Hydrogen Electrode – Reference Electrode – Indicator Electrode

Content

Reference electrodes
Hydrogen electrode HydroFlex
Mini-Hydrogen electrode
Hydrogen electrode HydroFlex – Activation
Storage
Exchange of the Hydrogen Cartridge
Cleaning
Troubleshooting
Application of Hydrogen electrode HydroFlex
How develop potentials
Measuring potentials – not least in schools
Potential – voltage – Electromotive force
Reference electrodes with salt bridge (reference electrode 2. kind)
Electrolyte bridge – Salt bridge – Electrolyte key
Diffusion potentials

Reference electrodes

Reference electrodes are needed for measuring electrochemical potentials. A reference electrode is a half cell with an electrode potential establishing fast, reproducible and long time constant. It’s necessary to mention which kind of reference electrode you are using for your measurements. For measurements confirming standards you has to use the Hydrogen Electrode. The hydrogen electrode must be connected as „ground“. „Ground“ is the common reference point and considered to have zero voltage.
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HydroFlex – THE Reference Electrode – Indicatorelectrode

Setup in crossview of hydrogen reference electrode HydroFlex.

Setup in crossview of hydrogen reference electrode HydroFlex.

The Hydrogen Reference Electrode HydroFlex is the reference electrode of first choice for monitoring electrochemical potentials conforming to standards. Only hydrogen reference electrodes are measuring directly the activity of hydrogen ions. This hydrogen electrode HydroFlex is made of PTFE. This electrode is as small as a pencil and thereby very handy. The total length is 12 cm only. The rod has a diameter of  8 mm. By activation of the hydrogen evolution in the head of the electrode (cartridge) the production of hydrogen begins. The gas fills the PTFE-tube and flows out through the platinum/palladium electrode.

The voltage drop can be measured at the golden socket at the electrode’s head.Your measurement devices should have an impedance of 5 MOhm and more. This hydrogen reference has a low resistance – you can use unshielded cables.
This hydrogen electrode can be used up to temperatures of 210°C – as long as only the PTFE-Tube is submitted this temperature.
The advantages of this practicable hydrogen reference electrode  are obvious. An internal hydrogen source (cartridge) – easy to replace delivers hydrogen for the electrode. The hydrogen electrode has no inner electrolyte and therefore, no Iionic outlet and no diffusion potentials. There is no electrolyte to refill – low maintance. The hydrogen electrode doesn’t contain toxic heavy metals. HydroFlex is made for use in concentrated fluoride containing media because it’s made of PTFE. It  works very well in highly concentrated alkaline and acidic media (pH -2 up to pH 16).
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Activation of Hydrogen electrode

Principle of reference electrode HydroFlex.

Principle of reference electrode HydroFlex.

For first use the hydrogen electrode must be activated. The Hydrogen evolution in the electrode’s head begins and the gas fills the PTFE-tube. It flows out through the platinum/palladium electrode. There a hydrogen potential appears – depending on the measured electrolyte. This voltage can be measured at the golden socket in the electrode’s head. Once activated, this reference electrode runs through for six months.

 

 

 

 

 

Activation of the Hydrogen electrode HydroFlex – Step by Step

For first use the hydrogen electrode must be activated. For the operation of the electrode it’s neccessary that you take the following steps:

Activation of hydrogen electrode HydroFlex - step by step

Activation of hydrogen electrode HydroFlex – step by step

Activation of Reference Electrode HydroFlex (Video)
Manual Hydrogen Reference Electrode HydroFlex

 

Your are interested ? Please contact us.

We offer HydroFlex and service worldwide (Europe, USA, Australia, Asia, New Zealand, Africa).

You can order HydroFlex directly in our Webshop or at eDAQ (article ET070).

 

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Mini-hydrogen electrode

mini-hydrogen electrode capillary diameter 1. 6 mm, capillay length 85 mm

mini-hydrogen electrode capillary diameter 1. 6 mm, capillay length 85 mm

For applications on confined space we minimised our hydrogen electrode. Our mini hydrogen electrode is supplied ready for use. No activation is neccessary.
The hydrogen source must be replaced after 12 month.
This electrode is for use up to  80°C .
It’s working without inner electrolyte.
We offer this one with a capillary made of PEEK. Length of the capillary is 85  mm, it’s diameter is 1.6 mm.
Your are interested ? Please contact us.

