DE2759605C3 - Device for the spectrophotometric investigation of products of electrochemical reactions - Google Patents

Description

.15 The invention relates to a device according to the Preamble of claim 1.

From a publication in "Analytical Chemistry" Volume 42, No. 4, pages 551 and 552 (1970) is a Method and device for spectrophotometric

V) recent investigation of electrochemically formed products known. In a vessel containing a solution for carrying out the electrochemical reaction, a rotatable electrode with a vertical axis of rotation protrudes below the liquid

«Keitsspiegel a. The electrode has a working surface perpendicular to the axis of rotation and is surrounded by a large number of optical fibers on the outer circumference of the work surface. The optical fibers are embedded in a hardened synthetic resin and after they run in the direction of the axis of rotation are deflected tapering, together with the power supply for the electrode through the bearing for the rotatable electrode through to a light detector. Electrode and fiber optic cable are in one

hi wave arranged in the form of a noble jet pipe, which in the camps. A fiber optic cable serving as a transmitter for monochromatic light ends at Gefäßboclen and is coaxial with the axis of rotation of the

Electrode on the electrode and those surrounding it Optical fibers directed. This device is used according to a so-called lock-in technique. The electrical voltage of the working electrode is periodically changed with square-wave pulses. The frequency -> is usually at least about 10 Hz, so the technology works sensibly. With this technology and this device approximate conclusions could be drawn about the Process of chemical reactions are drawn. However, it is mentioned that there is a lot of noise and ι ο low absorption values impair the accuracy of the examinations.

In contrast, the invention is based on the object of providing a device of the type mentioned at the beginning to create, which leads to results with higher informative value and investigations on both long-lived as well as short-lived products and secondary products, and their structure with regard to geometric and optical conditions are as stable as possible.

The object is achieved according to the invention in the device mentioned at the outset by the in the characterizing Part of the measures mentioned in claim 1 solved. Further developments of the invention are the subject matter of the subclaims.

It has been found that the imprecise results of the known method can be attributed, among other things, to this are that the lock-in technique described is used. The short impulses only generate one low modulation of the measuring effect because the characteristic times for setting stationary product κ> concentrations in the flow boundary layer are usually considerably greater than the mentioned pulse times. It Therefore, in the known method, only one second-order effect was regularly measured, its Size usually does not allow any conclusions to be drawn about the process J5 allows the reaction and has a correspondingly low informative value Invention under quasi-stationary conditions. The characteristic times for hiring stationary Product concentrations depend on the type of reaction. Therefore, it is preferable to wait about one second Switching on the current measured. The measurements can be carried out without having to switch off the power in the meantime be made. However, working with current or voltage pulses is preferred Duration of approx. 1 to 5 seconds in order to be able to carry out zero point determinations in the meantime. These Zero point determinations are particularly useful if a longer reaction time occurs By-product accumulates in the electrically conductive reaction solution and 00 if not observed Could lead to measurement errors.

It was also recognized that the thickness of the liquid layer through which the light is to be transmitted is preferably in a certain ratio to the Prandtl's flow boundary layer on the electrode and in particular to the thickness of the Nernstian diffusion boundary layer on the electrode, whereby further sources of error can be eliminated. First, the liquid layer should be so thick that the m> Pfandtische boundary layer can form along the surface of the disk electrode, which, depending on the viscosity of the liquid and the speed of the rotating disk, has a thickness of the order of magnitude of about ¹ mm or less. On the other hand, the thickness of the liquid layer should lie within a certain ratio to the thickness of the Nernst boundary layer, so that absorptions due to the accumulation of a product formed during the reaction in the reaction solution still remain in a reasonable ratio to the absorption within the Nernst boundary layer, within which this occurs the product formed by the electrochemical reaction initially stops. Since the Nernst boundary layer is an order of magnitude smaller than the Prandtl boundary layer and the concentration of the product formed within the Nernst boundary layer is usually around 10,000 χ higher than in the solution, the thickness of the transilluminated liquid layer can be kept around 100 χ greater than the Nernst boundary layer and about 10 χ larger than the Prandtl boundary layer, if one wants to achieve that the zero point shift of the absorption due to the enrichment of the product in the solution remains in the range of percent. As a rule, the thickness of the liquid layer is kept in the range from approx. 1 to 10 mm, preferably 2 to 5 mm.

