DE1598453B2 - ELECTRODE FOR THE POTENTIOMETRIC DETERMINATION OF SULFIDIONS IN A SOLUTION - Google Patents

Description

1 2

The invention relates to an electrode made of a

the outside in contact with the solution preferred embodiment by drawings

non-porous membrane and one with the inside of the explained. It shows

Membrane in contact, a fixed po- F i g. 1 is a schematic, shown in section Potential maintaining arrangement for the potential 5 side view of a for the determination of sulphide metrics Determination of sulfide ions in a ion in a solution with the suitable electrode sung. Features of the invention,

There are already several instrumental procedures, FIG. 2 is a partially sectioned schematic

with the help of which the determination of the sulfide ions in tables side view of a measuring arrangement with a

a solution carried out with quite a selectivity io electrode for the determination of sulfide ions,

can be. Frequently, therefore, spectrophotomic F i g. 3 is a graph showing the effec-

trical methods are used, but since in solution a change in pH value affects the response

Sensitive sulfide ions absorb only a little light, hold opposite the electrode according to the invention

usually have to. Reagents are added, sulfide ions illustrated.

which react with the sulf.dipnene and absorb light. It has been shown that by anodic deposition

Create connections. There are also polar electrodes made of silver in sulfide solutions

graphic and amperometric detectors are known, generally subject to fluctuations and not

which, however, result in a precise control of the current of the reproducible potentials, whereby the in the

test medium requirements, if the determination of the literature contained in the literature are confirmed

should take place continuously. Also the use of 20 den. Instead of using the electron transfer

potentiometric electrodes is already the proposed reaction

gen and by numerous experts for the sulfide 2Αε 4-S ⁼ - »Ae S 4- 2e ~ Ci

determination has been investigated (J. I ν e s and G. ■.. ν «

J an zt, "Reference Electrodes", Academic Press, the invention is based as an electron potential source

New York, 1961, Chapter VII). However, there has been 25 on the use of a membrane made of silver sulfide,

found that only silver and mercury redox which contain practically no free silver, which one

elements respond to sulfide ion activity, while such a reaction could undergo. The one below

Schwan electrodes made of these metals in accordance with its application in the

show tensions and non-reproducible voltages. poteniometric electrode technology used

Although electrodes of this type are used to determine 30 pressure "membrane" refers to a leaf or

of titration endpoints are usable, they lack a film-like structure, and indeed generally un-

the state required for absolute activity measurement - depending on its flexibility or curvature, that

mobility. defines two boundary surfaces, between which a

Electrodes for potentiometric determination of the charge transfer takes place.

The ion concentration of solutions can be determined according to FIG. 35 FIG. 1 illustrates an electrode 20 having FIGS

U.S. Patent 3,219,556 is also a non-porous one, comprising features of the invention that unite one on both

Have semiconductor material existing membrane. Ends of open elongated, hollow containers or

Such a semiconductor material, which usually has in shaft 22. This shaft 22 typically consists

Form very thin layers of, for example, 38 μ wise from a liquid-impermeable, practical

Thickness is used, also brings only a small 40 stiff, electrically insulating material, such as un-

Stability with itself. In addition, polyvinyl chloride, polytetrafluoroethylene, and glass increase with semi-soft items

conductor material durch.minor impurities of the or the like, the opposite of which the sulfide ions contain-

Conductivity considerable, so that in this regard the solutions with which the shaft 22 is in contact

considerable difficulties can arise, especially since it can be brought within practical limits before- \ J

is inert due to the technological cleaning of 45 semiconductors.

materials only an "almost" free of imperfections. One end of the shaft 22 is covered by an isolating conductor crystal lattice can be achieved. Another disc or membrane 24 covered or sealed-off disadvantage of semiconductor materials such as Ge or Si, tet, which consists of practically non-porous, high purity in their slight surface oxidation, which consists of sedentary silver sulfide, which is practically none takes place when the materials are stored in air and contains 5 ° free metallic silver. The membrane 24 the conductivity of the material or its purity can in itself be quite thick and, for example, a adversely affected. Have a thickness of about 6.35 mm; usually

In contrast, the object of the invention is preferred to the structure that is thinner. The membrane

Creation of an improved stable electrode, which can be 24 with the help of an adhesive, such as an epoxy

a relatively low resistivity and 55 resin glued across one end of the shaft 22

has a high cationic conductivity, which is, however, preferably, as shown, to one

