DD139463A1 - PROCESS FOR THE COULOMETRIC DETERMINATION OF UNSATURATED HYDROCARBONS - Google Patents

Abstract

The invention relates to a method for coulometric determination of unsaturated hydrocarbons in gaseous, solid and liquid media and aims at reducing the amount of work to be done per analysis by solving the problem of detecting and indicating the addition of unsaturated hydrocarbons by coulometric analysis in suitable solvents, have the corresponding proportions of sodium or potassium bromides, takes place. From the Coulomb values in the solutions, the presence of double bonds in the unsaturated hydrocarbons can be determined.

Description

Method for the coulometric determination of unsaturated hydrocarbons

Field of application of the invention

The invention relates to a process for the coulometric determination of unsaturated hydrocarbons with conjugated or isolated double bond arrangements in gaseous, solid and liquid media.

Characteristic of the known technical solutions

Significant analytical methods for the determination of hydrocarbons are known, which previously extended to the classic Bromzahl- or Dodzahlmethode and the determination of the conjugated hydrocarbons, which have a Diels-Alder reaction at the basis of the analytical methods.

In addition to the visual Bromzahlmethode, d, h., The iodometric back-titration of excess bromine, are mainly applied methods with electronic and coulometric end-point display, according to these known methods

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one monoolefinic double bonds. For the detection of conjugated diolefinic compounds, two main methods are used. A short, approximately 20-minute method for the determination of conjugated dienes, d. H. the reaction of dienes with tetracyanoethylene and back-titration of the latter with ethanolic cyclopentadiene solution and a more time-consuming method which consists in the reaction of the conjugated dienes with maleic anhydride in 16 hours and subsequent hydrolysis of unreacted anhydride and polarographic-cathodic reduction of maleic acid to succinic acid , Using this method, series of series analyzes are carried out, which are very time-consuming and involve a great deal of work.

The long reaction time, between the conjugated diene and the freshly sublimed maleic anhydride, of 16 hours at 100 ° C, does not permit faster performance of such analyzes. The subsequent hydrolysis of unreacted anhydride is also a time response and should be a shortening of the reaction time between anhydride and diene, this can be achieved by a known method by adding a catalyst, but detectable steric rearrangements occur so that existing ciscis forms in pass active cis-trans or trans-trans forms, or some isolated double-bond pairs, under the given conditions Diels-Alder syntheses and pretend a true diene number only pretend.

The known analytical methods are time-consuming and labor-intensive and thus offer only unfavorable conditions for rapid quantitative detection of conjugated dienes. The reaction of the dienes with tetracyanoethylene is not very time-consuming, but the disadvantage is that the

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necessary chemicals have to be brought before implementation with considerable labor and time, by additional purification methods, only in an underlying the reaction active form.

Also, the reaction of the dienes with maleic anhydride is a long-term reaction and there is also the need to convert the chemicals required for the Diels-Alder reaction prior to the reaction only with considerable effort into the active form.

It is not possible to achieve a rapid quantitative detection of the olefinic compounds with these methods since the time required for a single analysis is already very high and therefore a constant faster execution of investigations of the individual samples as well as in series analyzes is limited.

Object of the invention

The aim of the invention is to avoid the disadvantages described.

Explanation of the essence of the invention

- The technical problem which is solved by the invention.

The invention is based on the object to provide a method for the coulomeirischen determination of unsaturated hydrocarbons, which significantly reduces the labor and time required for this determination method, the purification of reaction components for the determination of unsaturated diolefin

nischer hydrocarbons largely avoids and provides meaningful data from the process of certain process processes and indicates the addition of unsaturated hydrocarbons.

Features of the invention.

The object is achieved in that the coulometric analysis for detecting olefinic or diolefinic components is used because of the high rate of addition of unsaturated double bonds to bromine.

The samples to be determined are analyzed in the micro range, the titrant bromine anodically generated in the solution and added at the moment of formation of the double bonds.

Since current intensity and time can be measured very accurately in the micro range, the sensitivity and reproducibility of this coulometric method depends only on the electrometric end point display.

It has now been found that in solvents of the following compositions and concentration, the olefinic hydrocarbons not only add the intermediate bromine bromine to the dibromo compound, but also form tetrabromic compounds in solvents when the requirement is established by varying bromide concentration.

In recognition of the favorable conditions, which provides a coulometric analysis, was in a solvent of the composition

800 ml of acetic acid (98 - 100 %)

100 ml of methanol

100 ml of distilled water

50 g sodium bromide

first determines the bromine number. In this solvent, the addition of the resulting bromine takes place only on a double bond pair, regardless of the number of double bonds present.

With a solvent of the composition:

12 g of mercury II chloride

8 g of potassium bromide

200 ml of water

400 ml of chloroform

400 ml of methanol

1080 ml of acetic acid (98 - .100 %)

the bromination of all double bonds to the tetrabromo compound takes place.

The bromination of the isolated double bonds to the tetrabromo compound is achieved when this solvent contains 14 g of potassium bromide. If there are no isolated double bonds in the sample, the bromination is restricted to a double bond pair and the normal bromine number determination value is obtained.

The endpoint indication is performed fully automatically by a control unit or the control unit of the coulometer.

It is performed by amperometry with two polarized platinum electrodes, d. h., The diffusion current at a given larger potential is measured during the titration.

The coulometric titration with amperometric end point display is in the mA ~ range, so that Parasit.ärströme be avoided and an unaltered Endpunktanzeige is guaranteed.

