WO1992004624A1

WO1992004624A1 - Process for determining the number of dissociable particles in fluids

Info

Publication number: WO1992004624A1
Application number: PCT/DE1991/000717
Authority: WO
Inventors: Wolfgang Dittrich
Original assignee: HÜLS Aktiengesellschaft
Priority date: 1990-09-10
Filing date: 1991-09-05
Publication date: 1992-03-19
Also published as: TW240293B; CA2091347A1; KR930702671A; FI931035A0; FI931035A; EP0548133A1; JPH06501097A; DE4028716A1; CN1059966A

Abstract

The invention relates to a process for determining the number of dissociable particles (ion pairs) in fluids in which an electric field is applied to a pair of electrodes in the fluid with a low injection capability for charge carriers, the electric current arising owing to the electric field is integrated and the integration figure found is used as a measure of the number of dissociable particles in the fluid.

Description

Method for determining the number of diss in liquids

The invention relates to a method for determining the number of dissociable particles (ion pairs) in liquids.

So far, electrochemical methods for determining ions in liquids are known, which are all based on a conductivity measurement of the liquid. In this method, an electrical voltage is applied to a pair of electrodes located in the liquid to be examined and the electrical current which then arises is measured by means of a measuring device.

Disassociable particles represent impurities in liquids and are generally in a dissociated and non-dissociated state.

A certain equilibrium is established between these two states. Only the currently dissociated particles can cause an electrical current flow as charge carriers.

The methods mentioned based on a conductivity measurement are therefore not able to exactly determine the number of dissociable particles in liquids, since during such a measurement some of the particles are always in undissociated form and therefore cannot contribute to the conductivity. In addition, when conventional electrodes are used, charge carriers are also continuously injected into the liquid, which also falsifies the result. The invention is therefore based on the object of specifying a method which enables a simple but very precise determination of the number of dissociable particles in a liquid.

This object is achieved according to the invention by a method in which an electric field is applied to a pair of electrodes in the liquid which has a low injection capacity for charge carriers,

in which the electric current which arises due to the electric field is then integrated over a certain period of time,

and in which the determined integration value is then used as a measure of the number of dissociable particles originally contained in the liquid.

Developments of the invention are the subject of the dependent claims.

In the method according to the invention, not only the particles already dissociated at the start of the method contribute to the measurement result, but also, since integration takes place over a certain period of time, the particles which are dissociated only later under the influence of the electric field.

In addition, the use of special electrodes which have a low injection capacity for charge carriers prevents additional charge carriers from entering the liquid and contributing to the current integral. This increases the accuracy of the measurement.

The process is preferably carried out until the current flow practically no longer drops. Since generally certain residual flows always remain, it cannot be waited until the flow comes to a complete standstill. If such residual currents are present, their contribution to the current integral must of course be disregarded.

After completion of the integration, it can be assumed that all particles capable of disassociation have dissociated and migrated to the electrodes as free ions. The remaining liquid is therefore in a highly pure state.

At each time, the following relationship is shown between the dissociated particles (ion pairs) N _£ extracted per unit volume during the action of the electric field of the liquid and the current I flowing:

T

O

where e stands for the elementary charge and V for the volume.

After a sufficiently long time, all particles capable of dissociation have dissociated and migrated to the electrodes, so that the current flow comes to a standstill. The number of disassociable particles _D present in the liquid per unit volume thus results

^N D ^{= N} E

.-_ *

In the statements made above, it is of course assumed that the liquid as such is difficult or not capable of disassociation at all and therefore essentially only impurities contribute to the current flow. The following shapes can be used as electrodes with low injection capacity.

On the one hand, so-called passivated electrodes can be used.

Electrodes are understood to be electrodes whose ability to inject electrodes has been greatly reduced by suitable pretreatment. The passivation can be achieved with commercially available brass electrodes, for example, in a very simplified manner as follows.

