This section of our website is intended to answer the most frequently asked questions and to enable you to develop a greater understanding of the coulometric technique in general and our Aquamax KF titrators in particular.
Our specialists and consultants have spent many years on Karl Fischer titrations – let them help you. If you have a sample analysis problem, or if you are not sure if Karl Fischer titration would be suitable, contact us today for free technical and application advice.
Simply click on any of the following questions to view the answer.
Detection system is the same for each method – only difference is the means by which the “active ingredient” (Iodine) is introduced. In the volumetric technique the iodine is introduced via a burette or similar dosing system. In the coulometric technique the iodine is produced in-situ by electrolysis.
Volumetric technique is generally better suited to high water contents and is most widely used in food, agriculture, industries.
Coulometry, being 1000 times more sensitive, is better for low water content determination.
The volumetric technique involves dissolving a sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. The iodine concentration of volumetric Karl Fischer reagents must be checked using standards.
In the coulometric technique the required amount of iodine is produced at the anode which then reacts with any water present. The production of iodine is directly proportional to the amount of electricity. No reagent calibration / standardisation is required.
According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. It is therefore possible to directly determine the amount of water present in a sample by measuring the electrolysis current in coulombs.
Karl Fischer titration is simply a means to measure water content of samples. Modern instruments, such as the Aquamax KF, use the coulometric principle, whereby the water present in the sample is coulometrically titrated to a predefined end point at which there is a minute excess of free iodine present. Stoichiometrically, 1 mole of water will react with 1 mole of iodine, so that 1 milligram of water is equivalent to 10.71 coulombs of electricity. Combining the coulometric technique with Karl Fischer titration, Aquamax KF titrators determine the water content of the sample by measuring the amount of electrolysis current necessary to produce the required iodine. This is an absolute technique which does not require calibration of the reagents.
Using the latest pulse current technology and our patented “ACE” control system, (Patent No.GB2370641), the Aquamax KF automatically selects the appropriate titration speed dependent upon the amount of water present in the sample. The titration speed is reduced as the end point is approached, and when the titration is completed the instrument prints out and displays the results.
There are various designs of titration vessels used on coulometric Karl Fischer titrators and they all have one thing in common, the interior of the titration vessel should be sealed from ingress of atmospheric moisture.
To achieve this, the most frequently used design is to have ground glass joints on the generator and detector electrodes which are then fitted into ground glass joints on the lid of the titration vessel. However it is well known that dry ground glass joints will have a tendency to stick to each other and then the problems begin.
The most expensive part of any coulometric Karl Fischer instrument is generally the glassware, especially the titration vessel and electrodes. If these become stuck then it can be a very costly exercise trying to separate them, often resulting in breakages.
To overcome this problem many manufacturers suggest smearing grease on the electrode joints or, alternatively, fit Teflon sleeves over the joints. Each of these methods are equally effective but they are also equally problematic. If grease is being used to seal the electrodes into place then it is necessary to turn the joint at least once per week to ensure that the grease has not dried out and the electrode stuck. This precaution is usually stated clearly in the user manual. Similarly, when the glassware is periodically cleaned and recharged with fresh reagents the teflon sleeve should also be cleaned, dried and refitted to the electrode joint. These sleeves are often lost or damaged which once again leads to electrodes being stuck in the titration vessel.
Originally designed for coulometer users on North Sea oil and gas platforms, the G.R. Scientific Low Drift Cell uses a special design of glass joint which will not stick or jam and is totally grease free. The connecting cap of the electrode is screwed down to compress the O-ring on to the top of the female joint which is part of the titration vessel, ensuring a perfect seal. To release the electrode simply unscrew the connecting cap and the loosening ring will lift the electrode free from the joint.
Hassle free assembly and disassembly – guaranteed.
