DE2650850A1 - Analysis method using solid electrolytes - with different electrolytes in tandem and unity ion transport number - Google Patents

Abstract

The determination of activity, concn. or partial pressure in solid, fluid or gaseous media by measurement of e.m.f. using solid electrolytes is based on temp. treatment and/or tandem connection of different electrolytes to bring the ion transport number of the electrolyte materials to 1 during the period of the measurement. With a zirconium dioxide electrolyte (4) in equilibrium under a low, reference pressure, hole conduction occurs. With a thorium oxide electrolyte (5) and high pressure equilibrium, excess-electron conduction can take place. If the electrolytes are placed in tandem and the ratio of the layer thicknesses (d1/do) is chosen so that the potential at the junction of the electrolytes is 1 no electron conduction can take place.

Description

Method for the determination of analytical data by means of improved Solid electrolytes The invention relates to a method for determining activities Concentrations or partial pressures in solid, liquid or gaseous media by means of EMF measurements with solid electrolytes. Such analytical determinations can be found in thermodynamics, chemistry and, above all, in the metallurgical sector increasingly important.

The measuring probe usually consists of a closed tube in which the solid electrolyte is introduced. It separates two spaces with different partial pressures or different concentrations. When measuring the EMF, the requirement is assumed that states of equilibrium exist and that there is pure ionic conduction.

In practice it has been found that when measuring with Solid electrolyte probes from different Reasons deviations occur. One of the main reasons for this is given by the electron conduction, which can also occur with such electrolytes.

In particular, if at low partial pressures next to the ionic conduction due to excess electrodes n-conduction and at higher pressures due to defective electrodes p-conduction occurs in the electrolyte. The arithmetical consideration takes place mainly by introducing the ion transfer number. The evaluation assumes an exact one Determination of the half-value pressures in advance and is therefore very time-consuming. Furthermore but the evaluation remains very uncertain, since equilibria are assumed, which may not even appear in the electrolyte during the measuring times.

The invention is based on the object of the electron conduction to switch off a pretreatment and / or a corresponding build-up of the solid electrolyte.

This is to ensure that the ion transfer number is always kept during the measurement 1 is present.

The object is achieved according to the invention in that the electrolyte materials for the duration of the measurement by temperature treatment and / or by series connection different electrolyte materials can be brought to the ion transfer number 1.

An expedient development of the inventive concept results also from the fact that the electrolyte material at a given external partial pressure before use for measurement by tempering with the specified partial pressure in the chemical equilibrium and is stabilized to the ion transfer number 1.

This ensures that the electron conductivity is opposite Measurement and reference pressure through the solid electrolyte that is not in equilibrium Setting of a stabilizing pressure before or during the measurement switched off will. There is another possibility to switch off the electron conductivity in that the measuring probe consists of two different solid electrolytes lying one behind the other consists. Such a tandem probe is illustrated below with the aid of schematic representations described with further details.

It shows: Fig0 1: Schematic representation of the measuring principle and the Cell voltage of an electrolyte probe; Fig. 2: Potential profile inside an electrolyte in a schematic representation; Fig. 3: Schematic representation of a multilayer electrolyte as a tandem probe.

Using the example of a solid electrolyte that conducts oxygen ions 1, the measuring principle is shown in FIG. 1 in a simplified manner. The electrolyte separates two rooms with different oxygen partial pressures or different oxygen concentrations. The two metallized, mostly platinum-coated interfaces of the electrolyte 1 form the anode 2 and the cathode 3.

An oxygen reference partial pressure pO is specified on the left and on the right let the pressure P1 be determined. In the example, let P0> p1. When the ions pass through One mole of oxygen ° 2 from left to right does the work in the pressure gradient Ap = RT ln P1 / Po obtained, but on the other hand the ions must counteract the Potential difference ECP - # oie work Ae = (# 1 - # 0). z. F, where F is the Faraday constant, z is the valency, equal to 4 based on 02.

means. In equilibrium, the work components A compensate each other + A = 0.

P e For the EMF we get: If the cell with the ionic resistance R. is loaded via an external resistor R, the emf is reduced to the e terminal voltage E * If the electrolyte also has an electron conduction, the associated electron resistance Re is now set in parallel with Ri instead of the previous external resistance as an electron conductivity ° e superimposed on the ionic conductivity oi.

where t. equal to the ion transfer number, it follows from equation 1.2 1.3 E * = E. ti If one sets P1 = p or pO = p + dp and leads t. as pressure-dependent in equation 1.2, one finally obtains the well-known equation The cell voltages measured by solid electrolyte probes can deviate from the EMF expected according to equation 1.1 for various reasons. One of the main causes is given when reactions like 2.1. - 1/2 02 gas + (") + 2 - where (.) = Ion vacancies 2.2 1/2 O2 gas + (") O-- + 2 + in the electrolyte at low oxygen partial pressures next to the ionic conduction due to excess electrons n conduction or at higher pressures due to defect electrons p-conduction occurs. As long as the concentrations (O--) and [(")] in the electrolyte remain sufficiently high and thus relatively constant, the above reaction equations with the distribution constants K ~ and K + 203 result in k = c2 pl / 2 where c = [(3] 2.4 k + = c + ² pl / 2 where c + = [c +] With the ionic conductivity Oi and the electron conductivity ° e = o + ° + applies to the transfer number ti = 1 / (1 + o- / oi + o + / oi).

