Determination of Nitrate in Plant Tissue using Ion Selective Electrodes

The nitrate in plant tissue is extracted by shaking with distilled water in the presence of an ion exchange resin. Chloride is a relatively high interference for the nitrate ISE so an additional sample conditioning may be necessary to minimise any chloride interference.

Equipment Required

1. Ion Meter, pH/mV Analyzer, or pH Meter with millivolt scale
2. Nitrate Combination Ion Selective Electrode
3. Beakers
4. Test tubes
5. Stoppered Erlenmeyer Flasks
6. Filter paper
7. Funnel
8. Glass tubing

Reagents Required

1. Dowex 50-X8 (50-100 mesh) hydrogen-saturated resin
2. Aluminium Sulphate Al2(SO4)
3. Dilute Barium Chloride BaCl2
4. Silver Nitrate AgNO3
5. Phenylmercuric Acetate
6. Dioxane
7. Sodium Chloride NaCl
8. Ionic Strength Adjustment Buffer (ISAB)
9. 1000ppm Nitrate standard

Aluminium Resin

Weigh out a known amount of Dowex 50-X8 (50-100 mesh) hydrogen-saturated resin and transfer to a beaker. Add 2.2g of Al2(SO4), per 10g of resin. Make a slurry by adding a small amount of distilled water. Filter the mixture under suction or by gravity. Test for excess aluminium salt by rinsing the mixture in the funnel with distilled water. Transfer a sample of the filtrate to a test tube and add 12 drops of dilute BaCl2: If precipitate forms, salt is still present in the resin mixture repeat the rinsing and testing until no precipitate forms. Store in a stoppered glass bottle.

Silver Resin

If plant tissue samples have a chloride level greater than 2% by weight, and a nitrate level less than 500 ppm NO¬3-. The addition of silver resin to the sample will remove chloride levels up to 6%. The silver resin is prepared in the same way as the aluminium resin, except that 7g of AgNO3, are added per 10g of resin, in place of the Al2(SO4): To test for excess silver salts in the mixture, rinse with distilled water. Add a small amount of NaCl to the filtrate. If no precipitate forms, the resin is salt free. If precipitate does form, repeat the rinsing and testing until no precipitate forms.

Preservation Solution

Dissolve 0.1g of phenylmercuric acetate in 20 ml of dioxane. Dilute to 100 ml with distilled water. Add 1ml of this solution to each litre of distilled water used to prepare all samples and standards.

Prepare standards of 100, 10 and 1 ppm by serial dilution of the 1000 ppm stock solution in distilled water and add 1ml of ISAB per 100ml of standard.

Sample Preparation

Dry a suitable quantity of plant tissue sample in an air-forced furnace as explained in Procedure No. 6.002 (a) of the Official Methods of the Association of Official Agricultural Chemists.

Weigh out 0.400g of dried, ground plant tissue and transfer to 125ml Erlenmeyer flask. Add 50ml of distilled water to the flask. Using 6mm glass tubing, collect approximately 1.5ml of aluminium resin in the tube by pressing the tube upright into the resin. Dispense the resin into the Erlenmeyer flask. (Repeat this procedure for the silver resin, if required due to chloride interference). Stopper flask and shake or stir for ten minutes. Filter the suspension through folded filter paper and collect the filtrate in a 100ml beaker and add 1ml of ISAB.

Method

Immerse the electrodes in each of the standards starting with the lowest and in increasing concentration steps, rinsing the electrodes with distilled water between standards. Plot a graph on lin/log graph paper of mV response against standard concentration. Immerse the electrodes in the sample, record the mV response and plot sample concentration from the graph.

Reference

‘Nitrate Determination in Plant Extracts by the Nitrate Electrode Carison, R.M., J. Ag. Food Chem., 1968, 16(5) 766

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Determination of Sodium in Food using Ion Selective Electrodes

The measurement of sodium in food is important due to the link between high blood pressure (hypertension) and sodium concentration. The sodium level is found by the technique of known addition.