Since publication of his “New method for the determination of water”, the name of Karl Fischer and his titration technique have been recognised throughout the world. The technique is one of the most widely used and reliable methods for the measurement of water content in a large range of samples. Karl Fischer titration is now employed as a standard method in most laboratories and can be subdivided into two main techniques: volumetric titration & coulometric titration. In 1935 the German scientist, Dr. Karl Fischer, developed a titrimetric determination of water content using a reagent which contained iodine, sulphur dioxide, anhydrous pyridine and anhydrous methanol. This volumetric technique involves dissolving a sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. However, even with the automatic or semi-automatic instruments commercially available there are certain problems associated with the technique. These problems can include long analysis time, reagent calibration required, high reagent consumption rate and large sample amount required. In 1959 Meyer & Boyd were first to apply coulometry to the Karl Fischer principle. In this method the sample is introduced into a mixture of pyridine/methanol which contains iodide ions and sulphur dioxide. Using electrolysis, iodine is produced at the anode which then reacts with any water present. The production of iodine is directly proportional to the amount of electricity. According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. It is therefore possible to directly determine the amount of water present in a sample by measuring the electrolysis current in coulombs. (Coulombs are a measurement of current multiplied by time). The water present in the anode compartment of the titration cell is coulometrically titrated to a predefined end-point at which there is a minute excess of free iodine present.
Combining coulometry with the Karl Fischer titration can provide many advantages over the volumetric technique. The main benefits that this coulometric technique offers include higher sensitivity, faster titrations, no reagent calibration required and economical operation. Indeed, unlike volumetric instruments where the solvent mixture is normally replaced after each titration, coulometers can determine the water content of multiple samples on one single charge of reagent.
The criteria governing reagent life are threefold:
The physical size of the titration cell usually allows for 50 – 60 ml of sample to be added. Sample volumes of 0.1 – 2.0 ml are typical for most oil and petroleum products therefore the maximum volume of 50-60 ml is not usually a limiting factor.
The second criteria governing reagent lifetime is the total amount of water that can be analysed before saturation. A standard charge of 100ml anode reagent can usually analyse upto 1 gram of water. Considering that the injected sample volume is normally quite small, and that usually the analysis is for the determination of low levels of water, then this water capacity should not become a limiting factor either.
Similar to all other Karl Fischer reagents, coulometric reagents deteriorate when left out in sunlight and with increases in temperature. Life expectancy for one charge of reagents when left in the titration cell can be 2-3 weeks although this is also dependent on the total amount of sample injected and the amount of water titrated.
Coulombs are a measurement of current multiplied by time.
The optimum pH range for Karl Fischer titrations is 5 – 7. This will allow the reaction to run quickly and stoichiometrically. Acids are produced during the titration of water and these are neutralised by the base present in the coulometric reagent.
The term Drift refers to the background moisture in the titration vessel. This could be caused by ingress of atmospheric moisture, chemical reactions between different samples or gradual breakdown of the reagents. Vast majority of high drift values are caused by the titration cell walls and generator electrodes not being cleaned and dried sufficiently. The drift value is normally displayed in micrograms of water per minute. This additional count is automatically subtracted and is assumed to be constant for the duration of the titration. For this reason, the rate of change of the drift value is more important than the actual value itself. Although it is possible for the titrator to be used at high drift values, it is advisable to wait until the drift value is below 20 mg/minute, and stable, before commencing, especially for low water content samples in the ppm ranges. The lower, and more stable the drift – the more accurate the result.
Although it is recommended that reagents are changed either in a fume cupboard or a well ventilated area, for routine operation it is not necessary to keep the titrator in a fume cupboard. The titration vessel design seals the unit from ingress of atmospheric moisture.
Many of the commercially available coulometric Karl Fischer titrators are now becoming so complicated that a considerable skill level is often required. However, the Aquamax KF titrators, although extremely versatile and suitable for laboratory use, have been designed with non-laboratory personnel in mind. Simple single button operation, easy to assemble glassware, pre-mixed reagents, all make these titrators suitable for almost anyone to use.
What routine maintenance is required? How often do I change the septa or desiccant?
Coulometers, such as the Aquamax KF, require very little maintenance. Ideally, the glassware should be cleaned and dried each time the reagents are replaced but this is not always possible. Likewise, the injection septa and possibly the desiccant should be changed periodically. The required frequency depends on the volume of testing. For example, a program running 100 samples per week will spend about 30 minutes per month cleaning the glassware. Otherwise there is very little to be done.
In the U.K., G.R. Scientific offer both in-house and on-site calibration / validation services for almost all makes and models of coulometric Karl Fischer titrators. For USA users, Penn Hills Lab Supply offer an in-house service at their premises in Pittsburgh. This service includes complete overhaul / cleaning of the glassware, checking all electronic settings using calibrated equipment and adjusting as required, replacing consumables such as septa, paper rolls, etc, running water standards and providing a comprehensive calibration certificate.