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Storage

After the measurements, rinse HydroFlex or the mini – hydrogen electrode thoroughly with water.

Please always provide the hydrogen electrode in a liquid – e.g. measurement solution, 1 mol/l hydrochloric acid, 1 mol/l sodium hydroxide solution – even if you do not measure. The hydrogen electrode must not be stored dry in the air!

We recommend not to change the runtime at HydroFlex

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Exchange of the Hydrogen Cartridge

The function of the hydrogen electrode is ensured, as hydrogen is produced ( runtime ) . After reaching the set duration, we recommend 6 months, the cartridge must be replaced at HydroFlex.

Exceeding the running time can cause damage to the gas diffusion electrode and must be avoided.

HydroFlex: With a SW21 – wrench, you can remove the old cartridge and replace it with a new one. When reinstalling the cartridge be careful with the right position of the sealing O – ring. This must not press out itself . Seal the screw thread with a universal grease (as Korasilon paste high viscosity ). The spent cartridge can be disposed of as waste battery .

The cartridge and the SW21 – wrench can be purchased through our Online-Store or contact us.

 

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Cleaning

In rare cases such visible deposits on the electrode, a complete cleaning of HydroFlex is required. Remove the cartridge. Bring the HydroFlex for 1h in 1 mol/l nitric acid. Then put the entire HydroFlex in water for 24 hours. Dry the Hydroflex at about 120° C (make HydroFlex upside ) for 24h. Now HydroFlex is again completely cleaned. Install a new cartridge and activate the HydroFlex.

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Troubleshooting

The potential is not displayed correctly

Cause 1: Bad connection
Check the measuring cable for visual damage such as corrosion, cracks and firmly seated connectors. Replace the cable.

Cause 2: No or low hydrogen production
Was Hydroflex electrode properly activated? If not, please activate Hydroflex.
Do you await the activation phase of 24 hours? If not, then wait 24 hours before you measure!
If the term of the Cartridge exceeded? If so, then please the electrode clean as described under Cleaning and install a new cartridge. This is to enable it until naturally before use.

Cause 3: Ion exchange is slow, for example, when changing from concentrated solutions to weakly concentrated solutions
Response time to wait – sometimes the balance of concentrations simply takes much longer than expected.
Check the potential in another electrolyte like 1 mol/l hydrochloric acid.

Cause 4: Air/oxygen occurs at the electrode
Avoid that gases such as air or oxygen reach the bottom of the shaft before or on the electrode. In this way, the hydrogen is displaced or reacted from and it can be set no hydrogen potential.

Fluctuating, noisy or vibrating Potentials

Cause 1: Hydrogen bubbles of the electrode itself.
From electrode continuously withdrawn from gas bubbles. These are sometimes very small, sometimes larger. Normally they dont’t disturb your measurements.
Forms at the bottom of the electrode from a large bubble, which sticks to the vessel wall?
Position the electrode, if possible, further away from the edge of the vessel or hang HydroFlex obliquely in the measurement vessel.

Cause 2: Initiated gases
Change the position of your gas inlet. Initiated gas bubbles that pass by close to HydroFlex can interfere with the measurement and lead to fluctuating potentials.

Cause 3: Potentiostat or measuring devices
Check your measuring device. If you measure in poorly conducting electrolyte, the gauges and potentiostat can very quickly reach their limits. More Information about noise in electrochemical measurements can be found here: What-can-cause-my-experiment-to-be-noisy

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Deposits (e.g., red or brown ) on the electrode

Cause 3: Electrode is contaminated and it sets itself a mixed potential
Check the runtime of the cartridge.
Wipe clean with a cloth or wash in 2 mol / l nitric acid and rinse with distilled water .If this is not sufficient, the electrode needs a cleaning as described under Cleaning.

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Application of Hydrogen electrode HydroFlex

So far we tested HydroFlex in these solutions. If you are looking for a solution not mentioned in the table above, it does not necessarily mean that HydroFlex can’t be used in your application. It only means that we did not yet test it. Please do not hesitate to contact us in this case!

You can use hydrogen reference electrode HydroFlex f.e. in this electrolytes.

You can use hydrogen reference electrode HydroFlex f.e. in this electrolytes.