Equipment Required

1. Ion Meter, pH/mV Analyzer, or pH Meter with millivolt scale
2. Sodium Combination Ion Selective Electrode (Glass or PVC)
3. Double Junction Reference Electrode (only required if a Mono electrode is being used)

Reagents

1. Sodium stock solution 2000 ppm
2. Prepare 100 ppm, 1000 ppm and 2000 ppm sodium standard solutions by serial dilution of the stock solution
3. Sentek Sodium ISAB

SAMPLE Preparation

Blend contents of sample including any water. Weigh a 10 g sample and dilute to 100 ml with DI ionised water. Mix and transfer to a clean, dry 150 ml beaker. Add 2ml of ISAB per 50ml of sample or standards. Stir all samples and standards during the measuring process.

Method

Take 100 ml of sample, immerse electrode and record the electrode potential mV. Pipette 5 ml of standard (see below table), stir thoroughly, allow for stabilisation and record the electrode potential mV.

Table

Calculation

Where:

Cu= Concentration of unknown ion
Cs= Concentration of Standard addition
Vs = Volume of Standard addition
V¬U= Volume of sample
ΔE = Change in electrode potential in mV
S = Slope of electrode in mV

The difference in the measurements mV2-mV1=ΔE. An accurate value of the electrode slope will be required, this can be found by producing a calibration curve using the 100 and 1000 ppm standards. To determine the mg per cent (mg/100 g of food), multiply the answer obtained from the equation for known addition by 10.

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Determination of Sulphide in Water using Ion Selective Electrodes

This method is applicable in essentially all waters including most industrial waste waters.

Equipment Required

1. Ion Meter, pH/mV Analyzer, or pH Meter with millivolt scale
2. Sulphide Combination Ion Selective Electrode

Reagents

1. Sodium hydroxide pellets
2. Ascorbic acid
3. Disodium EDTA
4. Sodium sulphide hydrate
5. Lead perchlorate
6. Zinc Acetate

Reagent and Standard Preparation

Sulphide Antioxidant Buffer Preparation (SOAB)

To approximately 600ml of distilled water in a 1000ml volumetric flask, add 80g reagent grade NaOH pellets, 35g of ascorbic acid, and 67g of disodium EDTA stir until everything dissolves. Dilute to the mark with distilled water. Freshly prepared SAOB, when stored in a tightly stoppered bottle, has a shelf life of approximately two weeks, if opened frequently. When oxidised, the solution turns dark brown and should be discarded. Light brown solutions are still usable.

Sodium Sulphide Standards

Sulphide standardizing solutions are prepared from reagent grade sodium sulphide hydrate, Na2S.9H2O. All sulphide solution preparation and measurement should be performed in a fume hood, to avoid breathing noxious fumes. Precise standards cannot be prepared by weighing the salt because of the large and variable water of hydration. Instead, prepare saturated Na2S solution by adding 100g of the Na2S.9H2O to 100ml of DI water, shake well, stopper securely, and allow to stand overnight. To prepare a stock sulphide solution, pipette 1ml of the saturated solution described above into 50ml of SAOB and dilute to 100ml using DI water.

Standardisation

The concentration of stock sulphide solution must be determined by electrode titration before constructing a calibration curve. Use a titrant of known concentration of 0.1M lead perchlorate. Take 50ml of stock sulphide solution, add this to 25ml SAOB and 25ml H2O and titrate using the sulphide electrode.

Calculation

The concentration of stock sulphide solution in mg/l (C) is given by: C= (Volume of 0.1M lead solution) x 64. Each day, prepare four calibration standards using 100ml volumetrics as follows:

A – 5.00ml of sulphide stock to 45ml of SAOB and make up to 100ml using DI water.
B – 1.00ml of sulphide stock 50ml of SAOB and make up to 100ml using DI water.
C – 2.00ml of calibration standard A 50ml of SAOB and make up to 100ml using DI water.
D -1.00ml of calibration standard A 50ml of SAOB and make up to 100ml using DI water.

The concentration of the calibrating standard is calculated from the concentration of the sulphide stock, as determined by titration. If the stock concentration is C mg/l, then the calibration standards have the following concentrations:

A= 0.05C
B= 0.01C
C= 0.001C
D= 0.0005C

Sample Preparation

Samples are treated prior to analysis with sulphide antioxidant buffer. Samples should be taken with a minimum of aeration to avoid air oxidation or loss of any volatiles. Samples can be preserved by adding 0.2ml of 2M zinc acetate (equivalent to 128 mg/L S=) and 0.05ml (1 drop) of 6M sodium hydroxide to a 100ml bottle, filling it completely with the sample, and stoppering it with no air bubbles trapped under the stopper. If the concentration of sulphide is greater than approximately 100mg/l, the amounts of both reagents should be increased. SAOB contains EDTA to redissolve the zinc and free the sulphide. The entire sample is used for analysis, and since the results will be given as mg sulphide per litre, the sample volume must be known.