You can check the performance of your titrator by periodically running water standards such as those supplied with Cou-Lo Formula reagents. Although in principle standardisation of a coulometer is not necessary since the water “titrated” is a direct function of the coulombs of electricity consumed, the ASTM methods for measuring water content of crude oil and petroleum products stipulate that the performance of the coulometer be regularly monitored by injecting 10 microlitre of pure water. The suggested interval is after every 10 determinations and the result obtained should be 10,000 +/- 200 microgram. The reagent should be changed if the result is outside these limits.
Most manufacturers use a similar configuration of titration cell in which the anode and cathode compartments are separated by a frit (diaphragm) and each compartment is charged with different reagents. Some companies now offer fritless generator electrodes for use with single solution coulometric reagents.
Probably the most widely available coulometric reagents are the Riedel de Haën Hydranal Coulomat products. They are used with the majority of commercial coulometers. Some coulometer manufacturers promote the fritless electrode configurations and single solution coulometric reagents, such as Hydranal Coulomat AG, as being preferable to the fritted type for many applications including for oil samples.
Although the ASTM and other methods stipulate using separate coulometric anode and cathode reagents in conjunction with fritted electrodes, we decided to ask the leading Karl Fischer formulation specialists for their opinion, especially with regard to crude oils and transformer oils samples.
Their replies stated;
“Hydranal Coulomat AG is not to be recommended for crude oil because it is very important to dissolve the tar in the crude oil in order to avoid a coating of the platinum pins. Using solubilizers such as chloroform or xylene in a reagent, it is necessary to use a cell with diaphragm (frit).”
On the subject of transformer / insulating oils they stated;
“Water content of approx 10ppm have to be analysed carefully and we would also prefer a cell with diaphragm (frit) and a reagent containing chloroform.”
It would appear that they agree with our long held opinion that coulometric generator electrodes which have a frit separating the anode and cathode chambers, in conjunction with separate anode and cathode reagents such as Cou-Lo Formula reagents, are much better for these applications. All Aquamax KF titrators are supplied with fritted type of electrode unless otherwise requested on the order.
This problem is effectively minimized through the Aquamax’s unique titration vessel design and patented ACE control system (automatically compensated errors).
Reagents, sample port septa and desiccant are the only consumables. Septa and desiccant cost very little and are replaced about once or twice per month. It may be helpful to keep a spare set of glassware on hand for breakage replacement.
The Aquamax KF has a built in battery, an in-car power adaptor and an optional carry case is available. Many Aquamax KF users carry their unit for use on the tailgate of a truck. The carry case also has a compartment for carrying the power pack, syringes, etc. Can I include on-site training with my Aquamax KF purchase? Although the Aquamax KF is simple to setup and operate, some customers choose to arrange for installation and training assistance. Our field specialists can help create a simple and efficient moisture testing program. Training of personnel includes instrument setup, theory, operation, maintenance and performance verification techniques.
Although the Aquamax KF is simple to setup and operate, some customers choose to arrange for installation and training assistance. Our field specialists can help create a simple and efficient moisture testing program. Training of personnel includes instrument setup, theory, operation, maintenance and performance verification techniques.
The toggle switch on front of the Aquamax KF enables stirrer speed to be adjusted. However, the speed that it starts at when first switched on is an optimum default value which we have found to be suitable for virtually all sample types. It should not be necessary for operator to change this setting.
It is not necessary to switch the unit off overnight but, if it is not going to be used for several days then we suggest that it is switched off. If it is not going to be used for two or three weeks then we suggest emptying the titration vessel, cleaning and drying it and assembling without replacing the reagents. On the day that you wish to start using titrator again you can simply add the new reagent and allow it to stabilise.
Most coulometric Karl Fischer titrator manufacturers only provide a 1 or 2 year warranty. At G.R. Scientific we are extremely proud of our manufacturing quality and workmanship, so much so that we now provide a full 5 years parts and labour warranty totally free of charge. A warranty certificate is enclosed with each Aquamax KF titrator.
With proper maintenance and careful handling of the glassware, repairs are rarely necessary. Electronic and software failure occurs in less than one instrument per 1000.
If you are out “on-site” and wish to leave a hard print copy of results with your customer whilst also bringing original hard copy back to your depot, simple hold down the Start key for three seconds and a duplicate print will be made.