The speed of rotation of the electrode can be varied within wide limits. The lower limit should be around '/ 2 until ! Rev / sec. lie so that the natural convection of the solution is overcome and the theoretically calculable Flow is formed on the rotating disc. If the number of revolutions is greatly increased, then it is increased diffusion flux through the Nernst boundary layer, associated with a decrease in thickness the Nernst boundary layer, which leads to a reduction in the amount of product detected and thus to one Decreasing the absorption values therefore leads to the upper limit being fluid and being replaced by the desired accuracy of the measurement results determined. As a rule, speeds of 100 rev / sec are not exceeded. Also other parameters such as current strength, temperature, concentration of the solution and wavelength of the monochromatic light can be adapted to the particular reaction to be investigated.

Another disadvantage of the known device mentioned at the outset is that the the fibers that conduct light from the electrode area to the receiver area or the fibers formed therefrom Rotate the Lic'akabel together with the electrode. Slight local differences in the arrangement Individual fibers in the fiber bundle and in the light guide lead in this way due to the rotation relative to the fixed receiver to increase the noise.

In contrast, the device according to the invention care has been taken that the optical parts that rotate with the electrode face in the direction of the • Jchtweges compared to the known device be kept very short, in particular through straight light guidance and plane-parallel design of the Entry and exit surfaces are optical defects caused by the rotation of the translucent ring could, are practically excluded. In addition, the ring is preferably made of an optical in contrast to the optical fibers bound by a synthetic resin known device. There is by the use of synthetic resin the area of application because of the chemical instability of the synthetic resin is limited in many solutions. Most synthetic resins are z. B. not resistant to alkaline solutions.

The device according to the invention is thus characterized in that the optically transparent body consists of an optically (omogeneous ring, whose the other end face is plane-parallel to the one end face and the receiver for receiving the also the

softer face of the ring penetrating light is fixed in place. The optically transparent king is preferably made of quartz glass, as well as the other optically conductive parts of the device, whereby a Working in the ultraviolet range is possible.

The diameter of the metallic disc electrode can be varied as required. In the case of electrodes with a diameter of more than 10 mm, it is advantageous to make the electrode ring-shaped, that is to say with an insulator in the center of the electrode. As a result, at a given current density, the current and thus the conversion can be kept low, fs it is also avoided that the paths of the products formed in the electrochemical reaction are undesirably rude before they reach the optically transparent King, in whose area the optical measurement only takes place can be carried out. By measuring at more than one wavelength of light, several I ¹ Co (IiIkIt * / iiulrit'h iinlprsnchi.

The embodiments of the invention result .ms the 1 IntcransprÜL'hen.

The invention is illustrated below with reference to Drawings explained in more detail. Is shows

I'ig. I a schematic representation of an execution company the device.

I ι g. 2 a diagram of the con / entration course perpendicular to the rotating disk electrode for two Thicknesses iV, and //. Of the diffusion layer.

Ii g. 3 the dependent wedge of the ratio of Extinction / 'to the current density / from the reverse angular velocity (ti for the oxidation mim NW - DiphenyIheucidine.

I ι g. ·! the dependence of the | - * \ tinkitoii I on the diffusion limit sirom on the root of the reciprocal angular velocity ei.

Γ 'g. 5 the ratio of the extinction / to the current density / as l-'iinclion of the reciprocal angled bullet indigkeil ο,

l'ig. h the ratio of the extinction ! '. of the stable product for the measured extinction / in -dependent wedge but the reciprocal root of the angular cohesion or the like.

ι ι g. / never like organs more experimental.> spectra iier Oxidation product lies N.N Diphcnylbenzidms Lind I ι g. No other form of guide for the device.

In the case of the I i g. Isl a keaklionsgcfäß I provided from a cup-shaped l'ntcrteil 2 and an attachment designed as a lid? is composed. The cup-shaped lower part has a lateral, sealable opening for connecting an optical fiber cable 4. coming from a monochromator. The fiber optic cable 4 opens into a ring lamp 6, held by an adjustable holder 5, which serves as a transmitter and in which the incoming light is directed vertically upwards. The ring lamp 6 has a free passage 7 in its center and is arranged within the cup-shaped lower part mi far above the floor so that the reaction solution can flow upwards through the passage from below. A rotatable disk electrode 8 with a vertical axis of rotation is arranged coaxially to the ring lamp, the drive shaft 9 of which extends upward through the cover 3. The diameter of the disk electrode essentially corresponds to r> nri-h

mpttpr

Ring light fi.