Conductivity due to the migration of silver ions around the circumference of the opening provided in the shaft 22

is caused. Around arranged O-ring 26 placed

According to the invention, this object is thereby achieved and is provided by an annular flange 27 on the

read that the membrane made of high-purity, silver-free 60 shaft 22 screwed cuff 28 in solid

Silver sulfide consists. System held against the O-ring 26. If the

Advantageously speaking, the cuff 28 according to the invention is rotated in the correct direction,

Electrode repeatably for sulfide ion activity, it displaces axially and urges the membrane 24 _;

so that the entire measuring device is continuously in sealing contact with the O-sealing ring 26 so that

Measurement voltage supplies which one end of the shaft 22 is sealed in a simple logarithmic 65. As well as

mixed relationship to the sulfide activity and through the O-ring 26 and the sleeve 28

a short response time to changes in the sulfide are preferably made of a plastic, such as poly-

activity ensures a delay-free regulation. vinyl chloride.

3 4

Inside the shaft 22 is in electric and is cooked for 2 hours. The process steps of physical contact with the inner surface of the membrane - the settling, decanting and washing in boiling burner 24 a charge transfer device with the water are repeated several times, and tightly sealed silver concentration either in metallic borrowed the precipitate is filtered off and provided in distilled or ionic form. In the case of the. 5 system water slurried, filtered, washed, and embodiment of the invention consists of this one, although initially with dilute nitric acid and then direction of a reference electrolyte 30, for example with further amounts of distilled water, and from it an aqueous saturated KCl-AgCl solution. Then dried under vacuum. The dried electrolyte 30 is an inner reference electrode 32, when the precipitate is finally _{slurried in CS 2} , for example a known Ag-AgCl element added to dissolve any residual sulfur, filtered, and immersed. This combination of the electrolyte 30 washed with the acetone and at 80 to 100 ° C in the air

Reference electrode 32 is a means by which it is dried. ,

Help the inner surface, d. H. those with the electrolyte 30 Although a variety of methods are known,

contacting surface of the membrane 24, around the Ag ₂ S powder to a non-porous membrane

If a practically stable or fixed potential is applied, 15 deform, the membrane 24 is preferably through

den can, suggesting the use of a solid silver, pressing under vacuum into thin cylindrical plates

concentration is due. If the elec- trolettes or disks are deformed; this procedure uh-

trolyt 30, for example, simply made of solid silver sprayed for the manufacture of KBr tablets for

the infrared spectroscopy occurs between the silver and the silver sulfide,

at a fixed potential evidently through 20 sintering or casting processes are themselves in controlled

Ion transfer on a charge transfer. Only extremely difficult to apply in atmosphere, what

. In such a case, the reference electrode used must be traced back to the fact that the Ag ₂ S tends to

trode32 simply a metal wire, like a copper one, to decompose near its melting point,

wire, because the silver-copper compound is a. Pressing takes place in a die made of hardened

Charge transfer caused by electrons. 25 _: and polished steel, being during compaction

The other open end of the shaft 22 has an annular cap 34 with an optionally a 'moderate negative pressure applied, an opening into which can be. To achieve non-porous disks, a conventional coaxial cable 36 is sealed, the minimum pressure of which should be of about 4220 to 5270 kg / cm ² inner conductor connected to the reference electrode 32. If the Ag ₂ S is sufficiently pure while its outer conductor is used as an electrostatic shield, the steel die molds are not used. · Grabbed.

The most important considerations in making a membrane, the two solutions from each other of the electrode according to FIG. 1 relate to the structure that separates, one of which contains silver ions with a fixed con-membrane 24 and the type of centration described above and the other is a sample solution seal between the electrode shaft 22 and the 35, creates a membrane according to the known Nernst equation 24. The other parts as well as the shape and electromotive force:
the size of the electrode 20 are not particularly critical

table and can, depending on the E _n . = const + - ■■ In ^ _Ag ⁺ , (1)

can be selected for their intended use. F.