This is extract from the Aquamax KF user manual;
SETTING THE DATE AND TIME
1. Switch on the Aquamax KF and wait for it to display the following:
2. Hold down the START key until the Aquamax KF displays:-
3. Use the decimal point key to move the cursor across the screen.
4. Use the numerical keys to set / alter the required number
5. When completed press the key to save the changes and exit
Under normal circumstances the titration cell can be used for a large number of samples before having to replenish the reagents. Once the reagents have been exhausted, or when the titration cell maximum volume has been reached, it should simply be necessary to:-
1. DISASSEMBLE titration cell (disconnect leads from titrator)
2. EMPTY the titration cell and generator electrode
3. RINSE all parts with methanol. Do not use brush on electrodes.
4. DRY all parts
5. REASSEMBLE glassware
6. RECHARGE with fresh reagents
If the cell is heavily contaminated then it may be necessary to clean it more thoroughly. For oil samples, cleaning with chloroform or xylene is suggested, whilst for salt deposits a water wash may be required. Use whichever solvent is most suited for the sample type. The titration vessel (not the electrodes) can even be cleaned with hot soapy water and a bottle-brush. However, after cleaning with suitable solvent, all glassware parts, should be rinsed inside and out with methanol. They can then be dried with a warm air blower, such as a domestic hair dryer, placed in a low temperature oven at 40 – 50 ° C, or left in a desiccator.
After being fully dried, reassemble the titration cell and charge with fresh reagents.
THE MORE THOROUGHLY THAT THE TITRATION CELL IS CLEANED AND DRIED, THE FASTER THE INSTRUMENT WILL STABILISE READY FOR OPERATION AFTER RECHARGING REAGENTS.
Under normal conditions, the Aquamax KF should be ready for operation within 5 – 10 minutes after reassembly, however it could take considerably longer to completely stabilise if the drying procedure has not been properly carried out.
After cleaning the glassware with suitable solvent, all glassware parts, should be rinsed inside and out with methanol. They can then be dried with a warm air blower, such as a domestic hair dryer, placed in a low temperature oven at 40 – 50 ° C, or left in a desiccator.
Although it is highly recommended that the glassware be cleaned and dried before replacing reagents, it is not always possible to achieve this, especially when away from the laboratory or workshop. In these circumstances we suggest emptying the reagents into a suitable container and re-assembling the glassware as quickly as possible. The least time that the generator electrode is exposed to atmospheric moisture then the faster the titration vessel will stabilise again after replacing reagents.
Aquamax KF reagents offer optimum performance with almost all models of coulometric Karl Fischer titrators. Our packaging concept has been based on advice from the HSE (Health & Safety Executive) to enable non-laboratory personnel to work more safely.
Aquamax KF anode reagent is suitable for most routine applications and is especially useful for water content determination of oil samples. This reagent contains the required amount of xylene and other solubilizers and is supplied in 100ml “single shot” bottles, no volume measurement or mixing with other solvents is required. Aquamax KF cathode reagent is supplied in “single shot” 5ml ampoules which have “safety snappers” pre-fitted thereby reducing risk to the operator.
Each pack contains 8 x 100ml bottles of anode reagent plus 8 x 5ml ampoules of cathode reagent. Weighing only 3 kilo, the total pack volume is less than one litre so they can be shipped as limited quantity.
This is extract from the Aquamax KF user manual; For most routine applications 100ml of Formula “A” (anode reagent) and 5ml of Formula “C” (cathode reagent) are used. When analysing Transformer oils, Crude oils and other petroleum products Formula “A” is especially suited as it contains other solvents to improve sample miscibility and solubility.
(When analysing samples of Ketones, amines or others which may interfere with the reaction it is advisable to use specialised reagents which can be obtained from various suppliers).