The disk electrode is exactly circular shaped Outer circumference seamlessly from a plane-parallel and optically compact quartz ring IO surrounded, the one above the light exit point of the ring lamp 6 arranged isl.

Also held by the cover 3 is an annular receiver 11 above the quartz ring 10 is arranged essentially in a mirror-image arrangement to the ring lamp 6 and through its coaxial passage 12, the drive shaft 9 runs. The receiver II is through an adjustable bracket 13 as well as the ring glow 6 in the reaction vessel held stationary. A fiber optic cable 4 leads from the receiver II through the wall of the attachment 3. that is connected to the light detector.

The reaction vessel can be made of glass. Ls can However, it can also be made of stainless steel, with it at the same time can form the counter electrode to the disk electrode 8. The remaining electrodes, like the reference electrode and the i'leklrisrhr Konlakl to the rotating Electrode are not shown. The l.ichiaustriitsfläche of the sender or the ring light 6 and the Einirittsflä before the receiver 11 are limited by diaphragms 15 The two panels can be replaced by a thin, optically opaque layer, e.g. B. made of metal on which Quar / ring to be replaced. In addition, these (-'lachen still covered by flat quartz rings 16. These form a permanent exposure to light and light and protect the parts behind.

The example of this was the oxidation of N.N 'Diphenylbenzidiu investigated, proceeded as follows:

I for the measurements ssurdc a "Olendc disks electrode used made of a Plalin-Iridium alloy tion with 10 weight percent iridium existed. The disk electrode with a radius r = f> .5 mm was the polished face of a cylinder that was screwed to the drive axle with a (iesvinde Electrode surrounded by a quartz ring mn an outer radius r-IOnim and a thickness J = - ι mm. The inner bore was at a tolerance adapted from around the outside of the electrode. The flat surfaces of the quartz ring had a pianparaiieicn optical> chiii'i. iJie rotating. ^ criciDC s was via an I: 2 ratio or reduction son driven by a 30 stepper motor, the speed of which is between 1.5 Hz and 120 Hz through control electronics regulated ssiirde. The drive system and the The axis of the rotating disk electrode is connected by a contact-free magnetic coupling. Through this ssurde the transmission of vibrations of the Drive on the electrode system avoided. The upper part of the rotating axle was as a hollow shaft worked into which an iron wire protruded. The electrical contact of the rotating / around resting System was made via a quartz silver drop.

Line glass / ridge with a plane cut was made airtight on the electrode holder via a Teflon seal attached. A stream of nitrogen was passed through the cell through the gas inlet and outlet capillaries. the The counter electrode made of a 2 cm platinum sheet was located in a secondary vessel that was passed through a frit ran away from the main vessel. A Haber-Luggin capillary. which ended below the disk electrode established the connection to the reference electrode.

The cell was filled with electrolyte solution the liquid level was 2 to 3 mm above the underside of the stationary disk electrode.

The optical components are / to protect against Solvent coated with Teflon. Brackets. The adjustment device and cover are made of stainless steel. Monochromatic light from a spectrophotometer reaches the ring light via the fiber optic cable. The glass fibers used for this are permeable in the wavelength range 360-800 nm. The one at the Outlets of the light, arranged in a ring, are covered with a thin disk of resin covered. There is a 2 to 5 mm high gap between the ring light and the rotating disk. the Ring light has a hollow core with 6.5 mm caclius. The dimensions of the gap and hollow core are chosen so that the flow of liquid to the rotating Disc is practically not disturbed.

^{0.1 M LiC-} Kh in acetonitrile was used as the base electrolyte. The acetonitrile was purified and dried by first passing it over calcium hydride. then it was rectified over di-phosphorus pentoxide and finally again over potassium hydride.

The first and last 10% of the distillate were discarded in each case. The N, N-diphenylbenzidine to be investigated was available p.a. and was recrystallized to n-Hcxane.

A silver wire in contact with OI M AgNO in acetonitrile was used as a reference electrode. Alk measurements were carried out at room temperature, usually 22 C.

The measuring electrode was polarized with potentiostatic or galvanostatic pulses from a potentiostat. The pulse duration was a few seconds. The change C in absorbance produced by the pulse was measured. When using the signal averaging technique, the potentiostat was controlled by the signal computer via a pulse generator. After repeating the measurement several times, the memory content of the signal calculator was recorded with a ^ VV 'recorder.

