Wood's Corner - pH Electrode Care
MeasureNet Spectrum Quarterly; July, 2005
Probes, we wish they were not so delicate...
Unfortunately for us, most phenomena that take place in the chemistry laboratory are beyond the ability of our natural senses to detect. We can tell if a solution is hot or cold, blue or red, or acidic or basic using our natural senses; but if we want to quantify these observations, we need to use some type of probe to measure the response.
A probe may be a device such as a thermometer costing a few dollars, or an expensive spectrometer costing hundreds of thousands of dollars. Regardless of cost, our man-made probes are delicate devices that were developed for one reason: to obtain information about a chemical or physical system. Therefore, it becomes very important for us to give our probes the proper care so they will correctly give us the needed information.
I decided to start our discussion on probe care with the pH electrode, as it is commonly called. The probe that MeasureNet Technology provides is a refillable Ag/AgCl type combination electrode. By combination electrode we mean that a single unit contains both the glass electrode and the reference electrode. In the laboratory, this probe is quite robust and ideal for the general H+ concentration measurements. The probe needs a limited amount of care, and will give you several years of service if taken care of properly.
When a new probe arrives at your laboratory, slide the rubber sealing ring down and fill the inside chamber with the saturated AgCl/KCl solution. Rinse the electrode tip with distilled water, blot it gently with a Kimwipe, and the probe is ready to be used.
You should never immerse the probe into solutions containing heavy metals, proteins, sulfides, tris-buffers, organic solutions, oils, or greases. These materials coat the glass bulb or poison the active sites on the bulb surface leading to probe failure. In some cases the probe may be restored, but before attempting to do so, contact us. Never rub the electrode surface, as this will destroy the hydrated gel layer. Be careful to blot the glass bulb gently if the need arises.
Do not store a pH probe in tap or distilled water. This will leach the ions from the hydrated gel layer of the glass and lead to electrode failure. Electrode storage solution may be purchased from chemical supply stores or prepared by dissolving 10g of KCl into 100ml of pH 4 phthalate buffer solution. Be sure to change your storage solution annually, as those little bugs will grow in almost anything.
With these few tips your pH probe will give you several years of reliable service.
Happy titrations,
Elwood Brooks, Ph.D. is a MeasureNet Senior Applications Specialist and motorcycle enthusiast. He can be reached at brooks@measurenet-tech.com