|Although reagents can be poured into the titration vessel whilst it is located on the titrator, we recommend that the vessel and electrodes are removed from the instrument whilst this procedure is performed to avoid reagent spillage onto the instrument casing. (Any spillage onto the instrument casing should be wiped off immediately to avoid damage or staining).|
|Remove the drying tube and injection septa. Using the funnel supplied, charge the titration vessel upto the lower line with Formula “A” reagent. For your convenience these reagents are supplied in “single shot” bottles which contain 100ml so it is not necessary to measure any volumes – simply pour in the complete bottle|
|Also using the funnel, charge the inner chamber of the generator electrode with Formula “C” reagent which are supplied in “single shot” 5 ml vials which havce “safety snappers” pre-fitted thereby reducing risk to the operator.(It is not necessary to clean the funnel between reagents.)|
|Reconnect the drying tube and injection septa so that the titration vessel is sealed from ingress of atmospheric moisture.Locate the complete titration vessel onto the titrator and connect the electrode leads onto the appropriate sockets|
This depends on the type and number of samples. Average reagent change is about once every two weeks. Talk with our experts about ways to optimize reagent use. Coulometers can determine the water content of multiple samples on one single charge of reagent. The criteria governing reagent life are threefold: The physical size of the titration cell usually allows for 50 – 60 ml of sample to be added. Sample volumes of 0.1 – 2.0 ml are typical for most oil and petroleum products therefore the maximum volume of 50-60 ml is not usually a limiting factor. The second criteria governing reagent lifetime is the total amount of water that can be analysed before saturation. A standard charge of 100ml anode reagent can usually analyse upto 1 gram of water. Considering that the injected sample volume is normally quite small, and that usually the analysis is for the determination of low levels of water, then this water capacity should not become a limiting factor either. Similar to all other Karl Fischer reagents, coulometric reagents deteriorate when left out in sunlight and with increases in temperature. Life expectancy for one charge of reagents when left in the titration cell can be 2-3 weeks although this is also dependent on the total amount of sample injected and the amount of water titrated.
The reagent will typically have to be replaced every two – three weeks. A reagent change uses 100 ml of anode reagent and one vial of cathode reagent. Divide the price of the reagent by the number of samples analysed to determine the cost per analysis.
Three factors govern reagent life as explained in another section. Usual indications that reagents are in need of being replaced are, darkening of the cathode reagent, progressively higher background drift values, slower titrations, unstable baseline as end point approached.
Although any Coulometric Karl Fischer reagent can be used with the Aquamax KF titrator, our own Aquamax KF reagent brand is formulated to promote optimal results. This is especially the case with oil samples due to the excellent dissolution of sample and reagent.
Some oils contain additives that can interfere with water measurements and therefore require a secondary technique employing an evaporator. Most commercially available evaporators can be easily interfaced with the Aquamax KF.
Water, acid and fine particulates in EHC (electrohydraulic control) fluids are the primary causes of failures. Water is the simplest of these to measure. Monitoring water content can provide major financial savings in fluid replacement and equipment failure
Both of these techniques are semi-quantitative at best and are sensitive to only large amounts of water. The Aquamax KF is reliable and accurate down to 1 ppm of water.
If samples are being analysed by W/w then the accuracy of the syringe is less important. However, if analysing by V/SG method, whereby a measured volume of sample is injected into the titration vessel, then the syringe and syringe technique of the operator, are of great importance. A good quality gas tight syringe with luer lock needle is recommended.
For samples such as transformer /insulating oils, which are viscous and contain low levels of moisture, we recommend inserting the syringe needle below the surface of the anode reagent before injecting the sample. This ensures that the sample is given enough time to disperse in the reagent and for the detector electrode to see any signal change. Please remember that no titration takes place until after the detector electrode has seen a signal change caused by the presence of water.
This is extract from the Aquamax KF user manual;
Program the Aquamax KF with parameters for the analysis. For Transformer Oil samples, these parameters are usually:-
Result Format = mg/kg or ppm
Calculation Mode = V/SG
Sample Volume = 1.0 ml
Sample SG = 0.875
1. Confirm that Aquamax KF is in “Ready” mode
2. Flush 1.0ml syringe several times (minimum 6 times) with sample
3. Fit luer needle and flush through with sample
4. Draw sample into syringe beyond the 1.0ml marking
5. Invert syringe so that any air bubbles can be ejected through the needle and adjust syringe plunger to the 1.0 ml mark
6. Wipe off excess sample from outside of needle using a clean, dry tissue or paper towel
7. Pierce needle through injection septa of titration vessel (1 – 2 cm)
8. Press START
9. Push needle into anode reagent and inject sample
10. Withdraw needle from titration vessel
11. Read result, in mg/kg (ppm) water, on display and printout
12. Repeat steps 2 – 11 if duplicate result required
There are various commercial water standards available, including our own Cou-Lo Formula standards. Some standards provide results as water content by weight, others by volume. As many Aquamax KF users require on-site operation of the titrator they do not always have a balance available. As such we decided to provide water standards based on 0.1 milligram per millilitre and 1.0 milligram per millilitre. This means that the operator, even when out from the laboratory, can check that his titrator is operating correctly by simply injecting 1.0 ml of standard which will give results of 100 or 1000 microgram counts
The Aquamax KF is programmed so that the calculated amount of water (ppm or percentage) can be displayed live during the titration. It is not necessary to wait until the end point has been reached to see if water content is going to be above specification
The Aquamax KF is programmed so that the sample data required for the calculation can be entered during the titration. This is particularly useful when analysing samples by W/w calculation (Weight) as the Tare weight can be entered whilst titration is still in progress.
