John Barron, Technical Director of Reagecon, offers his top 10 tips on how to measure pH accurately and consistently.
1. Selection of the pH Measurement System
The selection of an appropriate measurement system that includes metre, electrode, buffers and controls is the first prerequisite. To ensure the system is fit for purpose the following questions should be asked:
- Where is the measurement going to be performed (e.g. laboratory, air-conditioned environment, reactor vessel, beside a river) and the accuracy and criticality required? This determines the fitness for purpose of the system you select.
- Such a process may benefit from the completion of a brief checklist of the requirements and selecting appropriate equipment against this checklist.
- Checking that the equipment performs correctly before putting into service. Such check could include checking the measurement performance with pH buffers or checking the electrical inputs and outputs of the pH metre.
- For a more comprehensive guide on how to perform these checks please see The Selection and Use of Instruments for Accurate Conductivity Measurement which has information relevant to pH.
2. pH Metre Qualification
Whilst informal checks are advantageous and may be adequate and fit for purpose in many instances, it is of much greater benefit and in regulated industries, mandatory, to perform this process on a formal, documented basis by a process known as Equipment Qualification. Performing Equipment Qualification on a formal basis results in savings of time and money, is a key step in achieving the fundamental goals of obtaining pH measurements that are correct, that can be proven to be correct and fit for purpose. There are four main stages of Equipment Qualification:
- User Requirement Specification (USR)
- Design Qualification (DQ)
- Commission, which can be divided into Installation Qualification (IQ) and Operational Qualification (OQ)
- Performance Qualification (PQ)
The whole process of Equipment Qualification is well documented but a reference of particular value is the authors paper Equipment Qualification and its Application to Conductivity Measuring Systems, the content of which is more relevant to pH metres than any other published literature to date.
3. pH Method Validation
Equipment Qualification demonstrates that the measuring equipment is capable of producing test measurements that are fit for purpose. The full test regime employed on real life samples also be suitable for its application, if the results are to be fit for purpose. Therefore, the full test method must be validated. There are large volumes of published literature on method validation. The reader is referred to the authors paper on Equipment Qualification and its Application to Conductivity Measuring Systems or for a more detailed study. The authors paper The Quality of the Analytical Result presents substantial information on all of the metrological aspects of method validation including accuracy, precision, traceability, calibration, limits of detection (LOD), limits of quantification (LOQ), sensitivity and much more. Very detailed information is also presented on uncertainty of measurement.
4. pH Electrode Selection
There are a wide variety of options in terms of selecting the correct pH electrode in any measurement situation. In general, though, there are about 5 variables that need to be considered, which cover the vast majority of applications. Firstly, there is the physical configuration of the electrode in terms of thickness, length, robustness and durability. Electrodes can be procured and used that possess a wide range of specifications around these variables. For example, the electrode diaphragm and sensing membrane must be able to comfortably reach the sample and both need to be fully immersed in the sample. Is the electrode being used to measure pH in samples at elevated temperatures or above atmospheric pressure? Is the measurement being done in an on-line or process situation? Is cleaning in place (CIP), sterilisation in place (SIP) or autoclaving required? These are additional relevant questions that must be considered and answered around durability, robustness and ruggedness. This may also dictate the physical structure of the sensing membrane or its megaohm resistance. For example, if the measurement is being done on a flat material, such as leather, the membrane must be flat or spear shaped, if the sample is hard (e.g. meat or cheese).
The second variable is sample. Chemical composition is of critical consideration as is the possible necessity for the measurement to be done at elevated temperatures. More specifically, the sample must be compatible with the sensing membrane, the material in the body of the electrode and both the reference electrolyte and reference system of the electrode. There are many options available centred around these variables, but ultimately a combination of using good reference literature, experience and fit for purpose method validation will greatly enhance the prospect of correct selection of the best electrode option.
Compatibility with the sample is linked to the next two variables – correct choice of electrolyte and correct choice of reference system. Specific assistance with the correct choice of electrolyte can be obtained from the authors paper on this subject – Electrode Care and Maintenance. This paper helps the user to choose an electrolyte, that is compatible with the sample being measured, starting with the fact that a good electrolyte must be equimobile in terms of the cation and anion, have constant chloride activity, be of high electrical conductance and as non-chemically reactive as possible. The optimal electrolyte, where possible is Potassium Chloride (KCl), but there are many instances cited in the previously reference paper, where the use of KCl is not possible. So, it is in these instances where a different reference system or a double junction electrode may be necessary so as to facilitate a different reference electrolyte.
5. pH Electrode Care & Maintenance
To ensure accurate and reliable pH results, a routine care and maintenance programme should be adopted for the pH electrode system. This will also ensure prolonged working life and reduces the necessity for both corrective action and fault diagnosis. Prior to use, the electrode should be shaken down like a fever thermometer to dislodge air bubbles, if the filling solution (electrolyte) is in liquid form. It is important to remember never to touch the sensing membrane by hand or with soft tissue and the electrode should never be in contact with any substance, apart from the liquid sample other than when the sensing is being done on solid materials such as agar, leather or blotting paper.
When in use, the electrode should be calibrated using two buffers that bracket the expected value of the sample. Stirring can, if desired, be applied to the liquid, but this must be applied to both sample and calibration buffer. The electrode must be rinsed a number of times with water from a wash bottle between readings and the final rinsing should be with the solution to be measured.
