DE4312096B4 - Process for automated pH measurement and calibration and device for carrying out the process - Google Patents

Abstract

Method for automated pH measurement and calibration of ion-sensitive electrodes designed as single-rod electrodes, in particular glass electrodes, characterized in that the sensitive membrane of the ion-sensitive electrode in the measuring mode by a measuring solution flowing out of a first capillary at 0.01 ml / min to 5 ml / min is flowed without immersing in the measurement solution, the measurement signal is recorded and registered, after a time interval determined by the accuracy requirements, which is between 2 hours and 150 hours, flowed through in succession through further capillaries with calibration liquids in the same way as in the measurement mode, after a time of 1 min to 2 min the supply of the calibration liquid is stopped and the pH measurement is continued by the flow of measuring solution.

Description

The invention relates to a method for automated pH measurement and calibration of ion-sensitive Electrodes, in particular glass electrodes, and a device to carry out of the procedure. The designated method should - preferably using the device described - at the pH measurement in various areas of process chemistry but also in laboratory use Find.

It is already known pH measurements using glass electrodes. Not yet satisfactory dissolved The problem is the calibration of the glass electrodes before, during and after their use. The usual The procedure is to use pH electrodes for calibration and / or Recalibration from the process bath or the measurement solution in one or several containers to convict the calibration Solutions contain. In these containers the glass electrode remains until the surface of the desired Equilibria hired again.

Technical solutions have also been proposed to calibrate the electrode during or in addition to the process of the actual pH measurement. In the DE-PS 31 18 771 describes a procedure according to which an automatic calibration is carried out in such a way that, with the aid of compressed air, the measuring solution, cleaning and calibration solution are pressed sequentially into a small cavity with an immersing electrode.

The GB-PS2051371 describes an automatic calibration system for an online-coupled measuring device with ion-sensitive electrodes. A 3-way valve alternately feeds sample solution, buffer and standard solutions into a flow cell construction and the measured cell voltages are processed automatically.

In the U.S. Patent 4,260,950 describes a device in which the portable pH measuring device contains a buffer reservoir. The arrangement permits mutual positioning of the electrode in the measurement solution and in the buffer reservoir. The cell voltage measured in the buffer solution is the basis for the automatic calibration.

The solutions presented in the references assume that the calibration of the pH electrode takes place in a vessel separate from the measurement solution, the advantage being seen in the fact that after the calibration an automatic continuation of the pH measurement on the test object is made possible. This is also the opinion of the inventors of EP 0 080 680 , The technical solutions of US 4,490,236 and US 3,556,950 bring no progress. The last-mentioned US patent describes a method for the automated measurement of ion concentrations and for the calibration of an ion-sensitive electrode, in which the sensitive membrane of this electrode is flowed through by a measuring solution without being immersed in the measuring solution.

The disadvantage of the solutions described is in that only the principle of separation of the measuring device varies from the measurement solution and Part of it, however, quite cleverly - is modified. Thereby becomes the discontinuity obtained, which carries the calibration process into the measuring process as a whole.

The object of the invention is in the specification of a method for automated pH measurement and Calibration and creating a universally applicable electrode arrangement, the without change the electrode positioning in a simple manner both continuous as well as discontinuous pN measurements and calibrations. In the case of technical applications, it should be achieved that in one long-lasting measuring and monitoring process the glass electrode can be recalibrated without manual labor or time-consuming Control devices and mechanisms must be used.

The object is achieved according to the invention by the process sequence described below:
The ion-sensitive electrode, which is designed as a special combination electrode, is positioned stationary together with the flow capillaries for measuring and buffer solutions near the measuring medium

• 1. Bei geschlossener Pufferzufuhr strömt Meßlösung, die beispielsweise als Bypass von einem Prozeßstrom abgezweigt wird, auf die Glasmembran der ionensensitiven Elektrode. Die Glasmembranmulde wird dabei vollständig mit der Meßlösung ausgefüllt und elektrolytische Verbindung zum Diaphragma gewährleistet. Überschüssige Meßlösung fließt über den Membranrand ab.
• 2. Zur Kalibrierung wird die Zufuhr der Meßlösung unterbrochen. Dann erfolgt ein Anströmen mit Pufferlösung A (pH-Wert von A sollte möglichst mit dem pH-Wert des Innenpuffers der verwendeten Einstabmeßkette übereinstimmen) zum Abgleich des Asymmetriepotentials. Diese verdrängt nach kurzer Zeit die Meßlösung aus der Membranmulde. Das sich einstellende Gleichgewichtspotential wird registriert.
• 3. Die Pufferzufuhr A wird gestoppt und die Elektrode mit Puffer B angeströmt. Die stationäre Meßkettenspannung dient zum Bestimmen der Elektrodensteilheit, die wiederum automatisch zur korrekten pH-Anzeige erfaßt und verarbeitet wird.
• 4. Die Rückkehr zum Meßmodus erfolgt durch Schließen der Pufferzufuhr und Anströmen mit Meßlösung wie unter 1. ausgeführt. Der pH- Wert wird nun unter Berücksichtigung der Kalibrierdaten aus der Meßkettenspannung neu berechnet (im allgemeinen mikroprozessorkontrolliert).