Method

Prepare a calibration curve by immersing the electrode in each of the calibration standards, beginning with the weakest one, and record the stable millivolt potential reading developed by each one. Construct a graph using this data using semi log paper and placing the sulphide concentration scale on the logarithmic scale and the mV reading on the linear scale. Pipette the sample into an equal volume of SAOB, stir thoroughly without vortex and allow to stand for 3-5 minutes. Place the electrodes in the solution, record the stable electrode potential and determine the sulphide concentration of the sample from the calibration curve. Between samples, rinse the electrode with deionised water, blot dry, and immerse them in a “blank” solution of 50ml SAOB plus 50ml deionised water.

Expected Range: Samples containing 0.1 to 3200mg/l of sulphide may be analysed by this method. The concentration range may be extended by dilution of an appropriate aliquot.

Reference

Annual Book of ASTM Standards, Part 31.

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Determination of Chloride in Mayonnaise using Ion Selective Electrodes

To determine the cloride in mayonnaise, the chloride is extracted from the sample by dissolving in 1 molar nitric acid which also acts as an ionic strength adjustment buffer.

Equipment Required

1. Ion Meter, pH/mV Analyzer, or pH Meter with millivolt scale
2. Chloride Combination Ion Selective Electrode
3. Chloride 1000ppm Standard Solution
4. Nitric Acid Solution (1M)
5. Glassware: 250ml beaker,100ml volumetric flask, 250ml volumetric flask, 1Litre volumetric flask, graduated pipette
6. Deionised water

Nitric Acid Preparation

Make a 0.5M Nitric acid (HNO3) solution by taking 125ml of your 1M nitric acid and diluting with 125ml of deionised water and store in a clean 250ml bottle.

Standard Preparation

Prepare standards of 100, 10ppm by serial dilution of the 1000 ppm standard solution. This is best achieved by pipetting 10ml of the standard into a 100ml volumetric flask and diluting to the mark with the 0.5 molar HNO3. This is now a 100ppm standard, repeat this step using the freshly made 100ppm standard to make 10ppm.

Sample Preparation

Weigh accurately 1g of sample and disperse into 500 ml of 1M nitric acid in a 1L volumetric flask. Mix the solution vigorously to ensure complete mixing. Once homogeneous top up to the 1 litre mark with deionised water. Filter the sample solution (22µm filter would be ideal) into a clean stoppered flask.

Method

Pour 100ml of each of the standards and samples into clean beakers. Be sure the beakers are clean and make sure not to touch the inside of the beakers with bare hands as Chloride contamination from sweat etc. is common. Beakers that have been washed with softened water or tap water will be contaminated. In these cases, rinse the beakers with deionised water.

Immerse the electrode in each of the standards in increasing concentration and plot the response in mV vs log(concentration) or follow the calibration routine on the ion meter, if available, ensuring to rinse the electrodes with distilled water and dabbing off the excess water between standards. Measure the sample and if necessary plot onto the graph.

Using the ion meter calibration function, if available, will allow the result to be calculated and displayed directly. Some meters allow for up to a 5-point calibration if required.

If you do not have an Ion meter you can read the mV values using a pH/mV meter. Record the mV response and plot a graph of mV vs. log of Concentration.

Calculation

The result on the display will correspond to mg Cl in 1g of sample. To determine % Chloride in Mayonnaise divide the result by 10.

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Using Custom Sensors and Electrodes for Water Quality Monitoring

What is the Environment Act 2021 and what is Section 82?

The 2021 Environment Act is aimed at improving how water quality is measured, monitored, and managed in real-time, particularly in rivers, lakes, estuaries, and coastal waters. The Environment Act 2021 introduced several measures to improve environmental protections, and continuous water quality monitoring is a critical part of these efforts, especially for the management of wastewater and storm overflows. Section 82 of the Environment Act 2021 focuses on improving monitoring regarding water quality, specifically targeting how sewerage undertakers (i.e., water companies responsible for managing public sewer systems) handle wastewater discharges, including storm overflows, into rivers, streams, and coastal waters.

