precipitate forms at the dropping point. The precipitate is tempered (for eastern forest ripening ) on a sand bath overnight. At the expense of the small crystallites, larger crystals form, which can be more easily filtered off. The precipitate is filtered off, washed with water and ethanol and dried. The precipitate is then calcined to constant weight at 600 ° C. As a rule, this does not take longer than 2-3 hours.

{\ displaystyle \ mathrm {SO_ {4} ^ {2 -} + \ Ba ^ {2 +} \ longrightarrow \ BaSO_ {4} \ downarrow}}

(Precipitation and weighing form)

Determination of nickel (II)

An alcoholic solution of dimethylglyoxime is added dropwise to the aqueous nickel (II) -containing sample solution until no further reddish precipitate separates out. Then the ethanol is boiled off. Then it is filtered off through a glass frit. After washing the precipitate, it is dried in a drying cabinet until the mass is constant. Reaction gas equation, whereby dimethylglyoxime is abbreviated with H [DMG]:

{\ displaystyle \ mathrm {Ni ^ {2 +} + \ 2 \ H [DMG] \ rightarrow \ Ni [DMG] _ {2} \ downarrow + \ 2 \ H ^ {+}}}

Gravimetric methods for humidity measurement

The most frequently used method for measuring the water content of material samples is the gravimetric method (also called Darr-Weigh-Drying). The water content of the material sample is determined by the weight loss during drying .

After removal, the material sample is packed airtight and weighed. The sample is then dried in a drying oven at approx. 105 ° C. until a constant weight is achieved with successive weighings. The drying time and temperature depends on the material and is specified in the relevant standards. No chemically bound water may be released during drying. That is why the drying temperature of gypsum is only 40 ° C. After drying, the material sample is weighed again. The water content of the material sample can be determined from the difference between the weighings.

The gravimetric water content results from the mass of the wet sample and the mass of the dry sample : ${\ displaystyle W_ {g}}$ ${\ displaystyle m_ {f}}$ ${\ displaystyle m_ {t}}$

${\ displaystyle W_ {g} = {\ frac {m_ {f} -m_ {t}} {m_ {t}}}}$

advantages

The procedure for taking the sample and the drying procedure is recognized as a reference method. Many other methods are compared with this method or the indirect methods are calibrated . Advantages of this method are the relatively easy handling and the generally good accuracy.

disadvantage

A disadvantage of the gravimetric process is that chemical conversions through oxidation processes can occur when organic materials are dried . Due to the oxygen withdrawal, these can influence the subsequent weighing to higher values. Thermal decomposition with a resulting weight loss can also occur. In soils containing colloids such as loam or clay, the complete removal of the stored water is only possible by destroying the colloid structure.

view

The gravimetric method is not recommended for long-term field measurements. However, it is established as a reference method for laboratory measurements. The long drying times can be achieved by means of quick drying

be abbreviated.

Other direct measurement methods for determining the water content in soils have not caught on. This is partly due to the cumbersome handling, the required effort or the poor accuracy.

swell

Kupfer, K. 1997 Material moisture measurement: Basics, measuring methods, applications, standards, Expert Verlag
Hübner, C. 1999 “Development of high-frequency measuring methods for determining soil and snow moisture”, Scientific Reports, FZKA 6329, Research Center Karlsruhe
Völkner, S. 2003 "On the influence of spatially limited discontinuities on the time-dependent moisture distribution in external walls", dissertation, Ruhr University Bochum
Technical University of Braunschweig, Collaborative Research Center 477, "Ensuring the usability of buildings with the help of innovative building monitoring", 2000
Johannes Weber, Wolfram Köhler: Non-destructive investigation methods on stone monuments
Kaatze, U. 2007 “Aspects of electromagnetic aquametry of ionically conductive materials” in Technisches Messen, issue 5/2007, 261–267