Silver sulfide is unusual in that it is not 40

is only insoluble in water to a high degree (K _sp 10 ~ ⁵² ), in which A _Ag + the silver ion activity in the prosonder, at least in the form (ß), which it means in the case of low solution. However, since Ag ₂ S assumes an extremely insoluble temperature and is relative, A ^ _s * is determined by the presence of the fixed, low electrical mass resistance in the compound and can be determined by the quotient of the compound with an extraordinarily high cationic solubility product K _sv of Ag ₂ S and the sulphide capacity, ie, the electrical conductivity activity As ^{= of} the sample solution can be expressed,. through the silver sulfide crystal lattice is in dh

primarily by a migration of silver ions and _o g

not by a one on sulfide ions or electrons - ^ Ag ⁺ - -— = · (2)

dormant guiding mechanism caused. The inven- 50
proper silver sulfide membrane 24 must have a

roughly. have a high degree of purity, because that ^: Equation (1) can then be described as follows

Presence of foreign ions or molecules, ben:

such as Ag ₂ O, porosity and poor mechanical properties RT ^

shafts. The for the reaction with the SuI- 55 ^E ™ = ^const - ~ zj ^{ln As} · ( ³ )
fidion available membrane surface
should be as free as possible from elemental silver and

Sulfur, as both substances, especially the silver, .. This logarithmic relationship is based on in operation

cause unstable and deviating potentials. the manner still to be explained is observed.

A preferred method for producing the SiI-60 according to FIG. 2 becomes the electrode according to the invention

.bersulfids consists in the purest AgNO ₃ in aqueous 20 arranged in operation so that the outer surfaces of the

Solution of a small excess of aqueous Na membrane 24 with one to be tested, which is to be determined-

. Add trium thiosulfate, whereby Ag ₂ S precipitates. The solution 40 containing the sulfide ions in contact

Addition takes place at room temperature under good standing. Solution 40 also provides a standardized

Stir. 65 tension electrode 42 brought into contact.

The precipitate will take about 1 hour to settle. The reference electrode 42 is typically a long one left to stand, whereupon the solution decanted, the arrangement borrowed in a conventional glass solder fresh distilled water is added and 1 to the flask is accommodated and an Ag-AgCl electrode

in saturated KCl-AgCl solution, which by an asbestos fiber compound from an 1 molar NaOH solution is separated. The latter solution occupies the lower end of the piston and is about the usual fiber connection indicated at 44 coupled with the solution 40. Both the electrode 20 as also the reference electrode 42 are electrically connected to the relevant inputs of an electrometric device 46, which is preferably a conventional voltmeter with high input impedance.

In operation of the arrangement according to FIG. 2, under constant temperature conditions, a potential Eref of a practically fixed value arises between the reference electrode 42 and the solution 40, regardless of the sulfide concentration of the solution. Another potential E _m develops across the membrane 24 between the inner electrolyte 30 and the solution 40, but this potential depends on the activity or concentration of the sulfide ions in the solution 40 or changes logarithmically as a function thereof. Since the potential Eint between the reference electrode 32 and the electrolyte 30 also has a fixed value, the potential occurring between the electrodes 42 and 20 is the sum of E _m , E _re f and Emt and thus changes only as a function of E _m . The potential £ «sought can easily be measured on the electrometric device 46, as a result of which the presence and the activity of the sulfide ions in the solution 40 are indicated.

The electrode according to FIG. 1 was in an arrangement like that according to FIG. 2 if there are several Experiments to determine the nature of the sulfide ion response to those in the following examples way described:

example 1

Several test solutions were prepared by weighing approximately 24 g of Na ₂ S · 9 H ₂ O and dissolving it in 1 liter of 1 molar NaOH. Serial dilutions were made to lower concentrations using 1 molar NaOH solution as the diluent. This resulted in a series of samples with constant NaOH base concentration and varying amounts of Na ₂ S. Since the solutions had almost constant ion concentrations, the activity coefficient of the sulfide ions was also considered to be almost constant. The constant pH value determined by the NaOH base content also ensured that a constant fraction of the total sulfide present was present in the form of S ⁼ . Under these conditions, an evaluation of the measured potential as a function of log Na ₂ S should result in a straight line with a gradient determined by RTJ2F or 29 mV per change in Na ₂ S concentration by one decade. The following table of measurement results shows excellent agreement with theory.

The time response of electrode 20 is limited by solution transfer and is 1 minute or less per change in NaS concentration by a decade. Response time can be reduced significantly by providing a quick drain to the outside of the Ag ₂ S disk. At room temperature, the electrode 20 reproduces its initial potential in standard solutions over periods of several hours with an accuracy of approximately 1 mV. The actual changes based on the electrode 20 itself are even smaller, since some of the deviations can be traced back to the "standard" solutions, which in fact are not standards, because it is difficult to allow the stored alkali sulfide standard solutions to be oxidized by air Prevent sulfur and other products.