Sewerage undertakers are required to install and maintain monitoring equipment on storm overflows and combined sewer overflows to track how often they discharge untreated sewage into water bodies. Sewerage undertakers must ensure that any discharge from their sewer systems does not breach water quality standards, particularly in sensitive areas such as bathing waters and protected ecosystems.

The government guidance details the types of technologies and methodologies that sewerage undertakers should use to monitor water quality effectively. This includes specifications for:

• Flow meters and sensors for measuring discharge volumes.
• Water quality sensors for detecting pollutants (e.g., ammonia, pH, DO2, etc.).
• Data collection and management systems for storing and reporting monitoring data.

The legislation in place dictates that sewage undertakers must monitor their waste output upstream and downstream, of their asset being monitored, once per hour or every 15 minutes during a high-risk event. These changes are required to be rolled out as soon as practicably feasible and have started by 2025 with complete monitoring completed by 2035.

How can Sentek Help?

Electrochemical sensors play a significant role in supporting the Continuous Water Quality Monitoring Programme and compliance with the Environment Act 2021, particularly for sewerage undertakers in implementing Section 82. These sensors are designed to measure various chemical properties of water in real-time, offering precise and reliable data essential for maintaining water quality and detecting pollution. Sentek can provide a range of electrochemical sensors suitable for measuring common sewage works contaminants and key indicators of a water courses health:

• pH Levels
• Temperature
• Dissolved Oxygen
• Turbidity/Conductivity
• Ammonia – An increase in Ammonia is a common indicator of sewage contamination.
• Chlorine – Chlorine sensors can detect the presence of disinfectants that may be released from wastewater treatment plants.

Nitrate sensors help track nutrient levels, which can cause issues like eutrophication (excessive nutrient enrichment in water bodies leading to algal blooms and oxygen depletion).
As one of the leading UK sensor manufacturers, Sentek can support clients with design of custom electrochemical sensors, probes and sondes to meet their specific water quality monitoring requirements. These can be installed at combined sewer overflows and other wastewater discharge points to monitor when and how often untreated wastewater is being released into rivers or coastal waters. Sensors provide early detection of changes in water quality. For example, if a sensor detects a sudden rise in ammonia or a drop in dissolved oxygen, this could indicate a pollution event such as a sewage spill. Early detection allows companies to respond quickly and mitigate the environmental impact before it escalates.

Sentek electrochemical sensors have been used for long-term deployment across the globe, providing continuous monitoring that ensures companies remain compliant with the environmental regulations over time. By tracking trends and long-term changes in water quality, sewerage undertakers can identify problem areas, manage wastewater systems more effectively, and make informed decisions on infrastructure improvements.

Sensors from Sentek can be integrated with remote monitoring systems and data platforms, enabling seamless collection and transmission of water quality data. This integration is particularly useful for large-scale, continuous monitoring across multiple locations, such as river systems or coastal areas.

The sensors can also be incorporated into smart monitoring networks, where data is aggregated and analysed to provide insights into overall water quality trends and potential sources of pollution.

Sentek electrochemical sensors could significantly enhance water quality monitoring efforts by providing reliable, real-time data that helps sewerage undertakers meet their obligations under the Environment Act 2021 Section 82. These sensors enable proactive management of water resources, ensuring timely detection of pollution and helping to maintain high environmental standards.

Ammonia vs Ammonium

Ammonia is just one component in sewage that can be particularly harmful to aquatic life, it can be very challenging to monitor, particularly in the field. Ammonia ISE are available but presently are only practically useable in a lab-based setting.

Ammonia is only present as the deprotonated form NH3 in a basic pH greater than 7, below this it is mostly in the less harmful ammonium ion (NH4+). Ammonium is unable to cross cellular membranes so is therefore much less toxic than ammonia. As pH and temperature increases the proportion of ammonium being deprotonated to the smaller and more harmful ammonia increases so by monitoring NH4+, pH and temperature we can relatively accurately infer the ammonia concentration in the water. The table below shows the percentage of ammonia vs ammonium at varying pH and temperature.