Theoretical foundations

To be able to judge the sensitivity of the method, it is first important to know how

FvlinLtirin for AIn clakil * »* FIpLf rr» lvc *> r \ t - / ^ HiiL · t \ ic \ r \ Aftr

Speed and depends on the radii. The extinction should be in a ring between η and r; are measured outside the disk electrode with the radius n _P , where r; > / "i> ra applies.

In the diffusion boundary layer in front of the disk electrode, at the current density j ', the concentration of the product B falls from the concentration fei at the electrode at V = O within the Nernst diffusion boundary layer of thickness .Y = # in a first approximation linearly to the concentration E inside the solution. With the same diffusion coefficient of the starting material A and the stable product ß cannot be greater than the concentration ß ~ of the substance A. The current density j at the disk electrode is given by

J = -n,

) = -, _h FD _t ""

If the solution was not originally the product Π , then fo, - b = Ab =: fei. The thickness of the diffusion boundary layer on the rotating disk depends on the diffusion coefficients D \ and D / j of the two substances:

- I .hl D ₁ '

and

14b)

In (4) ο is the angular velocity and r is the kinematic viscosity.
From (2) - (4) follows

Id ₁ , ill

The set rth stored in the boundary layer at r <r » per surface unit is given for two cases by

. i is the area of the hatched triangles in Fig. 2a and b illustrated.

Fig. 2a and b show the diagram of the concentration curve perpendicular to the rotating disk electrode for two thicknesses i '>, and i7. the diffusion layer.

y. The product B with the concentration b and the diffusion coefficient Dn arises in an electrochemical reaction from A with the concentration a and the diffusion coefficient D. \.

a) under galvanostatic conditions at Stromei densities below the diffusion limiting current density and

b) under potentiostatic conditions at the diffusion limit current density.

In case a) it is below with a constant disk current f. the diffusion limit current

= Ir. ·. ■ / "2 = /.'V Zn ₁ FD ₁ ,

16a)

If under potentiostatic conditions in case b) the diffusion limit current densityc /,ΛΟΓ.sogilt

One obtains; ius (4b) and (6aI

and from (4b) and (6b)

(6b)

= 0.81

{7h |

In the further distance r> n, from the electrode, the surface concentration mi in the diffusion boundary layer decreases.

For a quantitative solution to this problem, one must calculate κι with the help of the differential equation of convective diffusion, the concentration curve b (r. Y) as a function of the radius r and the perpendicular distance from the disk under the initial conditions that for

• "(4 * -) =

and for

<C ₀ C '.

V '-.ν

· ■ = ι nl · I) ₁₁

is. As in the ring-shaped area / between the two electrodes of the rotating disc ring electrode, the concentration curve at r> n depends only on the radius and not on the speed. That's why

10

mn differs from (7a) and (7b) by only one yield for r> i \. ^r aklor k (r). which for rr _n <rn will be of the order of one.

If valid. of the Lambert-Beer law, the extinction Fo of the stable electrolysis product with the extinction coefficient e under galvanostatic conditions is described by

E = _n, m '= k {r) ■ 1.3 and potcntinstutischcn conditions in animals by DilTusionsurenzstromdichte / · "'" ■ - <"i" * ^r = airi O.SI /) - '■' D ₁ , '■' ι · ¹ "Ti-i ^ll

If the extinction coefficient, diffusion coefficient and kinematic viscosity are known, the yield k (r) can be determined experimentally. If kfrj is known, unknown extraction coefficients can be determined, since the other quantities in (8) can each be accessed using different methods.

Equation (8) allows it. assess the sensitivity of the method. It is possible. Changes in the extinction to measure / T = IO 'easily, For an extinction coefficient f = 10 ^: cm-' mol ^:. a diffusion coefficient I) = IO "'cms ^:. a kinematic viscosity)' =! <) ^: cm- \ and a speed ^ '= 1 s', concentrations of the order of bn = 10 ⁵ M are just detectable. By means of an improved measuring technique the sensitivity can be increased considerably.

In the event that the primary product B reacts further, the extinction is no longer described by (8), but has to be calculated separately for each type of reaction. If ß continues to react in an irreversible first order reaction on the pane

"2 <■ '

(10)

because because of the irreversibility b <h, should be.

The absorbance F of the product is now compared to a stable product

EE _n =

hi, / H ₀ = 1 - «, A-, 1.61

reduced.