One of the two calibration buffers can flow out alternately through the one capillary during calibration. This is made possible by a T-distributor at the top capillary end and solenoid valves. The commercially available buffer solutions are located in storage containers above the electrode, so that the buffer solutions flow out through the hydrostatic pressure. The measuring solution flows through the second capillary onto the glass membrane during the measuring process. The supply of buffer solution is prevented. The working sequence (a) pH measurement in the medium of interest - (b) calibration - (c) return to the measuring process is realized in the following steps:

• 1. When the buffer supply is closed, the measurement solution, which is branched off from a process stream as a bypass, for example, flows onto the glass membrane of the ion-sensitive electrode. The glass membrane trough is completely filled with the measuring solution and electrolytic connection to the diaphragm is guaranteed. Excess measurement solution flows out over the edge of the membrane.
• 2. The supply of the measurement solution is interrupted for calibration. Then flow with buffer solution A (pH value of A should be as close as possible to the pH of the internal buffer of the rod used electrode match) to adjust the asymmetry potential. After a short time, this displaces the measuring solution from the membrane trough. The equilibrium potential that arises is registered.
• 3. The buffer supply A is stopped and the electrode with buffer B flows against. The stationary electrode voltage is used to determine the electrode steepness, which in turn is automatically recorded and processed for the correct pH display.
• 4. Return to the measuring mode is done by closing the buffer supply and inflow with measuring solution as described under 1.. The pH value is now recalculated taking the calibration data from the electrode voltage into account (generally microprocessor-controlled).

The proposed procedure enables the programming of time intervals that repeat the calibration cycle, according to the points 2 and 3 allowed.

The results show that continuous maintenance-free pH measurement for industrial plants, but also on a laboratory scale with good accuracy guaranteed is.

To implement the method, a device according to the invention is proposed which has proven to be expedient:
The following based on the 1 The measuring arrangement described is used for the potentiometric measurement of ion concentrations and for the calibration of the ion-sensitive electrodes used, preferably glass electrodes, in single-rod design. The diaphragm ( 6 ) of the external reference system opens directly into the volume increment of the pH-sensitive glass membrane ( 1 ), which is trough-shaped in the preferred embodiment of the invention. Near the glass membrane are two capillaries ( 4 ) and ( 8th ) arranged. These are preferably fixed and are used to supply measurement and buffer solutions. To ensure mechanical stability, in particular to minimize the risk of breakage, the glass electrode with the fixed inflow capillaries is housed in a shielding sleeve in such a way that the inflow solutions have direct and unimpeded access to the glass membrane and on the bottom side excess measurement and calibration solutions that arise due to the process, is guaranteed.

• – Strömungsgeschwindigkeit der Meß- bzw. Kalibrierlösung: 0,01 ml/min... 5 ml/min.
• – Positionierung der Kapillare: nicht unbedingt zentrisch über der Membranmulde, Abstand ca. 1 mm bis 5 mm.

The pH measurement is carried out by using the 1 ) attached first capillary ( 4 ) Measuring solution is rinsed on the membrane surface. The membrane trough is completely filled with measuring solution in the measuring state (dead volume of the trough-shaped volume increment 50 ul-150 ul). As the electrolyte continues to flow in, a rapid renewal of the material to be measured and thus effective rinsing when changing between different measurement or calibration solutions is achieved. Excess electrolyte flows over the edge of the glass membrane into a waste container or is returned to the material to be measured. The following parameters can be varied within a wide range without any noticeable influence on the functioning of the system:

• - Flow rate of the measuring or calibration solution: 0.01 ml / min ... 5 ml / min.
• - Positioning of the capillary: not necessarily centrally over the membrane trough, distance approx. 1 mm to 5 mm.

• – Das Elektrodensystem verbleibt während Messung und Kalibrierung am gleichen Ort.
• – Die Gestaltung des Meßregimes (kontinuierlich, diskontinuierlich, periodischer Wechsel zwischen Meß- und Kalibrierlösungen ...) kann leicht über die Ansteuerung von Magnetventilen bzw. Pumpensystemen automatisiert werden.
• – Für Kalibrierung und Messung sind nur geringe Volumina notwendig (entsprechend den Totvolumina des Gesamtsystems und der Ansprechzeit der Elektrode sind zur definierten Signaleinstellung ca. 500 μl Kalibrierpufter erforderlich).
• – Aufgrund des Spüleffekts der jeweiligen Meß- bzw. Kalibrierlösung in der Fließanordnung werden pH-Verfälschungen durch das vorherige Meßgut ausgeschlossen.