This can be plotted onto a curve also shown here.

Schematic model of ammonium ion (NH4 + ) and neutral ammonia (NH3) proportions in function of the pH. At acidic and neutral pHs, NH4 + is the most abundant species. However, NH3 is dominant in highly alkaline solutions. Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Schematic-model-of-ammonium-ion-NH4-and-neutral-ammonia-NH3-proportions-in_fig2_358973331

Calibration and Maintenance

Sensors or sondes can be pre calibrated prior to installing but will need periodic recalibration to maintain a high degree of accuracy. Over time the sensor will begin to drift away from the calibration point. Each sensor type will drift at a different rate as they age – the longer the sensors go without calibration the greater degree of uncertainty the measurement will have.

Regular maintenance will also be required to ensure that the sensors or sondes are working at their best, giving that monitoring a water way will naturally lead to an amount of biological fouling to the sensors. Contamination of the sensor junctions or plates will lead to increased resistance which in turn will skew measurements, so it is important that these are periodically cleaned off and recalibrated.

For more information on the Environment Act 2021 Section 82 and sensors used to measure chemical properties of water, contact a member of the team.

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Determination of Sulphate in Water using Ion Selective Electrodes

This application note covers the determination of sulphate in water. As there is no ion selective electrode (ISE) for the direct measurement of sulphate, the analysis is performed using a sample subtraction technique with a lead ISE. The potential of a known concentration of lead can be measured before addition of an unknown concentration of sulphate. The decrease in measured potential of the lead standard will therefore be proportional to the amount of sulphate added. There is no limit to the concentration of sulphate being measured, provided that the concentration of lead standard used in the method, is greater than that of the sulphate in the sample. If the concentration of sulphate is greater than the range of the lead ISE, then some sample conditioning may be required.

Equipment Required

1. pH/mV Analyzer or pH Meter with millivolt scale
2. Lead Ion Selective Electrode
3. 1000ppm Lead Standard Solution 500ml
4. Glass Beaker 250ml
5. Glass 10 and 100ml Pipettes
6. Deionised water in a wash bottle

Probe Slope

To ensure the accurate determination of sulphate in water (or your sample) you will need to measure the slope of the lead ISE. To do this measure the mV values in 100ppm and followed by the 1000ppm lead standards. Subtract the value in 100ppm from the value in 1000ppm. The slope should be between 24 and 29mV.

Method

Pipette 100ml of a lead standard of known concentration into a clean glass beaker, 1000ppm standard would be suitable here. Place the lead ISE into the standard, gently stir and select mV mode, note the stable mV result (mV1). Pipette 10ml off the unknown sulphate sample into the standard, gently stir using the electrode and after approximately 1 minute note the stable mV result (mV2).

Remove the lead ISE, rinse thoroughly with deionised water and replace the protective cap.

Calculation

Calculate ΔE by subtracting mV2 from mV1. The unknown sulphate concentration can be calculated using the following equation:

Cu = Concentration of the unknown sulphate
Cs = Concentration of Pb2+
Vs = Volume of Pb2+ standard
Vu = Volume of the sulphate sample
ΔE = Change in electrode potential in mV (mV2-mV1)
S = Slope of the electrode in mV

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Determination of Calcium in Milk using Ion Selective Electrodes

This application note covers the determination of Calcium in milk. Calcium ions are responsible for an important role in the stability of casein in milk. Measurement of Calcium concentration is made by sample addition.

Equipment Required

1. lon analyser with a Calcium ISE function, or pH meter with millivolt scale.
2. Calcium ion selective electrode

Reagents

1. Calcium stock solution 0.1M Ca2+(4000 ppm): Dissolve 11.1 g of CaCl2 into distilled water in a volumetric flask and dilute to 1000 ml.
2. Prepare calcium standard solutions of 10 ppm, 100 ppm and 1000 ppm by serial dilution of the stock solution (approx. 100 ml of each).
3. lonic strength adjustment buffer (ISAB)4.0M KCI: Dissolve 298.2 g of potassium chloride into distilled water in a volumetric flask and dilute to 1000 ml.
4. Outer filling solution 1.0M KNO3: Dissolve 101.1 g of potassium nitrate into distilled water in a volumetric flask and dilute to 1000 ml.