The pair of electrodes to be passivated is placed in a cell with propylene carbonate (PC) and a field strength of, for example, 10 kV / c is applied to them. After a while, the electrodes are passivated. In the case of brass electrodes and PC, the passivation can be recognized by the brown discoloration of the electrode surfaces. The electrodes passivated in this way can then be used for the method according to the invention.

Another way of reducing the injectability of commercially available untreated (bare) electrodes is to cover them with ion-exchange membranes in the reverse direction. The membrane prevents the ion from escaping into the liquid volume.

Finally, it was also found that the injectability of electrodes can be reduced by placing a layer of a material with a high dielectric constant between the untreated (bare) commercially available electrodes and the liquid to be examined. It would therefore be obvious to coat the electrodes with such a material. However, since experiments have shown that ceramics in particular Preventing an injection, but it cannot be made into such a form without difficulties and problems, the following arrangement was preferred in the investigations carried out so far:

Between the liquid and electrodes positioned a disc is made up of ceramic, so that the fluid between the two plates is ^■ and the electrodes of the plates can be fixed to the sides facing away from each of the liquid.

The present method is preferably carried out by applying a field of a few kV / cm. As a result, the method can be carried out in a sufficiently short time and it can be assumed that all particles capable of dissociation are dissociated.

Although not yet proven, it can be assumed that complete dissociation of all particles capable of disassociation only begins at a certain field strength. When propylene carbonet (PC) was used as the liquid and passivated electrodes, extensive dissociation was observed even at field strengths around 100 volts / cm. In principle, there is a connection that the method proceeds faster with large field strengths, since the field velocity k of the particles increases due to large field strengths.

Commercial devices designed for this purpose can be used for measuring and recording the self-adjusting stream.

These are, for example, storage oscilloscopes or current measuring devices with a writing device. From the current curves recorded by these devices determine the resulting current integral by estimating or using planimetry. Of course, devices can also be used that are able to determine the integration value independently. The method according to the invention is preferably used for liquids with high dielectric constants.

Since the dissociable particles represent soiling of the liquid to be examined, the number of dissociable particles can be used to predict the cleaning time for the liquid.

If I assume that the current drop follows a certain function, for example an exponential function, then by determining the relevant time constant T, a reliable prediction of the current integral to be expected is possible by means of a short-term measurement. In this case, the method does not therefore have to be continued until the point in time at which the current has almost dropped to the value of the unavoidable residual currents. The observation of the current drop and a time constant estimate can be carried out using conventional methods. Perform storage oscilloscopes particularly well. The integral of an exponential function is known to be the product of the time constant and the initial value. The following relationship thus results for the number of dissociable particles per unit volume:

if j indicates the time constant of the exponential function and I _Q the initial current value iεt.

An exemplary embodiment is discussed below, in which the number of dissociable particles in propylene carbonate (PC) is determined.

Fig. 1 shows an apparatus for performing the method. 2 shows a suitable measuring cell for the method. 3 shows the course of the current which arises during the method.

The measuring cell 1 shown in FIG. 1 serves to hold the propylene carbonate 3 to be examined. A pair of passivated electrodes 2 is connected to a voltage source 5. A current recording device 4 is connected between one of the electrodes and the voltage source. The recorder 6 of the current recording device prints out the temporal Stro veriauf. The start of the process is initiated by closing switch 7.

The electrodes are at a distance of 0.5 cm from one another, for example. The applied voltage is between 100 and 1000 volts. The resulting field has a strength of 200 to 2000 volts / cm.

The following main impurities were found for the propylene carbonate by analysis:

Sodium chloride, tetramethylammonium bromide, propanediol, water.

These impurities are mostly in the ppm range. The ratio between dissociated particles N and dissociable particles N _D before the creation of a field is called the degree of dissociation <_1 (O = N / N _D ). In the event of contamination of the propylene carbonate with water 10, with propanediol 10 ~ ⁴ and with sodium chloride and tetramethylammonium bromide, this is between 0.1 and 1. Fig. 2 shows a specific embodiment for a measuring cell. It can be seen that it is a closed measuring cell 2. The electrodes 1 contained therein are covered with ion exchange membranes 3. Alternatively, the passivated electrodes indicated in FIG. 1 could of course be used.