The precipitation of a sparingly soluble product B can be a heterogeneous reaction of the first order. Then it arises

_{k; iho} _

where Z) _{5 is} the saturation concentration of bist. If the over-saturation is not too high, the product will essentially only precipitate at the disk electrode, since the solution in the diffusion boundary layer = ¹ quickly dilutes for r> r ₀

It follows

E ₁₁ E = J "I +

[I ■ (■ A, I /), -

(8a)

(Hh)

13)

Analogous relationships for a heterogeneous second order reaction with respect to B can be derived without difficulty. For the exact determination of the rate constant of a homogeneous first-order subsequent reaction, however, a solution of the differential equation for the convective diffusion is necessary, taking the reaction into account. The relative absorbance, E / Et, is expected to decrease linearly with the logarithm of the angular velocity. For a homogeneous second order reaction one probably cannot give an explicit solution. However, working curves can be calculated using digital simulation. The homogeneous reactions of B will be dealt with in more detail in later work.

Measurement results

The new method was applied to the study of the oxidation kinetics of some aromatic amines. The oxidation of N.N'-diphenylbenzidine B in acetonitrile without the addition of acid produces in reversible reaction the corresponding ijmichinoni-

, -, B .— =? SH ⁺ + f "L · ',. = 0.320 V (14)

and in the further course the quinonediimine Q

Oh;

L /, = 0.448 V
(15)

According to Cauquis et al. and according to our own independent measurements, S is a radical cation in the neutral milieu and Q as a dication with a dichinoid structure.

In the presence of acid, a polarographic wave with a double step height is observed as soon as in the kinetically strongly inhibited equilibrium

W + H ^H

(16)

predominantly the cation BH ⁺ is present, which accordingly

bra

QH ₂ ⁺ * + H ⁺ +

(17)

is reversibly oxidized ^{to OhT + I.} The half-wave potential for the oxidation of 03 mM benzidine in the presence of 50 mM HClO ₄ was [/ i; ₂ = 0.55V. the

Extinkiioti of Q at /. = 580 nm is in A b b. 3 in Plotted as a function of the reciprocal angular velocity.

A b b. 3 shows the dependence of the ratio of the extinction E to the current density j on the reciprocal angular velocity ω for the oxidation of Ν, Ν'-diphenylbenzidine to the quinonediimine in 0.1 M LiCIO ₄ + 50mMHClOj.

Open symbols: light ring at η = / - "= 6.5 mm to η = 7.5 mm.

Closed symbols: aperture η = 7.8 mm to Γ2 = 8.8 mm.

Squares: / = 75.2 μΑ cm - '. Circles: ./= 37.6 μΑ cm - '.

The measurements with the light ring close to the electrode gave a yield k (r) = 0.78 ^{for Fo = 5.8500 · 10 7} cm ² mol ', regardless of the chosen constant current densities, which were, however, smaller than the diffusion limiting current densities. On a ring further out with η = 7.8 mm and / ■> = 8.8 mm, the yield has already dropped considerably to k (rj = 0.55. The inclinations of the straight lines in A b b Determine% and 2%. The mean error for k (r) results mainly from the uncertainty in the determination of the extinction coefficient with a mean error of ± 5%. Other smaller sources of error are inaccuracies in the initial weight, the kinematic viscosity

ν = 4.8 10 ¹ Cm ² S - 'and the diffusion coefficient D _n ,! ⁴ = Drjii, = 7.6 10 "em's', which is calculated from diffusion limit current densities.

If the diffusion limit current is reached under potentiostatic conditions, a linear one is obtained Dependence of the extinction on the reciprocal wort! from the angular velocity, like A b b. 4 shows.

A b b. 4 shows the dependence of the extinction £ at the diffusion limit current on the root of the reciprocal angular velocity in. Light ring at / ¹ I = / o = 6.5 mm to /-2=7.5 mm potential of the disk electrode £ / = 0.85 V. ( 0) 550 μΜ and (D) 170μΜ Ν, Ν'-diphenylbenzidine and 0.1 M LiClO4 + 50 niM HCIO4 in acetonitrile.

For two different concentrations of B is obtained

ΓΠ2 ^η d '* ¹ AiisKpiJtp L-Sr-) —0 8! "nri i-ir \ - 7Q Hip agree within the accuracy of measurement well with the yields from experiments under galvanostatic conditions.

In neutral solutions, S is stable. However, it owns a lower solubility than B or Q. so that after exceeding the saturation concentration at the Disc electrode deposited wild.