HydroFlex as RHE (Reversible Hydrogen Electrode – Indicator electrode)
The most common use of HydroFlex in the daily lab routine certainly is the application as RHE. You simply dip HydroFlex into your solution, directly. The advantages are obvious. You don’t need a liquid junction, you don’t have diffusion potentials and you don’t contaminate your solution by ions flowing out of your reference system. As HydroFlex needs no maintanance except the regular exchange of the H2-Cartridge every 6 months, it is very well applicable for long-term tests.
HydroFlex as NHE (Normal Hydrogen Electrode)
HydroFlex works as a NHE, if you put it into hydrochloric acid at a concentration of 1 mol/l and ambient pressure. The potential differences between a NHE and a standard hydrogen electrode are small while the experimental complexity is easily manageable.
HydroFlex as SHE (Standard Hydrogen Electrode)
The standard hydrogen electrode is the most important reference electrode, because it’s potential is defined as zero point of the electrochemical series.
For solutions in protic solutions, the universal reference electrode for which, under standard conditions, the standard electrode potential H+/H2 is zero at all temperatures.“ (IUPAC Compendium of Chemical Terminology, Goldbook, Version 2.3.3., 24.03.2014)
standard hydrogen electrode: „The standard hydrogen electrode consists of a platinum electrode in contact with a solution of H+ at unit activity and saturated with H2 gas with a fugacity referred to the standard presse p of 105 Pa.“ (Quantities, units and symbols in physical chemistry, IUPAC Green Book, 3rd edn, 2nd printing, IUPAC & RSC Publishing, Cambridge, 2008). Since 1982 the IUPAC declares the standard pressure as 1.000 bar (100 kPa) vor. Before 1982 the standard pressure was defined as 1.01325 bar (101.325 kPa = 1atm)2. that’s the reason you will find this value, it’s prefered in electrochemistry. In this case HydroFlex has to be dipped into a solution with hydrogen ion activity of 1 mol/l. A hydroclorid acid with molar concentration if 1.184 mol/l has got an activity of 1. The temperature has to be 298,15 K (77 °F) and the pressure has to be 1013,3 hPa (14,697 psi).As the verification of these standard conditions is challenging, the use of HydroFlex as SHE usually is irrelevant in the daily routine.
HydroFlex as pH Electrode

Hydrogen electrode Hydroflex as pH electrode combined with a silver reference electrode

Hydrogen electrode Hydroflex as pH electrode combined with a silver reference electrode

In combination with a reference electrode in a two electrode arrangement HydroFlex can of course also be used as a pH electrode . Even better is the combination of two hydrogen electrodes to a Hydrogen pH Electrode.

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Principles of electrochemical potential measurement

How develop potentials?

Voltages or potentials develop between an electrical conductor (Electrode) and an ionic conductor (Electrolyte), once they are connected. They are exchanging charges to reach a balanced state. When an iron plate is immersed in a copper saline solution, a copper deposit will form on the iron after some time. However, if you turn the experiment around and immerse a copper plate in an iron saline solution, nothing happens.
How do these two metals differ?
When the metal and the liquid are in contact with each other, equilibrium is formed between the two. From the metal which is immersed in a liquid of its own or any other metal ions, metal ions dissolve from its lattice. This is known as solution pressure, resulting from the difference between the lattice energy and hydration energy. In return, metal ions are forced back into the metal lattice due to the deposition pressure.

Schematic deposition copper on iron.

Schematic deposition copper on iron.

So in the example above, this means that iron ions go into solution, while the copper ions present in the solution absorb the remaining electrons in the iron and are deposited as copper on the iron. If the experiment is turned around, neither copper goes into solution nor are iron ions deposited. The solution pressure of iron is obviously higher than that of copper. If the solution pressure is dominant, an electron excess forms on the metal rod. If the deposition pressure outweighs, a shortage of electrons forms on the metal rod. A dynamic equilibrium forms. The deposition of the metal ions is supported by electrical pressure, meaning the electrical attraction of the negatively charged metal. Solution pressure, deposition pressure and electrical pressure lead to a state of minimum energy, where electrical work is done, which in turn is manifested by a voltage on the metal rod.
This absolute equilibrium voltage as such, cannot be determined experimentally, since only voltage differences are experimentally accessible. This requires a second electrode, the comparison electrode, or better, reference electrode (indifferent electrode). Even their absolute equilibrium voltage cannot be measured individually. To compare the voltages of various metals in their solutions with each other, they have to be measured against the same reference electrode. Here an electrode should be used whose equilibrium stress sets in quickly and reproducibly.
In the past, a platinized platinum electrode bathed with hydrogen gas and immersed into hydrochloric acid has established itself. The equilibrium voltage of this electrode was defined at all temperatures to 0,000 volts at standard conditions (hydronium ion activity of the hydrochloric acid 1 mol/l; hydrogen gas 1,013 bar) is defined at all temperatures to 0,000 volts. Only then the hydrogen electrode is referred to as a standard hydrogen electrode.
If now the voltage differences of various metals in their metal saline solutions are measured against the standard hydrogen electrode, various voltages smaller or larger than 0,000 V are shown.