A necessarily constant amperometric endpoint display is achieved with stabilized platinum electrodes by electrolytically generating the intermediate reagent for a short time connected to the generator cathode. The titrations are carried out in the micro range, wherein the solvent used at the same time serves for the anodic generation of the intermediate reagent bromine by means of a generator probe. The generator cathode is immersed in a 10% sulfuric acid which is separated from the solvent by a G. frit.

Before the start of the actual titration, the final potential and also the polarization potential are determined for the coulometric bromine number determination, the endpoint potential being determined by a sample titration and the mV-VVert lying at the inflection point of the curve being applied as the potential value to the indicator electrode.

The polarization potential can then be taken from the current-voltage curve of the pre-electrolyzed intermediate reagent solution. To record this curve, in the solution which has a slight excess of the electrolytically

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contain titrant, the indicator potential varies, and the resulting current is measured. The potential to be selected for the titration lies in the flat part (inflection point) of the curve.

embodiment

The invention will be explained in more detail using an exemplary embodiment.

An olefin sample should be analyzed for the content of unsaturated hydrocarbons by weighing this sample into a small (50 ml) volumetric flask and make up to the mark with benzene (p.A.). A certain amount (in ml) is made up of a first solvent

800 ml of acetic acid (98 - 100 % ± g)

100 ml of methanol

100 ml of distilled water

50 g sodium bromide

metered and the indicator electrodes connected to the control unit, the generator electrodes to the potentiostat and to the integrator unit. The generator cathode is immersed in a vessel separated from the solvent by a G. frit and filled with 10% sulfuric acid. At the beginning of the titration, bromine is generated at the generator anode, which is immediately added by all olefinic components. With the coulomb obtained at the titration endpoint, the calculation of the bromine consumption or the calculation of the bromine number can be carried out according to the following example:

  - 8th -

0.082. i. t

Bromine number = -! -

i = current in mA t = time in sec w = weight of the sample in mg

Subsequently, an equally determined amount of the same sample is added to a second solvent having the following composition

1020 ml of acetic acid (98-100 % ± g) 400 ml of methanol 400 ml of chloroform 200 ml of distilled water 12 g of mercuric chloride 8 g of potassium bromide

metered.

The endpoint indication is exactly as described above.

If unsaturated conjugated double bond hydrocarbons are still present in this sample, then addition of the bromine to the tetrabromo compound takes place in this solvent.

From the additional consumption of Coulomb in the second solvent compared to the consumption of Coulomb in the first solvent is now the calculation of the content of conjugated dienes, d. that is, a bromine number corresponding to this content is obtained.

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The sample is additionally made in a third solution consisting of

1020 ml of acetic acid (98-100%) 400 ml of methanol 400 ml of chloroform 200 ml of distilled water

12 g of mercuric chloride

14 g potassium bromide

be brominated, if in addition to conjugated double bonds also isolated double bond pairs are present in the sample.

This potassium bromide concentration provides a bromine number value corresponding to the mono- and isolated bond pairs.

From the difference between this value and the bromine number of the first solution, the proportion of hydrocarbons with isolated pairs of bonds is obtained. '

As an example, a working sample according to standard method and coulometric method is to be investigated:

Bromine number (standard) 54.6 g Br./100 g

Diene number (standard) 27.2 g Br./100 g

Diene number (coulometry) 26.9 g Br./100 g

Bromine number (Coulometry) 53.4 g Br./100 g

Weighed : 84.25 mg / 1 ml of benzene

Coulomb value in the first solution: 1320 mcb = 51.5 Coulomb value in the third solution: 1370 mcb = 53.4

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The small (<C100 mcb) difference of the values of the first solution to the third solution indicates that no unsaturated hydrocarbons with isolated double bond pairs are present in the sample.

After finding the

2070 mcb Coulomb value in the second solution = ₂ 050 mcb

the difference takes place

2070 - 1370 = 700 mcb / 25

= 27.2 g Br./100 g

2050 - 1370 = 680 mcb / 25

= 26.5 g Br./100 g

The calculation procedure for determining the analysis results changes if the proportion of monoolefins to serve exceeds the numerical value 4: 1.

This value ratio is determined during the coulometric titrations and must be taken into account in the Bromine Number determination.

The difference takes place only when a quarter of the Bromzahlwertes the first or third solution has been deducted from the value of the second solution, if no isolated double bond pairs are present.

Claims (1)

2 0587 invention claim A method for the coulometric determination of unsaturated hydrocarbons in gaseous, solid or liquid media, each with a solvent containing 800 ml of acetic acid, 100 ml of methanol and 100 ml of distilled water and a further solvent of the composition 1020 ml of acetic acid, 400 ml of methanol, 400 ml of chloroform, 200 ml of distilled water and 12 g of mercury-II chloride are combined and that these solvents still contain a proportion of bromide, which is anodically converted to bromine in the solution and is added by den'im medium existing double bonds immediately, characterized in that an olefin sample is contacted in succession directly with a solvent of different bromide proportions and the first solvent contains 50 g of sodium bromide and the addition of the resulting bromine is carried out only on a double bond pair of the olefinic components present and that subsequently the same P Robe is placed in a second solvent containing 8 g of potassium bromide, bromination of all double bonds to the tetrabromide compound is carried out and further mixing of the sample in the solvent two, with a proportion of 14 g of potassium bromide, then, when a bromination of the contained in the sample and arranged in isolation arranged double bonds to the tetrabromide compound is to be achieved.