The analytical advantages of the apparatus are clear from the following properties:

• - The electrode system remains in the same place during measurement and calibration.
• - The design of the measuring regime (continuous, discontinuous, periodic change between measuring and calibration solutions ...) can easily be automated by controlling solenoid valves or pump systems.
• - Only small volumes are required for calibration and measurement (approx. 500 μl calibration buffer is required for the defined signal setting in accordance with the dead volumes of the overall system and the response time of the electrode).
• - Due to the rinsing effect of the respective measuring or calibration solution in the flow arrangement, pH falsifications by the previous sample are excluded.

• 1. Überprüfung der Richtigkeit und Reproduzierbarkeit der vorgenommenen pH-Messungen
• 2. Messung realer Proben über längere Zeiträume und Rekalibrierung des Elektrodensystems

The function of the measuring arrangement will be demonstrated below by experimental examples:

• 1. Checking the correctness and reproducibility of the pH measurements made
• 2. Measurement of real samples over longer periods and recalibration of the electrode system

to 1.) NBS buffer solutions with pH values of 7.00 and 4.51 were mutually turned from storage containers by opening and closing onto the membrane surface ( 1 ) rinsed. Previously, the electrode system used had been calibrated in a conventional manner by completely immersing the single-rod chain using the buffer solutions mentioned above.

How 2 shows, with the arrangement according to the invention the pH values of the standard buffer solutions are reproducibly adjusted with a precision of 0.01 pN units.

to 2.) 3 documents the functioning of the calibration, measurement and recalibration of the measuring system using pH measurements in a technical rinsing bath. The pH of the rinsing bath was a constant 5.23 during the test period (multiple checks using a reference pH measuring system).

• – Probelösung strömt auf die Membran (85 mV)
• – Anströmen der Membran mit Puffer pH = 4,51 (124.5mV)
• – Anströmen der Membran mit Puffer pH = 7,00 (-12 mV)
• – Probelösung strömt auf die Membran (85 mV)

the 1 , Measuring and Calibration cycle in 3 are based on the following operations:

• - Sample solution flows onto the membrane ( 85 mV)
• - flow onto the membrane with buffer pH = 4.51 (124.5mV)
• - flow onto the membrane with buffer pH = 7.00 (-12 mV)
• - Sample solution flows onto the membrane ( 85 mV)

The parameters of the calibration function U = Uconst - S • pH are determined from the two measurements with calibration buffers (Uconst. = 371.6 mV, S = 54.8 mV). This gives the sample a pH of 5.22.

To 2 The measuring cycle described above was repeated for days. How out 3 evident, changes in the electrode function occurred. The parameters of the electrode function are now Uconst. = 367.3 mV and S = 54.6 mV. With this electrode function newly determined by the calibration, a pH value of 5.22 is again determined for the sample solution.

The examples given prove that in terms of the correctness and reproducibility of the pH measurement Properties of the arrangement according to the invention.

The 1 shows a schematic diagram of the measuring arrangement for automated pH measurement, the reference numerals meaning the following:

1

pH-sensitive Glass membrane;

2

reservoir the external reference system;

3

inner derivative;

4

capillary to flow towards with measuring solution (ML);

5

outer lead electrode;

6

 diaphragm;

7

coaxial cable;

8th

capillary to flow towards with buffer solutions PI and P;

The 2 shows a pH value - time diagram to illustrate the reproducibility of the calibration with NBS buffers pH = 7.00 and 4.51

The 3 shows an electrode voltage - time diagram to demonstrate the calibration, recalibration and acquisition of measured values over a period of 48 h.

Claims (5)

Method for automated pH measurement and calibration of ion-sensitive electrodes designed as single-rod electrodes, in particular glass electrodes, characterized in that the sensitive membrane of the ion-sensitive electrode in the measuring mode by a measuring solution flowing out of a first capillary at 0.01 ml / min to 5 ml / min is flowed without immersing in the measurement solution, the measurement signal is recorded and registered, after a time interval determined by the accuracy requirements, which is between 2 hours and 150 hours, flowed through in succession through further capillaries with calibration liquids in the same way as in the measurement mode, after a time of 1 min to 2 min the supply of the calibration liquid is stopped and the pH measurement is continued by the flow of measuring solution. Method according to claim 1, characterized in that the time intervals calibration by automatically controlled inflow of calibration solutions depending on the application can be set. Method according to claim 1 and 2, characterized in that the supply from measuring and calibration solutions by controlling solenoid valves and / or pump systems becomes. Method according to claims 1 to 3, characterized in that for pH measurements with high Requirements for the correctness of the electrode system and the supplied solutions are thermostated. Device for the automated calibration of combination electrodes, characterized in that the sensitive membrane ( 1 ) the combination electrode is trough-shaped, a diaphragm ( 6 ) of the external reference system in electrolytic contact with the trough-shaped membrane ( 1 ), one capillary or several capillaries ( 4 . 8th ) compared to the sensitive membrane ( 1 ) are fixed in such a way that liquid flowing out under pressure onto the sensitive membrane ( 1 ) flows.