Method

Take 100 ml of standard and immerse the electrodes. Record the electrode potential mV1, and then pipette 10 ml of sample solution into sample, stir thoroughly and allow for stabilisation. Record the electrode potential mV2. The change in electrode potential ΔE is given by mV2-mV1. Use the sample addition formula below to calculate the concentration of unknown.

Sample Addition

In this case a small known volume of the sample is added to a known volume of a more dilute standard solution and the electrode potential difference ΔE measured. The equation in this case is:

This technique finds application when the samples to be measured are of rather high concentration and the standard addition procedure produces very little change in the electrode potential.

Calculation

The slope of the electrode needs to be accurately known. This can be achieved by taking two standards 100 and 1000 ppm Ca2+ and record the electrode potential difference between the two. The theoretical value for Ca2+ISE is +29.2 mV. The result from the sample addition formula is a direct measurement.

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Electrodes For Electrochemical Analysis and pH Control in Wastewater Treatment

Any wastewater produced in industry must be analysed, treated and decontaminated before it can be reused or released. It is important to set limits on the water quality to ensure that legislation is adhered to. Electrochemical analysis and pH control of wastewater pre and post treatment is needed to ensure that the treatment has had the desired effect, and the waste is safe to discharge.

How you treat wastewater really depends on the contaminants and their concentrations. A range of tools are available for quick in-lab or inline electrochemical analysis of water such as conductivity, pH and various different target ions can be measured using ISE. By using a selection or maybe just one of these electrodes you can take a more targeted approach to wastewater treatment allowing you to use the correct chemicals in the correct volumes to treat the contamination, ultimately reducing costs.

Batch or continuous pH monitoring and control?

Most water treatment systems are integrated into existing industrial facilities and need to be constantly adjusted based on environmental, operational and legal factors. As such, continuous monitoring control measures must be implemented at all times. This can be either done manually by qualified technicians or by automated instrumentation. Manual control measures involve testing the pH levels in water at least once a day and making any manual adjustments accordingly. Some facilities where the pH levels must be kept at a certain level at all times, precise automatic monitoring systems need to be used. Sentek produce a range of sensors suitable for inline monitoring these are specifically designed to withstand high pressures and temperatures so that they are suitable for CIP procedures. By using continuous monitoring, a system can automatically dose the wastewater to allow effluent to be safely withing the desired range at all times and can automatically alert operators when outside of defined limits to ensure that legal limits aren’t breached, and the facility is safe at all times. By using a feedback loop a facility can dose a precise amount to constantly stay within range minimising waste chemicals. Long term data monitoring can also give the operators the opportunity to analyse wastewater trends and optimize water treatment.

pH control in wastewater

Measuring pH levels in wastewater is one of the simplest and most common ways to determine the quality of the water and the best way to treat it. pH is measuring specific ions; either H+ or OH- in water. In the UK, drinking water must be between pH 6.5 and 9.0, this can be quickly measured either inline or by an operator using a pH electrode. Using pH measurement can take the guess work out of how to treat waste streams. You can deliver an exact amount of chemicals to balance the pH that is safe to dispose of. Water can be treated by manipulating the pH to remove certain impurities, for example a range of heavy metals can be removed by adjusting the pH of solution into a basic range to form metal hydroxides; these are insoluble compounds which can be easily filtered out before then adjusting the waste stream back into an acceptable range.

Chemical treatment efficiency – Many chemical processes used in wastewater treatment, such as coagulation and flocculation, depend on specific pH levels to work effectively. For instance, certain coagulants will work best at specific pH levels to aggregate and settle out contaminants.

Achieving optimal biological activity – Wastewater treatment often relies on beneficial microorganisms to break down contaminants. These microorganisms have an optimal pH range where they are most active. If the pH deviates significantly from this range, the microbial activity can decrease, leading to reduced treatment efficiency. By adjusting the pH to acid levels, you can quickly kill bacteria and inhibit further growth of organic matter until you are ready to adjust back to safe levels for disposal.

Infrastructure Protection – Maintaining the right pH level can help protect wastewater infrastructure. Highly acidic or basic wastewater can corrode pipes, tanks, and other system components, leading to premature equipment failure and increased maintenance costs.