The Aquamax KF has two different delay times which are operator programmable. Start Delay enables additional time to be added before electrolysis current is applied and the titration starts to count. This is useful if the sample is very viscous or is slow to release water Minimum Titration Time allows the titration to commence and to count as normal but does not enable the end point to be reached until at least the programmed minimum time has elapsed. This is useful when using a vaporiser or sometimes when analysing gas samples which have very low water content.
Older Aquamax KF units can be re-programmed with our latest versions of software for very little cost. Alternatively, we can also offer substantial trade-in discounts if your old titrator, regardless of make or model, is given in part exchange. Contact us for details.
Our glassblowers have specialised in manufacturing coulometric Karl Fischer electrodes, vessels, etc, for many years. We can provide equivalent glassware for almost all makes and models of coulometric Karl Fischer units at a very competitive price. Contact us for details.
Contact us for details about your nearest distributor.
Our specialists and consultants have spent many years on Karl Fischer titrations – let them help you. If you have a sample analysis problem, or if you are not sure if Karl Fischer titration would be suitable, contact us for free technical and application advice.
This is a measurement of the hydrogen ion concentration of a solution.
Sorenson found that the strength (potenz) of a hydrogen ion solution is a logarithmic function.
pH = -log (H+)
At equilibrium, at 25 degrees centigrade, the ionic product of water is 10-14 Molar.
H20 = H+ +OH- = 10-14
Therefore the concentration of hyrogen ions at equilibrium is 10-7M.
Using the Sorenson equation, the value of pH at equilibrium is 7.
So pH7 is neutral.
Greater than pH7 is alkali and less than pH7 is acidic.
The glass electrode response is governed by the Nernst Equation.
E = E0 + 2.3 (RT/nF) log H+
E = mV output from the electrode
E0 = Zero offset for the electrode
R = Ideal gas constant
F = Faraday constant
T = Temperature (ºK)
N = Ionic Charge (+1 for hydrogen)
By using Nernst, you can calculate that for each 1 unit change in pH, the measured mV value will change by 59.16mV at 25C.
The equation is a fundamental to pH measurement.
E = E0 – 2.3(RT/nF)log (Activity)
but pH = -log ( H+ ), the Sorenson equation so at 25°C, by substituting constant values.
The mV on meter µ 59.16.pH
So for every 1 unit change in pH, the mV reading on the meter will change by 59.16.
The graph for the Nernst equation is straight.
At ideal conditions, it should pass through 0mV, the iso potential point.
Temperature has an effect on the pH value.
There is a Temperature function in the Nernst equation.
This is why, for exact measurements, you must compensate for T.
This can be done with an ATC system.
Use an Automatic Temperature Compensation probe or an electrode with built in ATC system.
Basic systems have MTC, the Manual system, where you input the room temperature at the time of measurement.
The graph passes through 0.
The temperature effects are greater at the extreme ends of the pH scale.
A basic system comprises a meter, a measuring electrode sensitive to pH concentrations, solutions to calibrate the system (pH buffers) and an arm to hold the electrode in the solution.
You can use half cells (separate pH and reference electrodes) or combination options to measure the hydrogen ion concentration of a solution.
A combination electrode has two chambers, the pH sensing system and reference chamber.
The pH system is made of a glass that is sensitive to hydrogen ion concentrations.
There are different types of combination electrodes. The basic pH sensing system does not change.
The sealed chamber contains a pH7 buffer solution. The glass bulb measures the potential, mV, of the solution. The meter converts the mV signal to a pH value on the display.
The reference type can change, according to the sample being measured.
The reference filling medium can be liquid or gel.
It may have a single or double junction.
The electrode may have a glass or plastic housing.
The pH glass may be shaped to meet application requirements.
