PRO6138 — flat pH composite electrode: a flat-membrane pH electrode with an unbreakable polycarbonate body for surfaces — skin, leather, paper, textile, cut food surfaces and protein-containing samples. It has a refillable Ag/AgCl reference, a single fabric junction and a BNC (Q9) connector for the meter.
How the flat pH composite electrode is built
The reference system is Ag/AgCl. It is refilled with 3 M KCl, so the flat pH composite electrode can be kept in service by topping up the electrolyte.
Technical parameters
| Model / order code | PRO6138 |
| Electrode type | pH composite (combination) electrode |
| Measuring range | pH 0 – 14 |
| Working temperature | 5 – 60 °C |
| pH sensor | Flat glass membrane |
| Body material | Polycarbonate |
| Reference system | Ag/AgCl |
| Junction | Single fabric |
| Fill solution | 3 M KCl |
| Temperature sensor | None |
| Dimensions | Φ12 × 120 mm |
| Connector | BNC (Q9) |
Full data sheet: Technical Specification (PDF), two pages.
Is this the right electrode for your sample?
The flat pH composite electrode is recommended for: meat, fish; fruit, vegetables; paste; plasma, gastric juice, urine; pharmaceutical formulations; protein-containing samples, enzyme solutions, agar media; leather, paper, textile; skin.
Some samples need a different design. The selection by sample in the electrode catalogue shows which electrode is recommended for which sample; the most frequent cases are:
- Samples above 60 °C: an electrode specified to 80 °C, or the high-temperature electrode to 120 °C.
- No refilling wanted: a non-refillable, low-maintenance electrode.
- One sensor instead of two: the 3-in-1 pH composite electrode with a built-in temperature sensor (ATC).
- Liquid samples in a beaker: a general-purpose electrode with a spherical bulb.
One electrode for benchtop and portable pH meters
The flat pH composite electrode does not belong to one meter. The same electrode works on a benchtop meter in the laboratory and on a portable meter in the field — what decides is the input of the meter, not its housing. Its BNC (Q9) connector fits every meter with a BNC input; see the pH meters in the catalogue.
Because the flat pH composite electrode has no temperature sensor, the meter compensates temperature from a separate temperature probe or from a value entered by hand. After moving the electrode to another meter, calibrate again: calibration data are stored in the meter, not in the electrode.
From sample to result
Preparing the surface
Measure on a clean, flat part of the surface. A dry surface — skin, leather, paper, textile — gives no reading, so wet the spot with a few drops of distilled water and place the membrane on it. Use the same amount of water and the same contact time for every reading, so that results can be compared.
Preparing the flat pH composite electrode before calibration and measurement
- Remove the protective cap from the flat membrane at the bottom of the electrode and rinse the tip with distilled water; blot the drops, do not rub — rubbing charges the glass and slows the response.
- Check the electrolyte level; if it is low, top up with 3 M KCl solution.
- Leave the refill hole open during calibration and measurement.
Calibrating the flat pH composite electrode
- Prepare fresh buffers at the same temperature as the samples and start the calibration on the meter.
- Immerse the tip in the first buffer, pH 7.00; wait for a stable reading and confirm the point.
- Rinse the tip with distilled water and blot the drops.
- Immerse the tip in the second buffer (pH 4.01 for acidic samples, pH 10.01 for alkaline); wait for a stable reading and confirm the point.
- The meter shows the electrode slope. If it is outside the limits given in the meter manual, clean the electrode, take fresh buffers and repeat the calibration.
Measuring
Keep the refill hole open while measuring and the electrolyte level above the sample. Press the flat membrane lightly against the wetted surface so that the whole membrane and the junction touch it, wait for a stable value and read pH together with the temperature. Rinse the membrane between measurements.
pH on skin, leather, paper and textile
The flat pH composite electrode measures where there is only a surface and no liquid sample.
Skin and cosmetics: the pH of the skin surface is measured in dermatological and cosmetic testing, before and after a product is applied.
Leather, paper, textile: surface pH is checked in production and in incoming control, because acid or alkali left in the material affects its durability.
Microbiological culture media: the flat membrane is placed directly on the surface of solidified agar, so the pH of a poured plate can be checked without cutting or melting the medium.
Cut food surfaces: meat, fish, fruit and vegetables can be measured on a fresh cut without preparing an extract.
pH meters for the laboratory and for the field are in the electrochemistry catalogue.
Care, storage and accessories
Storing the electrode
Never store the flat pH composite electrode dry and do not keep it in distilled water. Between measurements and overnight, keep the tip in the protective cap filled with 3 M KCl storage solution and close the refill hole. Top up the reference with 3 M KCl filling solution when the level drops.
When the reading is slow or drifts
The usual causes are a membrane that has dried out, a blocked junction, old buffers or a low electrolyte level. Soak the tip in storage solution, top up the 3 M KCl, use fresh buffers and calibrate again.
Accessories
- pH buffers in sachets: single-use portions, always fresh, practical for field work and occasional calibration.
- 3 M KCl solution: for refilling the reference and for storing the electrode.
Video: preparing, using and storing the electrode
The film shows treatment before first use, connecting the flat pH composite electrode to the instrument, rinsing between samples and storage after measurement.
Video: how the electrode holder is installed and used
Buffers, solutions, cables and holders are in the accessories section of the electrochemistry catalogue.
Frequently asked questions
Does the flat pH composite electrode fit my meter?
The flat pH composite electrode fits any meter with a BNC input. Meters with another input type need a different connection, so tell us the meter model before you order.
How often should I calibrate the flat pH composite electrode?
For routine work, calibrate at the start of each working day and whenever the sample type or temperature changes noticeably. For occasional measurements, calibrate before each series.
Where can I read more about electrodes?
The electrode catalogue has an electrode guide on sensor shapes, body materials and junctions, a full selection matrix and an overview of connectors. The electrochemistry overview covers pH, conductivity, dissolved oxygen and ion measurement.