Response behavior in mV Concentration Na ₂ S Reference electrode (Moles) ^; -750.5 5 x ⁴ ΙΟ- -761.5 1 × 10- ³ -783 5 × 10- ² -792 1 io- ² -819 1 · 10 ¹

Example2

Measurements were made to determine the sulfide concentration range made, over which ■ the electrode 20 is able to measure the sulfide activity.

For this purpose, two titrations were carried out simultaneously in the same solution with a pH electrode and the electrode 20 according to the invention, the solution initially containing 1 mol of NaOH and 1 mol of Na ₂ S ■. The titration was carried out with 1 mol of HCl. The data obtained were based on the method shown in FIG. 3 evaluated way shown.

In Fig. 3 the logarithm of the S ⁼ activity is plotted as a function of the pH value. In the individual response behavior of each electrode, namely one to the pH value and the other to the sulfide, two deviating gradients occurred, which are assumed to be due to a stepwise conversion from S ⁼ to HS and finally to H ₂ S are due. The curve according to F i g. 3 appears to contain two straight line segments, the slope of which is a change in S ⁼ activity of about one decimal place per change in solutions of high pH, ie from pH> 7, by a decade and two decimal places per change in solutions Low pH, i.e. pH <7, to indicate a decade. The linear relationship with approaches the value 1 approaching pH values indicates that the erfindungsgemißs electrode 20 even when free SulfiJionenkonzentrationen up down seems to be measured to 10 ^"17 moles of the sulfide.

The silver sulfide membrane 24 has a much lower resistance than the usual pH glass electrode and works at temperatures up to the freezing point of most aqueous solutions. The performance at high temperatures is in large part limited by the internal pressures generated by an aqueous filler solution or the liquid electrolyte 30 located inside. However, if there is little loss of freedom to adjust the isopotential /? 3 ⁼ to an optimal value, the solution filled into the interior can be replaced by a metallic silver coating that is directly connected to the inner surface of the membrane 24 and the reference electrode 32. As a result, the practically applicable upper temperature limit can be raised to about 175 °; At this temperature the Ag ₂ S changes into its solid state («) with simultaneous mechanical rupture of the membrane 24.

The electrode 20 of the present invention is quite specific in its response to sulfide ions. The extraordinary insolubility of Ag ₂ S prevents interference due to metathetic reactions with all types of ions present in the aqueous solution with the exception of mercury ions, which, however, cannot be present to any significant extent in solutions which contain significant amounts of sulfide ions. The electrode 20 is generally not attacked by organic substances and complex-forming ions, which are known to attack other silver salt electrodes, such as the Ag-AgCl reference electrode.

Of course, the electrode 20 can be made in other configurations. Advantageously, high density silver sulfide, as described above, is readily machinable into a variety of shapes. For example, a flow-through sulfide electrode can be made by pressing Ag ₂ S into a cylindrical shape and providing it with an axial central bore so that a hollow tube with a wall thickness of up to about 6.35 mm is formed. The outer surface of the tube can then be electrically connected to a wire at a fixed contact potential, for example by covering the tube with an annular silver strip and attaching the wire to the latter. The outer surface is then sealed with insulation. As the solution 40 containing sulfide ions is passed through the interior of the tube, the desired potential is created across the tube from the interface with the solution to the silver streak.

Claims (6)

Patent claims: 1. Electrode consisting of a non-porous one which is in contact with the outside of the solution Membrane and one with the inside of the membrane in contact, a fixed potential maintaining arrangement for potentiometric determination of sulfide ions in one Solution, characterized in that the membrane (24) is made of high-purity, silver-free Silver sulfide consists. 2. Electrode according to claim 1, characterized in that the membrane (24) made of highly compressed Material consists. 3. Electrode according to claim 1 or 2, characterized in that the membrane (24) is sealing arranged on the open end of a hollow body (22) made of electrically non-conductive material is. 4. Electrode according to claim 3, characterized in that in the interior of the hollow body (22) a Reference electrode (32) is arranged, which is via a solution (30) with a practically fixed silver ion concentration is in electrical contact with the membrane (24). 5. Electrode according to claim 3, characterized in that the inside of the hollow body (22) arranged reference electrode (32) over a body (30) made of metallic silver in electrical Is in contact with the membrane (24). 6. Electrode according to claim 4 or 5, characterized in that the reference electrode (32) is a Metal wire is. 1 sheet of drawings 109 532/323