For example, flat surface or spear type.
They must be easy to clean.
They must be cleaned carefully without scratching the glass to remove excess sample.
Always keep the glass bulb wet.
Soak in water overnight or use liquid storage sleeve for long term storage.
Calibrate the system frequently to check pH performance.
Always top up reference electrolyte.
These are solutions whose concentration of hydrogen ions do not change.
They do have fixed shelf life.
The most common are pH4, 7 and 10.
They are available as ready made solutions in bottles or sachets.
Powders or capsules are made up in water according to instructions.
Use the pH buffers to calibrate the system. It is best to do this on a daily basis or before each batch of new samples. A two point calibration is favoured. However, a three point calibration, across the pH range, is chosen when sample pH is truly unknown.
When a system is calibrated, the meter may have the ability to give a readout of slope value (in mV) or efficiency (usually as a %).
This is used as a test of electrode performance.
A poor slope value may mean that the electrode needs cleaning, the reference fill solution topped up or, (for very precise analysis to the third decimal place) replacing.
The meters can be small hand held or bench laboratory type.
Each type converts the measured mV readings to pH values.
These values are displayed on the meter screen.
Many have features for printing, data logging and memory functions.
The most common electrode connector is BNC.
These analyses do not obey Nernst because the hydrogen ions are not ionic. They are typically covalent/organic mixtures.
mV readings do not obey Nernst and can be outside typical values from normal usage.
As organic compounds can remove water molecules from the glass bulb of the electrode, reconditioning before each analysis is vital.
Choose the right electrode for the job. Size, shape, connector type.
Always look after the electrode.
Keep clean, hydrated and ready for use.
Calibrate the system daily for optimum system performance.
This is a measure of total ionic concentration.
The higher the concentration of ions, the higher the current passed through the solution.
The meter is usually an auto ranging system to allow for a wide range of applications.
The ideal conductivity cell measures the ability of current to flow through volume of solution between two plates
Plates 1cm square, and separated by 1cm.
The practical implementation of the ideal conductivity cell is very difficult.
Modern cells are built into glass or plastic
The plates may be curved and separated by varying distances.
The conductivity (C) is calculated from the actual conductance (G), measured between the plates, multiplied by the cell constant (K).
Any cell can be represented by an equivalent cell in this case the formula can be expanded to:
C = (G L) / A
C = Conductivity (S/cm)
G = Conductance (S)
L = Distance between plates (cm)
A = Area of plates (cm2)
A precise cell constant or K factor is given to a cell by comparison to a standard cell.
If ad.c.voltage is applied to the plates. Then polarisation and electrolysis will occur. This will cause bubbles to form on the plates.
To overcome this problem an a.c. voltage is applied to the plates which are also coated with a black platinised coating that will absorb the products of electrolysis on alternative half cycles.
The standard probe has a K factor close to 1.00.
The K=1.00 probe is suitable for most applications
Two other kinds of probe are available K=0.1 and K=10.
K=0.1 is used for low conductivity solutions.
The plates are moved closer together so that the lower level of activity gives a higher signal
The K=10 cell is used in situations where the top of the range is required
The meter may be hand held or bench laboratory type.
Electrodes can be glass or plastic type.
It is common in instrumentation to require calibration of multiple points over a small range.
Conductivity has a wide dynamic range over many orders of magnitude and can often be calibrated at a single point.
Calibration is performed by measuring solutions of a known concentration of potassium chloride
Conductivity is used to measure the overall concentration of conducting ions in solution.
This can be used to measure the purity of a water sample, the most common application.
In addition to this a number of derived measurements are based on conductivity.
Total dissolved solids is also a measurement of purity which is based on conductivity.
Salinity is another measurement which can be performed.
Conductivity measurements are referenced to a temperature.
This is usually 25 degrees but can be 18 or 20.
The system will have a variable temperature coefficient but a standard value, set in memory. This is usually sufficient for most applications.
Defined as the weight of sample remaining after the gravimetric analysis of a solution.
This a tedious experiment with a great possibility for error.
An approximation is multiplied by the conductivity measurement.
The factor which is used for this is called the EC ratio, set to 0.6. A standard can be used.
The measurement of Salinity is another application of conductivity.
A more complex equation is applied in this case but the measurement is a version of theTDSmeasurement.
The units are different and the range is limited by the standard method which is applied (2-40 Practical Salinity Units).