A b b. 5 shows that under galvanostatic conditions at low angular velocities the extinction of S is always lower than the yield k (r) corresponds to. At high angular velocities, on the other hand, a straight line is obtained which, with the extinction coefficient es = 3.25 ■ 1O ⁷ Cm ³ ITIoI- ¹ at λ = 455 mm, gives the same value k (r) = 0.79 , which already results from A b b . 3 and 4 was determined. The extinction Eq of a stable product is obtained under these conditions.

Fig. 5 shows the ratio of extinction E to current density j as a function of the reciprocal angular velocity ω for the oxidation of 10 ~ ³ M Ν, Ν'-diphenylbenzidine to semiquinonimine in a neutral 0.1 M LiCIO.1 solution in acetonitrile at (o ) J = 37.6 μΑοΐη- ² and (G) 7 = 75.2 μΑατι- ² . The straight line corresponds to the respective extinction En of a stable product.

If, as in Fig. 6, the EoIE ratio from Fig.

5 against the square root of the reciprocal speed. according to (13), the rate constant results from the slope for the precipitation, Αϊ = 2.3 10-'cm s' and from the ordinate segment and the slope

6 shows the ratio of the extinction E _{n of} the stable product to the measured extinction E as a function of the reciprocal root of the angular velocity ω. Same experiment as in A b b. 5.

Since the concentration of the semiquinonimine in the solution is negligible, 6, - ^ is practically equal to the saturation concentration. For high speeds E _n IE = I. This deviation from (13) is based on the fact that a disk covered with solid product was assumed in deriving this relationship. The dissolution of the product leads to E> E _n . In fact, in the steady state at high speeds, the electrode is not covered with S because of b <6.

Oxidation of the diphenyibenzidine also provides B ' an example of a fast, homogeneous reaction of the second order. With positive potentials that Correspond to the limit current of the oxidation of B to Q, the quinonediimine Q reacts with Γ in neutral solution. to the semiquinonimine S:

Q + - B - 2 S USI

The reaction (18) takes place within the diffusion boundary layer. F i g. 7 gives the normalized spectra at two different angular speeds again.

F i g. 7 shows experimental spectra of the oxidation products of Ν, Ν'-diphenylbenzidine in 0.1 M L1CIO4 / CHiCN at various angular velocities ω. Limit current density j at the potential of the disk electrode £ / = 0.62 V and the angular velocities (a) ti) = 31, SS " ¹ i'.nd (b) o) = 189 s ¹ ; dotted: spectrum of the semiquinonimine with absorption maxima at A. = 455nrr u'id A = 480nm. Dashed: spectrum of the) quinonediimine with absorption maxima at λ = 405ηηι and λ = 580 nm.

Lie at the higher angular velocity

8Π0 / λ ntiH

Hpr Knr

nnmärpn

Product Q before, while at the lower Wi kel speed only 45 to 50% are available. A rate constant of the order of 10 ⁴ M ¹ S ¹ can be estimated for reaction (13). In a rough simplification, neglecting the concentration profile perpendicular to the disk, mean concentrations in the boundary layer were assumed and mean reaction times were estimated from the radius ratios and the speed.

The overlapping absorption bands of the substances are used for quantitative analysis of the spectra B and S split. This does not cause any problems in the present example, since the spectra of the pure substances are known. In other cases z. B. the unknown absorption spectrum of a short-lived intermediate from the known forms of the spectra of stable reactants and determine the total spectra measured at different angular velocities and current densities. Even if several unknown products occur, the division into the individual spectra is in principle possible.

In the case of the in FIG. 8 another embodiment of the device shown is the cup-shaped lower part 2 ' of the reaction vessel 1 'made of quartz glass. The rotatable electrode 8 ', the optical ring 10'. the

The drive for the electrode and the receiver are designed in a similar white in this embodiment as in the embodiment according to FIG. 1. As a sender 20 for the light, however, prism optics are provided which are essentially outside the reaction vessel is arranged Oas light reaches the side of the reaction vessel in a prism 2t, from which it is deflected vertically upwards towards the bottom of the vessel. An immediately following one widened prism 22 arranged below the vessel wall 2 'with a conical inner and outer surface the light beam into a light ring that is passed through the vessel wall. The light ring is in forwarded bundled to a body 23 which has the shape of a hollow cylinder and which is a higher one The refractive index than the surrounding solution has Der Holilzylindej · 23 is at a small distance above the Vessel bottom coaxial with the axis of rotation of the electrode 8 '

and arranged coaxially with the prism 22. The spacing is sufficient for a Ring flow of the electrolyte solution through the interior 7 of the hollow cylinder and on the rotatable Electrode 8 'can form past and corresponds approximately to the distance between the exit point of the light ar the top of the body 23 and the optical ring 10 '. The hollow cylinder 23 can also be directly on the Be placed on the bottom of the vessel, as indicated by the

