pH monitoring – a permanent headache? Solutions to common problems

What is pH?

In chemistry, pH, also referred to as acidity, historically denotes “potential of Hydrogen” (or “power of hydrogen”). It is a scale used to specify the acidity or basicity (alkalinity) of an aqueous solution. The scale is logarithmic and denotes the pH equal to -log10c where c is the hydrogen ion concentration in moles per litre.  Acidic solutions (solutions with higher concentrations of hydrogen (H+) ions) are measured to have lower pH values than basic or alkaline solutions. The pH scale runs from 0 to 14, with the neutral point being 7.

How do you measure pH?

When you think of “pH monitoring” you might think of litmus testing – but it is not always that simple. When measuring the pH of a waterbody, the pH monitoring cell is simple but easily affected.

A typical modern pH meter measures pH by measuring the potential difference across a pH sensitive membrane such as glass. It does this by measuring the potential inside the membrane using a Ag/AgCl electrode within a potentially neutral electrolyte such as Potassium Chloride (KCl) against a reference electrode, again Ag/AgCl within a KCl electrolyte.

In order to connect the reference electrode to the medium being tested, a porous junction between the reference electrode and the medium is incorporated in the sensor, through which KCl electrolyte permeates to ensure an electrical connection. This junction can be made from ceramic material, PTFE or be open.

The potential between the two electrodes is measured and converted to a pH reading using the Nernst equation.

Also available are ISFET (Ion Selective Field Effect Transistors) sensors. These use direct contact between the medium and a MOS arrangement transistor, removing the need for a glass membrane. A reference however is still required, along with the reference junction.

Several parameters affect pH

Several factors can have an effect on your sensor, such as contamination/dirt, flow rate, conductivity and  temperature.

Contamination of the sensor electrolyte (by diffusion into the sensor) or dirt build up on the reference junction can affect the potential matching of the reference electrode to the medium, or alter the potential within the sensor, affecting the overall potential measurement.

The rate of flow going past your sensor will affect your pH reading. A too-high velocity washes the electrolyte away from the junction reducing the potential matching between the reference electrode and the medium, so you don’t have the necessary bridge of ions to get your reading. Thankfully, you can do something about flow rate – a suitable stilling area or flow through system should result in less velocity going past the sensor. Most manufacturers of pH sensors will not provide information relating to the maximum velocity of the medium that can be withstood by the sensor. This is in part due to the interaction with other factors such as temperature, conductivity, changes of pH, response time of the pH sensor and contaminants within the medium. A number of flow through holders will however have a maximum flowrate assigned to them, which in turns limits the velocity past the sensor.

Next, there is the temperature to consider. Your pH sensors, particularly those incorporating glass membranes, are sensitive to temperature changes. As such, most sensors have a temperature sensor incorporated into them to provide temperature compensation. However, very low temperatures can make the sensor slower to take measurements. This can be due to the electrolyte thickening reducing the flow through the reference for example, or due to changes within the medium being tested. Response times and accuracy can both be affected by cold conditions.

You also have to consider the conductivity of the liquid. We’re talking very low conductivity – an instance where the ions are so diluted that you either get a very slow or an inaccurate measurement. You have a couple of options to fix this. You could use pressurised forced electrolyte. Another option is to calibrate the sensor with specific low conductivity buffers which will potentially compensate for the low conductivity medium.

If you have a very low temperature and conductivity, you are unlikely to get a good pH reading.

How could flow affect a pH measurement?

Say you have a small water treatment site, in an upland area with old source water. Hypothetically, conductivity is at an acceptable level to take what would usually be accurate readings, but the site is still getting an erratic pH reading.

Where could things be going wrong here?

This can actually be due to varying flow going through the flow-through holder.

Say the water is chlorinated for disinfection purposes, passes through a contact system and then passes into a small service reservoir, controlled to fill when there is a draw on it by level. Whenever water is drawn by the network, the flows into the service reservoir will increase, and when that draw stops, the flow stops. This will cause variations in the flow through the disinfection process and pH measurement system.

If operators observe the flow running at lowest flow rate, they may alter the fast flow system to increase that flow. However, when the flow then increases, the flowrate past the pH sensor increases to a level that reduces the accuracy of the measurement.

The solution could be constant head flow through systems, which would provide more constant conditions for a more stable and repeatable pH measurement.

Another example is a site high up in the hills, with a very cold source water, typically 1 °C. This temperature could drop below even this in winter.