Dissolved Oxygen Electrodes

The biological oxygen demand (BOD) and chemical oxygen demand (COD) is used to estimate the impact discharging the waste stream will have in the short term to the receiving body of water. These can be rough indicators on the quality of the water and whether or not it is safe to discharge. BOD is a measure of the oxygen used for all of the metabolic activity of the microorganisms in the water and this is often used as a measure of how polluted a river system is. A higher polluted waste stream may have more organic material for biological life to grow on which in turn will consume more oxygen. A pristine river is considered to have a BOD at 1mg/L or less whilst untreated sewage is 600mg/L or higher. So, by measuring the BOD the effectiveness of water treatment can be analysed. This is monitored by measuring the dissolved oxygen in a sample and incubating in a sealed container for 5 days before remeasuring the dissolved oxygen content.

COD similar to BOD as both measure the relative oxygen demand of the waste stream however, the COD is a measure of the amount of oxygen that is consumed by reactions occurring in the water. This can be tested by measuring the dissolved oxygen content before oxidising all of the organic matter into carbon dioxide using strong oxidisers in highly acidic conditions then remeasuring the dissolved oxygen content.

ISE (ion selective electrodes)

Ion selective electrodes work similarly to pH electrodes and will allow the user to measure specific ion concentration in their effluent stream. By continuous measurement of target pollutants or treatment reagents the effluent treatment efficiency can be monitored. There are a range of ions that are considered pollutants by environmental agencies which can be detected and measured directly using our electrochemical sensors.

Conductivity electrodes

Conductivity is an extremely simple low maintenance method to determine contamination levels of your water. Conductivity cannot give you the concentration of specific ions except in some very well controlled experiments but will give an indication of the overall ion concentration. By measuring the conductivity of a solution this will give us a good overall picture of what is dissolved in the water. An increase in conductivity indicates an increase in ions or charge conducting solids in the waste stream.

ORP/ redox electrodes

Oxidation reduction potential (ORP) is the measure of a solutions ability to accept or donate electrons. An oxidiser will readily donate electrons whilst a reducer will readily accept electrons. An ORP electrode is a low maintenance electrode that can be used to measure the overall oxidation/reduction potential of an effluent stream. Chlorine for example is a very common cleaning agent it is also a very strong oxidising agent by measuring the oxidation potential you can see how much of the chlorine has reacted in the solution and if further dosing is required. Another example could be Ferric (III) sulphate, this is a common coagulant used in water treatment; this is also a reducing agent so the dosage can be monitored and controlled using an ORP electrode.

Choosing an electrode

To get the most accurate measurement of the quality of water there are several electrodes you could choose:

• pH electrodes – This is the most common way to measure water quality. The electrode tests how acidic or alkaline the waste stream is. Often a waste stream can be treated entirely by adjusting this parameter.
• Redox electrodes – These measure the relative oxidising or reduction potential of a sample. This can be useful in measuring disinfection techniques and how effective they are.
• Conductivity electrodes – This electrode measures the ability of the water to conduct an electrical current. Higher quality water, the lower its ability to conduct a charge, a high conductance is an indication of ions or conducting solids.
• Ion selective electrodes – Used to measure the concentration of specific ions in water. By using a range of these ISE, you could potentially detect particular ions present in your manufacturing stream that are making it through into your wastewater.

Contact our technical team today and find out if we have any electrodes for you.

Choosing the right electrode for analysis of wastewater and treatment is a combination of understanding the specific challenges of your industry and matching them to the features and specifications of available electrodes. Once a suitable set of electrodes or chosen, these will allow you to monitor your specific wastewater to ensure environmental compliance. Regular maintenance, calibration, and periodic evaluations ensure that the chosen electrode provides accurate and consistent results over time.

These electrodes and other bespoke options are available to purchase from Sentek. Download our water and wastewater treatment brochure, or view electrodes online that are suitable for electrocheimcal analysis and pH conrol in wastewater treatment.

Not sure what you need, or want to know more about pH analysis in wastewater treatment? Get in touch with us today to speak to a specialist.