Which adjusting during the ^process' current waveform is shown in Fig. 3.

After the switch 7 in FIG. 1 is closed, a field will build up on the passivated electrodes 2. The current resulting thereby, which is mainly a dissociation current, is detected by the current recording device 4 and is shown in FIG. 3. It can be seen in FIG. 3 that the current takes an exponentially decreasing course. After about 5 minutes, the current reaches a value I _R which it practically no longer falls below and for which the residual current which is always present is responsible.

When determining the integral enclosed by the current curve between the times 0 and 5 minutes, the integration component shown hatched in FIG. 3 and caused by the residual currents must not be included.

It can also be seen that the current curve in FIG. 3 has a kink in the front area. This is a result of two overlapping processes. Initially, the particles that are already in dissociated form mainly contribute to the current flow. At the beginning, this leads to a high current flow I _Q , which drops very quickly with a time constant T->. The current flow is thus brought about by the constantly newly disassociated particles. The decrease in this dominant stream proportion occurs

correspondingly a time constant T ₂ •

By the recorder 6 shown in Fig. 1, the

Loss of power recorded. From the recorded

The course of the current can easily be determined by leveling out the area enclosed by it.

As already said above, the area indicated by hatching in FIG. 3 should not be included in the current integral, since this portion is caused by the residual currents.

If the current integral is known, the particle number per initially present in the liquid is obtained

Volume unit N _D :

with Q as the total charge due to the dissociation flow.

For propylene carbonate with an initial conductivity of 55 '* 10 ^"10 S / cm, the value for _D is 6 * 10 ¹³

1 / cπr 3

The two values given represent a concrete measurement result. In principle, of course, there is no fixed relationship between conductivity and the number of dissociable particles N _D , since the initial conductivity is essentially determined by the type of contamination and the corresponding degrees of disassociation of these particles.

The present method has proven to be extremely advantageous when working in connection with the cleaning of propylene carbonate while simultaneously producing passivated electrodes.

It was possible to determine the degree of contamination of the propylene carbonate easily and very precisely with the method according to the invention. In particular, it was 1 .

a prediction of the cleaning time for the propylene carbonate is possible.

It can be assumed that the present method proves to be extremely useful, particularly when determining the number of dissociable particles in liquids with high dielectric constants, such as are used in electrostatic machines.

Claims

1. Method for determining the number of disassociable particles (ion pairs) in liquids,

in which an electric field is applied to a pair of electrodes in the liquid which has a low injectability for charge carriers,

in which the electrical current arising due to the electrical field is then integrated,

and in which the determined integration value is then used as a measure of the number of disassociable particles originally contained in the liquid.

2. The method according to claim 1, characterized in that the integration is carried out until the current essentially no longer drops.

3. The method according to claim 1 or 2, characterized in that a pair of passivated electrodes is used.

4. The method according to claim l or 2, characterized in that a pair of electrodes is used with ion exchange membranes coated in the reverse direction.

5. The method according to claim 1 or 2, characterized in that one uses bare electrodes and dieεe of the liquid through a layer of a material with a high dielectric constant, preferably 1 ∑

made of ceramic, separates.

6. The method according to one of the preceding claims, characterized in that an electric field with a field strength of more than 100 volts / cm is applied.

7. Use of the method according to at least one of the preceding claims used for liquids with high dielectric constants.

8. Use of the method according to claim 7 for propylene carbonate.

9. Use of the method according to claim 6 or

7, to determine cleaning times of liquids.

._

10. The device for carrying out the method according to at least one of claims 1 to 6, characterized by a container for holding the liquid, at least two electrodes arranged in the container and spaced apart from one another, which electrodes have a small diameter

Injection sensitivity, a voltage source that is electrically connected to the electrodes, a current detection device for measuring and

Record currents that are in series with the

Voltage source and the electrodes is connected.

11. The device according to claim 10, characterized in that the Stromerfaεεungεeinrichtung is a Speicheroεzilloεkop iεt.