Say the conductivity varies between 3-5 micro siemens and the combination of this and the cold temperature makes measurement very difficult. There is no singular fundamental solution for this but a calibration with low electrolyte buffers or forced electrolytes to stabilise the measurements, may assist.

You could use a pressurised sensor which would leech electrolyte, maintaining a stabilised reading even at a low conductivity.

What to consider when choosing a pH sensor:

There are many factors that have to be considered when choosing a pH sensor and associated meter.

• Whilst the majority of applications are covered by standard pH sensors, a number of applications require far more consideration when choosing a suitable pH sensor.
• Factors affecting the sensor choice include:
• Pressure and temperature of medium
• Chemical compatibility of the sensor material with the medium
• pH range – extremes of pH can be difficult to measure accurately for any period of time
• Conductivity
• Sample type – fluidity, cleanliness, chemicals
• Interferences – biological or chemical
• Mounting/installation details
• Physical dimensions
• Sensor material/type – an ISFET sensor will be physically and sometime chemically more robust than a glass sensor
• Reference type
• Response time
• Maintenance requirements
• Calibration

Note: A pressurised electrolyte system can assist with many potential problems such as low conductivity, higher flows, biological build up and dirt in the flow.

Installation of pH monitors

Your pH sensors can be installed in a variety of ways depending on the details of the system they are being installed into.

Three common examples are:

• Dip systems – usually used when installing in an open tank or channel. The sensor is secured to the end of a pole or other mounting system and installed from above, into the tank or channel. Consideration will be required in terms of physical protection of the sensor (such as a stilling tube arrangement, cover or a different material of manufacture) or velocities in channels.
• Retractable holder – used when installing into pipework. The retractable holder is designed to allow the removal of the sensor from the pipework, often under pressure, whilst not allowing the medium to escape from the pipe. The pressure within the pipe will assist in the selection of the type of device.
• Flow through holder – this type of system is generally designed to suite the sensor type. It provides a vessel that flow is brought to via pipework, and then drained from via pipework, providing a constantly wetted holder with idea flows. These are commonly used on systems that have a small flow taken from a main flow and then discharged to waste, such as in measurement in water treatment works.

Calibration of pH monitors

In order to operate and measure accurately, pH sensors need periodic calibration. The frequency of the calibration will depend entirely on the nature of your medium being measured, and the regulatory requirements associated with the measurement.

A calibration should be performed following every clean.

Suitable buffers complying with the NIST standard for buffers should always be used, and the NIST details included/programmed into the pH meter. A link to the NIST website is as follows Standards | NIST Buffers will have their specific NIST details attached to the container, detailing the pH/temperature relationship for that buffer.

There are a number of points to consider when choosing and looking after buffers, when you are calibrating:

• Buffers should meet the relevant NIST standard
• Buffers should be chosen that relate to the pH levels being measured – for example, if you are measuring pH 5, use a pH 4 and a pH 7 buffer
• Higher pH buffers are relatively unstable (for example pH 10 buffer) and should not be used after the first use
• Observe the buffer shelf life
• Never put the sensor into the buffer container – always pour a small amont of the buffer into a separate beaker
• If you are measuring pH in a low conductivity medium, consider using special low conductivity buffers

How do I maintain my pH monitors?

Maintenance of pH sensors mainly involves keeping the sensor clean and regularly calibrated. Calibration is discussed above.

Cleaning frequency and requirements will depend on the medium being tested and the installation type. General cleaning is with a soft cloth and water. If there is a buildup on the sensor, particularly the reference junction, then a diluted acid such as 5% HCl or a citric acid can be used.

If the sensor is slow to respond, then the sensor can be sat in a beaker of electrolyte (KCl) for a period of time to recharge.

Note that a flow with solids such as sand will degrade the sensor quicker than a clean, solid free flow.

Maintenance intervals are completely site, application, and regulation driven.

If you have fairly clean effluent without a really strong regulatory push you may choose to go perhaps up to 3 months between necessary cleaning and maintenance. However, if you are measuring on the outlet from a water treatment works, you will have to calibrate on a weekly basis for example due to regulatory requirements.

If you are unsure on the frequency required, a suggestion is to start off conducting maintenance regularly – perhaps weekly or monthly – and then to alter the maintenance interval based on the condition of the sensor when maintained.

If you have any questions about your choice of pH monitor, get in touch with our personable experts.