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Introduction to Voltammetry

Voltammetry is an electrochemical technique based on the measurement of the current produced by an analyte as a function of the potential applied to a working electrode. In many cases the applied potential is varied or the current is monitored over a period of time. The applied potential forces a change in the concentration of an electroactive species at the electrode surface by electrochemically reducing or oxidising it. Analytical chemists routinely use voltammetric techniques for the quantitative determination of a variety of dissolved inorganic and organic substances. The analytical advantages of the various voltammetric methods include excellent sensitivity with a very large useful linear concentration range for both inorganic and organic species. There are also many other uses including fundamental studies of oxidation and reduction processes in various media, adsorption processes on surfaces, and electron transfer and reaction mechanisms. Voltammetric methods are also applied to the determination of compounds of pharmaceutical interest and, when coupled with HPLC, they are effective tools for the analysis of complex mixtures.

The electrochemical cell, where the voltammetric process is carried out, consists of a working electrode, a reference electrode, and usually a counter electrode. In general, the working electrode provides the interface across which a charge can be transferred or its effects felt. In practice, it can be important to have a working electrode with known dimensions and surface characteristics. The auxiliary electrode can be almost anything as long as it doesn’t react with the bulk of the analyte solution and conducts well.

The reference electrode should provide a reversible half-reaction with Nernstian behaviour, be constant over time, and be easy to assemble and maintain. The most commonly used reference electrodes for aqueous solutions are the calomel electrode, and the silver/silver chloride electrode.

Voltammetry has developed very rapidly, and several types have been introduced with high efficiency, sensitivity, and selectivity. Of the numerous types of voltammetric techniques the more common ones are:

Linear sweep voltammetry – in which the current at the working electrode is measured while the potential between the working electrode and the reference electrode is swept linearly with time. The potential at which oxidation or reduction of the analyte occurs at the working electrode is shown as a peak or trough in the current.

Cyclic voltammetry – in which the current at the working electrode is measured while the potential between the working electrode and the reference electrode is swept linearly with time. Unlike with linear sweep voltammetry when a set potential is reached the potential at the working electrode is swept in the opposite direction to return to the initial potential. A cyclic voltammogram is obtained by plotting the current at the working electrode against the applied voltage. The sweeps maybe repeated many times.

Stripping voltammetry – in which there is pre-concentration of an analyte on an electrode, followed by a potential sweep to selectively oxidize or reduce the analyte, with the current generated proportional to the amount of analyte present on the electrode.

The original dropping mercury electrode was originally the working electrode of choice, however, the use of mercury electrodes has fallen out of favour because of the toxicity of mercury. Other electrodes that have gained favour for use in voltammetric methods include carbon paste, glassy carbon, platinum and gold. In addition, with a glassy carbon electrode it is possible to create a thin film of mercury on the electrode by reducing Hg2+. This provides the advantages of mercury electrodes described earlier without the necessity for using large quantities of mercury.

Sentek produces a range of working electrodes as well as calomel and silver/silver chloride reference electrodes. Sentek can also supply customised electrodes to meet many requirements.

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Most Read Articles

Before we dive into 2023, we wanted to look back and revisit some of our most popular news articles from last year. We covered many topics on our blog in 2022, from the launch of our all-new website to tips on how to choose the correct conductivity electrode.

Read on for a summary of our 5 most popular blogs from last year.

#1 Coming Soon! Stay tuned for our all-new website. In January, we celebrated the launch of our all-new website. A full overhaul of the site is designed with an intuitive layout, increased range of products, and enhanced functionality to make it easier for users to find what they need, quickly. Read more >

#2 The Use of Disposable and Non-Disposable pH Sensors in Medicine Production. Monitoring and controlling pH is critical to biopharmaceutical manufacturing to ensure the quality and yield of the final product. In this article we look at the use of single-use or disposable bioprocessing equipment. Learn more >

#3 Introducing a new look for Sentek electrodes. Exciting news! We’re launching a sleek new cap design and an innovative zero-leakage refill seal – no more dried KCl solution! The next generation design will be rolled out across the range of sensors, including pH, reference, conductivity and ISE electrodes. Read more >

#4 Choosing a Conductivity Electrode. Choosing the right conductivity electrode is important to ensure that the right product is being used to give the most accurate readings. In this informative article, we cover the factors to consider when choosing a sensor, as well as application advice when using the probe. Learn more >

#5 The Importance of Lab Testing for Food Products. Here we answer the question ‘Why is laboratory testing for beverages and food products essential?’, exploring how pH affects food and outline some food testing processes. Learn more >

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