After measurement, the electrode should be stored in pH 7 buffer, if the storage period is only overnight, but if storage is longer it must be stored in especially formulated storage solution. Irrespective of whether buffer or storage solution used, it must be kept wet at all times. During usage electrodes can suffer from contamination to the membrane and/or diaphragm, either of which may result in measurement errors or show response. A detailed maintenance regime is the authors paper, Care, Maintenance and Fault Diagnosis for pH Electrodes, which presents specific electrode treatment, depending on what the specific contaminant may be (e.g. protein, sulphides, oils etc.)
6. pH Electrode Fault Diagnosis and Remediation
There are three main components contained within a pH measuring system, metre, solutions (calibration buffers and sample) and electrodes. Any of these three components can cause errors during measurement, but it is the electrode and more specifically the reference electrode that causes up to 80% of errors or problems. The first steps involve fault identification and this author has co-authored a seminal paper on this topic that can be distilled into a set of easy to follow steps, using three main variables (Eo, slope and drift). Once the fault has been diagnosed, a set of easy remedial steps are presented depending on what component of the reference system has contributed to the problem. If, having completed the necessary checks, the electrode is still showing drift or exhibiting erratic behaviour, the problem then is almost certainly due to an inappropriate selection of electrode for its intended application. Typical examples of lack of compatibility between electrode and application include measuring in a highly concentrated mineral type sample, causing a phenomenon called “liquid junction potential error”, a low ionic strength sample, which may be an aqueous or non-aqueous matrix or where the sample may have a high concentration of heavy materials. Details of these issues and many others can also be found in Care, Maintenance and Fault Diagnosis for pH Electrodes.
7. Temperature Control of the pH Measurement System
Accurate measurement of pH data has been a long-standing problem due to the effects of temperature. The effects of temperature can be divided into 2 main categories: temperature effects that diminish the accuracy and speed of the electrode response and temperature coefficient of variation that effects either the calibration buffers, control materials or the sample. Some of these effects have been described or alluded to in previous literature, but prior to the definitive work that has been carried out and published by the Reagecon technical services department, The Effects of Temperature on pH Measurement, no complete classification or account has ever been published, nor a definite set of guidelines on the significance of the various temperature errors, or how these errors can be reduced or eliminated. The paper provides the ultimate set of explanations of the various effects of temperature on pH with step by step easy to follow remedial actions.
8. pH Buffer Solutions
The correct selection, use, application and metrology of pH buffers is an imperative for accurate and fit for purpose pH measurement. As well as calibrating the pH measurement system, these products may be used as quality control materials, for method validation and for instrument qualification. A rigorous and consistent Standard Operating Procedure (SOP) should be in place so as to ensure consistency and comparability of measurement, the effects of the temperature coefficient of variation on the buffer must be accounted for and the buffers can be used for electrode fault identification and fault diagnosis. There are a certain set of characteristics that should be sought in pH buffer selection and certain metrological parameters such as traceability, stability, accreditation and certification that are mandatory for the achievement of fit for purpose results. The types of buffers used, how various buffers have evolved, specifications, packaging options and the features and benefits attributable to various buffer families are important in the selection decision process.
Because, there are a lot of options open to the user and there is very little published literature on pH buffers, we have prepared a detailed, easy to follow publication to enable the analyst to make the best possible decisions so as to ensure an optimal outcome of their pH measurement practice. Although there are many good producers of pH buffers, because of familiarity and close association with the author, the pH products produced by Reagecon, are used for example purposes, throughout the publication. It is highly recommended that this paper be read in full.
9. pH Sample Management
In all analytical measurement, the correct application of the science of sampling is of profound importance. Poor application will have a deleterious effect on the quality and fitness for purpose of the analytical result. This science, can be subdivided into sampling strategy, sampling equipment, containers, sample transport, preservation, storage and treatment prior to testing. In the case of pH measurements, analysis may be done in a process stream, on line, in situ, or in the laboratory. Our technical services department has been occupied with the science of sampling for many years resulting in a paper “Practical Considerations for Sampling” which is applicable to most analytical samples that are ultimately tested in the laboratory including pH. pH samples are particularly labile, principally due to the effects of carbon dioxide on samples in the alkaline region. The paper covers all aspects of sampling for a wide variety of analytes and measurements. Particular emphasis is placed on sampling planning and sampling strategy, finishing with substantial detail on sample chain of custody.
10. Role of Accreditation in pH Measurement
The acquisition and maintenance of accreditations and certifications continues to play a major role in the commercial production of high-quality pH buffer solutions. For a producer of pH buffers, both accreditations and certifications are an integral part of the customer value proposition. Sometimes there is confusion pertaining to the differences between the two and sometimes there is confusion relating to what exactly is accredited in the case of a producer holding ISO 17025 accreditation. This author has published a valuable paper on the Role of Accreditations in the Production and Use of Analytical Volumetric Solutions (Titrants). The information and advice contained within this publication is applicable to the manufacture of most standards and calibrants, including pH buffers and reference materials. The specific applications are covered in detail, as are the reasons for using standards certified by an ISO 17025 accredited company. The scope of the 17025 accreditation is also covered as are the fundamental differences between ISO 17025 and ISO 17034.