If, according to H. Schmalzried, one sets o- / oi = α-c- and o + / oi = α + o + and defines α- # k- # p-1/4 and α + # k + # p + -1 / 4, we get after integrating equation 1.4.

It follows that ti (p) = 1/2 for p- = p # p + or p + = p # p.

For p + # (po; p1) the first term in Eq. 2.5 vanishes; is further p- # (p1; p1), then Eq. 2.5 in 1.1 over. -As can be seen from the derivation, Equation 2.5 applies only if also inside the electrolyte compared to the partial pressures present at its outer limits equilibrium prevails, d. that is, if in the electrolyte according to equations 2.1 and 2.2 as a function the external partial pressure of oxygen is removed or installed until the distributions for equations 2.3 or 2.4 are available.

In Fig0 2 is the potential profile between the electrodes in the interior Electrolyte 1 shown schematically as a function of the location coordinate x in the event that electrolyte surfaces are parallel to the field against oxygen exchange are isolated. As long as t. = 1 remains, the potential changes linearly with x resp. with In p, where p = p (x) is the equilibrium partial pressure assigned to the interior po # p (x) # P1 if ti (x) <1, the potential remains opposite the otherwise existing linear relationship. The setting of the equilibria 2.3 or 2.4 along the position coordinate x can be used as an exchange reaction according to 2.1 or 2.2 of course not happen spontaneously. The greater the spatial distance x (p1 - x (pO) is, the longer the transition time for the establishment of equilibrium will be last inside the electrolyte.

In Fig0 3, the tandem probe is described in more detail.

With it, for example, the oxygen equilibrium partial pressure should P1 at a temperature above 1000 ° C in a steel melt in a range of 10-10 up to 10-26 atm against a reference pressure Po of 1 atm. With the zirconium dioxide electrolyte 4 In chemical equilibrium with the low MeB pressures, electron conduction would be defective conversely, in the thorium oxide electrolyte 5 would be in equilibrium with reference pressures Excess electron conduction occurs above approx. 10 atm. For suppression Both electron lines are added in the present example both electrolytes, where thorium oxide 5 has the index 1 and zirconium oxide 4 the index o, to a so-called Tandem probe together, the layer thicknesses d1 and d being dimensioned so that for the logarithmically formed mean value P1 '= 10-18 atm the contact point of the two electrolytes has a potential of 1.0, its equilibrium partial pressure P 1, ö would not allow electron conduction in either of the two electrolytes.

In the present example, P 1, o is optimally set to 10 8 atm.

For the potential values at the electrolyte ends, the following relationships apply with tion = 1: The layer thickness ratio V = d1 / d0 is determined from the P1, o given above as 3.2 V = d1 / d0 = # 1 / # 2. (log P0 / P1.0) / (log P1.0 / P1) by the ratio V determined in this way is the ion transfer number for the mean measured pressure P defined above; optimized to 1. The optimization does not yet apply without further ado for the upper and lower value P1 in the measurement interval provided.

At the lower limit P1 = 10-26 atm, the result is P1.0 = 10-11.55 atm at the upper limit P1 = 10-10 atm the result is P1.0 = 10-4.44 atm, whereby the Values P1.0 simply result from equation 3.2. the value P1.0 = 10-11.55 atm at high temperatures in the zirconium dioxide and the upper value P1.0 = 10-4.44 atm especially fear electron conduction in thorium dioxide.

In the present example, therefore, both electrolytes are in accordance with the invention now to be tempered at the previously optimal fictitious pressure P = 10 8 atm, so that 1, o both electrolytes with respect to the chemical oxygen potential to P1,0 and thus for the later measurement on t. = 1 are stabilized.

ion All the advantages achieved with the invention exist in particular that in the previously known measuring method for the individual test media a large amount of time had to be expended for the parameter determination. Here but with the pretreated or specifically designed measuring probes only the ion conductivity is included in the evaluation, these correction measurements can be omitted. Furthermore are This makes absolute measurements possible and the evaluation is easier.

EXPECTATIONS

Claims (1)

CLAIMS io method for determining activities, concentrations or partial pressures in solid, liquid or gaseous media using EMF measurements with solid electrolytes, in that the electrolyte materials (i) for the duration of the measurement by temperature treatment and / or by series connection various electrolyte materials can be brought to the ion transfer number 1 2e method according to claim 1, characterized in that the electrolyte material (i) at a given external partial pressure before use for measurement by tempering with the given partial pressure in the chemical equilibrium and so on the Ion transfer number 1 is stabilized. 3. The method according to claim 1, characterized in that different Solid electrolytes (4; 5) are combined to form a so-called tandem probe. 4. The method according to claim 3, characterized in that the layer thickness ratio of the electrolytes (4; 5) combined to form tandem probes -is chosen so that in the intended Application range of measuring and reference pressure at the point of contact between the two electrolytes equilibrium partial pressure associated with the drop in the measuring potential in the electrolyte an optimal ion transfer number for both electrolytes even without tempering (4; 5) is reached 0