METTLER TOLEDO : pH Essential Guidelines

Introduction

 This guide is intended as a first introduction to the practice of pH measure-ments. The basic knowledge that is needed to understand a pH measure-ment is given. The tools necessary for pH measurements are explained and a step-by-step guide to perform pH measurements is provided. Finally, some practical tips & hints are given, as well as a troubleshooting guide for pH measurements.

  

1. What is pH?

 

Probably you have already heard the word pH or the terms acidic or alka-line. But, what is pH exactly? And why do we classify an everyday liquid like vinegar as being acidic?

 

pH is a chemical property of an aqueous solution, which indicates its degree of acidity or alkalinity. pH values can vary from 0 to 14, where pH values from 0 to 7 are called acidic and pH values from 7 to 14 are termed alkaline, while a pH value of 7 is neutral.

 

The quantitative difference between acidic and alkaline substances can be determined by performing pH value measurements. A few examples of pH values of everyday substances and chemicals are given in the following figure:

 

 

2. Where and Why is pH Measured?

 pH measurement is a widely applied analytical technique in various fields. These examples describe the importance of pH in an extensive range of applications, demonstrating why it is so often determined:

 

Furthermore, pH can be verified in different steps of the same process. For instance, the pH values at different stages of beer brewing will have a direct effect on the quality and taste of the beer:

 

 

3. Types of pH Measurement

 There are mainly two systems to measure pH: pH indicators and pH me-ters. However, not all of them provide the same accuracy and quality level. A comparison between two widely used methods is shown below:

 

4. Essentials for a pH Measurement

 The tools necessary for pH measurements are relatively uncomplicated and provide reliable measurements when used correctly:

 

1. pH electrode

 

A pH electrode, which typically is a combination of a glass electrode and a reference electrode, is immersed into the sample to measure its pH. The right pH electrode should be used for every application.

 

2. pH buffers

 

Before measuring pH, two or more reference solutions of known pH values must be used for the calibration of the pH electrode.

 

3. Sample

 

This is the solution to be measured, which needs to be an aqueous solu-tion or to contain enough water for the pH measurement to be possible.

 

4. pH meter

 

The pH meter is a potentiometer that measures the voltage difference be-tween the glass electrode and the reference electrode, and calculates the pH value.

 

 

pH electrodes have a very important role in providing correct pH readings, since they are responsible for the actual pH measurement. The variety of pH electrodes is as diverse as the applications they are used for. Only the right combination of junction, electrolyte, shape and type of the membrane glass, and shaft material, make an electrode perfectly suited for a specific application. A pH electrode consists of the following parts:

1. SafeLock™

For refillable sensors only: prevents evaporation and leakage during storage and transportation. It must be opened during measurements.

2. Shaft Material

The sensor’s robustness is dependent on the right shaft material. Glass is highly resistant to chemicals and allows measurements at high tempera-tures. When mechanical robustness is key, plastic is the preferred material.

3. Reference Electrolyte

Sensors with a liquid electrolyte provide fast results. The ones with a poly-mer or gel electrolyte are not refillable and hence low maintenance.

4. Reference System

Provides a stable potential with which a measured pH potential can be compared.

5. Junctions

The junction is the connection between the reference electrolyte and the sample.

6. Membrane Glass

The membrane is the pH sensing part of the sensor. Its shape and glass composition are optimized to ensure best results in different applications.

7. Temperature Probe

The pH value of a solution is temperature-dependent. Thus, the temperature should be measured with every pH value. The temperature probe is either integrated in the pH electrode or an external one can be used.

8. Wetting Cap

To keep the glass membrane hydrated. Sensors should always be stored in aqueous and ion-rich solutions.

 

. Good pH Measurement Practices

Measuring pH is generally considered an easy, fast, and simple process, but so many things can go wrong. Different measurement and maintenance practices are essential to ensure the reliability and quality of pH measure-ments:

 

1. •Select the correct electrode for your sample
   pH electrodes play a very impor-tant role in performing correct pH value determinations, since they are responsible for the actual pH measurement. Based on your ap-plication, select the most suitable electrode.
2. •Rinse, but do not wipe the electrode
   After rising the electrode with dis-tilled water to remove salt deposits or any contamination, dab it dry with a tissue, but never wipe it. The rough surface of the paper tissue can scratch the pH-sensitive glass membrane, and/or it can create an electrostatic charge that might cause the measured signal to be-come very unstable.
3. . Stir the sample
   When measuring pH, gently stir the sample to ensure that it is homoge-neous. It is important that calibra-tion and measurement are done under the same conditions. Do not use the electrode to stir.
4. •Use the correct calibration buffers
   The buffers used for calibration must be selected according to the sample's pH. For instance, if a sample is expected to have a pH of 7.45, the calibration must include pH buffers 7.00 and 9.21 (or similar). Use a minimum of two fresh buffers for calibration.
5. •Check your calibration results
   The electrode calibration provides valuable data on the electrode con-dition. The slope of the calibration curve should lie between 95% and 102% of the theoretical value. Another measure of a good calibration is the offset at the zero point (0 mV at pH 7), which should remain relatively stable and should not exceed ±30 mV.
6. •Calibrate your pH electrode daily
   Regular calibrations will lead to more accurate results. Some appli-cations might require a calibration before every measurement, but in general it is enough to calibrate every 24 hours.

 7.Measure the temperature of the sample

 

Every sample has a different pH behavior depending on the tem-perature, therefore the temperature should be measured. The tempera-ture probe is either integrated in the pH electrode or an external one can be used. If the electrode and the sample have different tempera-tures, give the system enough time to equilibrate. To compare the pH of different samples, they should always be measured at the same temperature.

 

9.Keep the electrode cleaned Using a dirty electrode is one of the typical sources of error in pH measurement. Make sure that the electrode is always clean and well maintained.

 

8.Use fresh buffers for calibration

 

Check the expiry date of the cali-bration buffers before use. Never calibrate the sensor directly in the bottle. Do not re-use calibration buffers and never pour them back in the bottle. Close the bottles im-mediately after use and store them at room temperature.

 

 10.Store the electrode in a proper solution

Electrodes should always be stored in aqueous and ion-rich solutions. The electrode should never be stored dry or in distilled water as this will affect the pH-sensitive glass membrane and thus shorten the lifetime of the electrode. If not sure which storage solution to use, check the electrode's manual.