ίο indicated by dashed lines in Fig.8 is lateral Openings 24 in the hollow cylinder 23 allow the solution to pass through. The light becomes partial here passed directly into the hollow cylinder 23 at the points that lie directly on the bottom of the vessel and partially

is about the solution in the side openings. With this: Modification is to be expected with only slightly increased reflection losses.

For this purpose 7 sheets of drawings

Claims (16)

Patent claims: 1. Device for the spectrophotometric investigation of products of electrochemical reactions with a) a vessel for receiving at least one reaction starting material containing dissolved Electrolytes, b) an electrode rotatably mounted in the vessel and having a flat circular one Face, c) an optically transmissive one that surrounds the electrode in a ring shape and rotates with it Body, one end face of which is flush with the end face of the electrode, d) a counter electrode and a reference electrode, e) Devices for generating a current flow between the electrode and the counter electrode, Q a tunable monochromatic one arranged at a distance from the optically transparent body Light source for illuminating the body in a direction essentially perpendicular to the face of the electrode, g) a recipient for that by the one Light entered on the frontal surface of the body,
characterized in that h) the optically transparent body consists of an optically homogeneous ring (10, 10 '), the other end face of which is plane-parallel to the one end face, and i) the receiver (4 '. 11) for receiving the other end face of the ring (10, 10') penetrating light is arranged stationary 2. Device according to claim 1, characterized in that that the electrode (8) made of a corrosion-resistant material, preferably a Plalin-Iridium alloy with approx. 10% iridium. 3. Apparatus according to claim 1 or 2, characterized in that the ring (10, 10 ') consists of one one-piece quartz ring 4. Device according to one of the preceding »claims, characterized in that the radial Thickness of the ring (10, 10 ') is at least about 1 mm 5. Device according to one of the preceding claims, characterized in that the outer The diameter of the ring (10, 10 ') is approx. 10 to 50 mm. 6. Device according to one of the preceding claims, characterized in that the thickness of the ring (10,10 ') in the direction of the light path approx is up to 10 mm, 7. Device according to one of the preceding claims, characterized in that the end face of the electrode (3, 8 ') has a concentric central region made of insulating material. 8. Device according to one of the preceding claims, characterized in that the end face of the electrode (8, 8 ') has a diameter of approximately 1 to 50 mm and preferably has an electrically active area of approximately 0.01 to 10 cm ² . 9. Device according to one of the preceding claims, characterized in that the electrode (8, 8 ') can be rubbed in via a magnetic coupling is. 10. Device according to one of the preceding claims, characterized in that the light source (6) at a distance of approx. I to 10 mm, preferably 2 to 5 mm from the ring (10,10 ') held ■ i is 11. Device according to one of the preceding Claims, characterized in that the axis of rotation of the rotatable electrode (8, 8 ') is vertical is arranged, preferably the light source κι (6) below the electrode (8, 8 ') and the receiver (11) above the electrode (8, 8') are arranged. 12. Device according to one of the preceding claims, characterized in that the rotatable Electrode (8, 8 ') is arranged in the vessel (1) at a height which is in the region of the liquid level lies 13. Device according to one of the preceding claims, characterized in that between the optical ring (10, 10 ') and the receiver (U) are preferably adjustable or exchangeable optical aperture (15) is provided 14. Device according to one of the preceding Claims, characterized in that the light source and / or the receiver have ring-shaped light-guiding elements (6, 11) which are inside the Vessel (1) are arranged coaxially to the axis of rotation of the electrode (8, 8 ') and its cross-sectional area preferably the cross-sectional area of the ring in (10,10 ') is adapted 15. The device according to claim 14, characterized in that the light-conducting elements (6, 11) fiber optic cables (4, 4 ') for supply and discharge connected by light. vs. 16. The device according to claim 14, characterized characterized in that the light-conducting element (23) of the light source is preceded by a prism through which the cross-section of an incident light beam is shaped like a ring.