CH661799A5 - TEMPERATURE-INDEPENDENT RADIO MEASURING CHAIN FOR POTENTIOMETRIC MEASUREMENTS. - Google Patents

Description

The invention relates to a combination electrode according to the preamble of claim 1.

Single-rod measuring chains intended for potentiometric measurements, in which a measuring electrode and a reference electrode are integrated and which are used in particular for pH measurements, have long been known. If such combination electrodes are exposed to changing temperatures, a longer time generally passes, e.g. about 20 minutes until all parts of the measuring chain have adjusted to the changed temperature and the reference system consisting of the reference electrode and the reference electrolyte has the same temperature as the lead system which forms the actual measuring electrode, comprising a lead electrode and an internal buffer. In the period in which the two systems have different temperatures, usable measurement results cannot be obtained because the existing temperature differences lead to incorrect measurements.

In order to avoid such incorrect measurements, attempts have been made to use derivation and reference systems with negligibly low temperature coefficients, for example by using mixtures of redox systems as internal buffers and as reference electrolytes, the temperature coefficients of which are coordinated in such a way that they can be neglected. It is disadvantageous that numerous redox systems, e.g. Triiodide / iodide, a complicated structure is required. In addition, it has the disadvantage that a simple refilling of the reference electrolyte is not possible with such redox systems and that this can therefore only be connected to the measurement solution via a refillable electrolyte bridge. However, in order to prevent mixing of the reference electrolyte with the bridge electrolyte as far as possible, the inner diaphragm between the reference and the bridge electrolyte must have a relatively high electrical resistance (R> 5 kQ), which is not compatible with the standards applicable to pH meters is.

The object of the invention is therefore to create a combination electrode whose measuring accuracy is not influenced by the effect of larger temperature differences and whose response time to temperature changes is negligible.

The object is achieved according to the invention by the combination electrode defined in the characterizing part of claim 1.

This combination electrode has the advantage that it can be used without redox systems with the disadvantages mentioned above and that it has a simple and uncomplicated structure. The symmetrical design and / or arrangement of the derivation and reference system can be achieved in a simple manner and is not restricted to individual specific electrode materials and / or electrolyte compositions. Even with larger temperature differences between the measurement solution and the electrode, measurement results are obtained with great accuracy with practically no delay.

An electrochemically symmetrical design of the lead system and the reference system can be achieved, for example, by using a lead system which is formed from an Ag / AgCl electrode and a buffered 3-molar KCl solution as an internal buffer, and a reference system which is also made of an Ag / AgCl electrode and a 3-molar KCl solution is used as the reference electrolyte, the lead electrode and the reference electrode having the same geometric dimensions. Also suitable is a drainage system which is formed from an Ag / AgCl electrode and a partially organic, buffered KCl solution as an internal buffer. The organic solvent must be miscible with water. The KCl concentration is chosen so that the temperature coefficient of the voltage of this lead system corresponds to that of the reference system.

Preferred embodiments of the combination electrode of the type mentioned at the outset are described in claims 2 to 5.

The measurement accuracy to be achieved can be further improved if the derivation system and the reference system are arranged symmetrically in accordance with the configuration according to claim 2. This symmetrical arrangement is preferably achieved with the aid of the configuration according to claim 3.

The refinements according to claims 4 and 5 have the advantage that practically identical conditions are created for the temperature distribution and heat dissipation in the dissipation system on the one hand and in the reference system on the other hand, so that both systems heat up or cool down at the same speed and are therefore always at the same temperature .

An embodiment is described below with reference to the drawing, which shows a combination electrode in longitudinal section.

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The figure shows a combination electrode 2 with a shaft-shaped housing 4 made of electrically insulating material, e.g. Glass or plastic, such as epoxy. A lead system 6 and a reference system 8 are arranged symmetrically to the longitudinal axis of the housing 4 in the housing 4. The lead system 6 comprising the lead electrode 10 and an inner buffer 12 is surrounded by a first tubular envelope 14 made of glass. The reference system 8 comprising a reference electrode 16 and a reference electrolyte 18 is at least partially covered by a second tubular envelope 20, e.g. made of glass, surrounded. The envelopes 14 and 20 are geometrically defined by the envelopes 21, e.g. a shrink tube, fixed.

The tubular envelopes 14 and 20 are preferably of identical design and have the same diameter, the diameter being at most 3 mm. The first tubular envelope 14 and the second tubular envelope 20 serve to create the same conditions for the lead system 6 and the reference system 8.

The lead electrode 10 is connected via a first insulated line 22 to a connection element 24 in the head part 26 of the measuring chain. The reference electrode 16 is connected via a second insulated line 28 to a connection element 30 likewise arranged in the head part 26 of the measuring chain. The lines 22 and 28 are expediently made of platinum wire or insulated silver wire.

In the lower part of the measuring chain there is an ion-sensitive membrane 32, e.g. made of ion-sensitive glass. In addition, a diaphragm 34, e.g. a porous ceramic pin is provided, via which the reference electrolyte 18 is brought into electrolytic contact with the measurement solution when immersed in it.

Instead of the specified materials for the ciekiroden. the reference electrolyte and the inner buffer can also be used other materials common for measuring chains for potentiometric measurements, e.g. Kalomelelek-5 electrodes, reference electrolytes and / or internal buffer of different composition. In addition, gelled electrolytes can also be used instead of liquid electrolyte solutions. The selection is based on the respective application areas and the associated conditions and can be made by the user in individual cases without difficulty.

Reference numerals

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1 sheet of drawings

Claims (5)

661799 PATENT CLAIMS 1. Temperature-independent combination electrode for potentiometric measurements, in particular pH measurements, with a housing made of an electrically insulating material as a shaft, a drainage system consisting of an electrode and a buffer solution, which is separated from a measurement solution by an ion-sensitive membrane, which is only permeable to the ions to be measured, and a reference system, which is arranged within the housing and consists of a reference electrode and a reference electrolyte, which can be brought into contact with the measurement solution directly or via a bridge electrolyte, the lead system and the reference system being formed electrochemically symmetrically, characterized in that the derivation system (6) and the reference system (8) have identical temperature coefficients and are arranged symmetrically within the housing (4). 2. combination electrode according to claim 1, characterized in that the derivation system (6) and the reference system (8) are arranged symmetrically to the longitudinal axis of the housing (4). 3. combination electrode according to one of claims 1 or 2, characterized in that the lead system (6) from a lead electrode (10) and an inner buffer (12) at least partially from a first tubular casing (14) and the reference system (8) from one Reference electrode (16) and a reference electrolyte (18) are at least partially surrounded by a second tubular sheath (20). 4. combination electrode according to claim 3, characterized in that the first tubular casing (14) and the second tubular casing (20) are of identical design and have the same diameter. 5. combination electrode according to claim 1, characterized in that the discharge electrode (10) via a first wire-shaped, preferably provided with insulation line (22) and the reference electrode (16) via a second wire-shaped, preferably provided with insulation ( 28) are connected to connection elements (24, 30) in the head part (26) of the measuring chain or outside the housing (4).