Measuring standard potentials with HydroFlex as Normal Hydrogen electrode.

Measuring standard potentials with HydroFlex as Normal Hydrogen electrode.

The expected equilibrium potentials of different systems under standard conditions (25°C, metal ion activity of 1 mol/l) are to be collected in the so-called electrochemical series. Metals with equilibrium potentials less than 0.000 V are denoted as ignoble. When the equilibrium potentials are greater than 0.000 V, the metals are called noble.The expected equilibrium potentials of different systems under standard conditions (25°C, metal ion activity of 1 mol/l) are to be collected in the so-called electrochemical series. Metals with equilibrium potentials less than 0,000 V are denoted as ignoble. When the equilibrium potentials are greater than 0.000 V, the metals are called noble.
According to the electrochemical series can be specified equilibrium potentials not only for metals but also for non- metals and their ions and ionic compounds and their reloading .When a reactant is oxidized , is formed at the same time a reduction product , and vice versa. There are therefore always ahead of redox couples . The equilibrium potential is a measure of the oxidation or reduction capacity . The more negative this potential , the easier they emit electrons . They are oxidized and thus act as a reducing agent . The higher the potential , the more difficult they are to oxidize . Instead, they are easier to reduce and act as an oxidant . It can be calculated from values the expected voltages for different combinations of half cells . Predictions about possible or not possible chemical reactions in aqueous solutions are possible .
The convention E = E(right) – E(left) is significant for calculation of standard potentials.

electrochemical series

electrochemical series

electrochemical series for download

Activity and concentration – both terms are used in electrochemistry – but what is the difference?

The molar concentration c indicates the amount n of a dissolved species in a defined volume and therefore has the unit mol/l. Further customary in chemistry is the term molarity M for molar concentration in mol / l. The molar concentration is a temperature-dependent variable, because the volume is temperature dependent. One can relate the amount of substance of a dissolved species on the weight of the solvent in kg. Then we obtain the so-called molality b with the unit mol/kg. The molality is independent of temperature. One finds the molalities mainly in older table works. Furthermore, we find the concept of activity of a species. The activity of a substance is a thermodynamic quantity, a kind of corrected concentration. In a solution, the charged ions of solvation shells are (hydrate in water as solvent) surrounded and so off from each other. The ions are less effective. The activity therefore corresponds to the remaining effective concentration which is lower due to the interaction between the ions in the solution in the rule rather than the concentration. Only in very dilute solutions activity and concentration are approximately equal. In highly concentrated solutions go against the activities can achieve even higher values than the concentration because of insufficient solvent molecules are present to all ions completely solvate. The difference between activity and concentration is reflected in a correction factor, called the activity coefficient – y, γ or f -, depending on the form in which the molar concentration will indicate again.
General definition of the type (does not match the definition to physicochemical definition) accoridng to Jander, Jahr; Maßanalyse; 15. Auflage, 1989, de Gruyter:
a = y c c = molar concentration in mol/L (Molarity)

a = γ c c = molar concentration mol/kg (Molality)

a = f c c = Content of solute mole fraction

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Measuring potentials – not least in schools

The measurement of potentials is done practically without power. The electrode with the potential to be determined is connected in a corresponding solution by means of a measuring device with a reference electrode. Depending on the measurement conditions specific requirements are given to the cell itself. However, the instruments used can influence the measurements. The measured potentials can be influenced and falsified by input impedances of the measuring devices, depending on the resistance of the reference electrode itself. Using conventional references you have to use divices with high impedances because the references are high-impedance themselves. The Hydrogen reference electrode HydroFlex has got a low impedance allowing the use of simple devices with 10 MOhm impedances – asumed the second electrode and electrolyte are also of low resistance.
In our opinion Hydroflex belongs to every school , but can be of simple systems the standard potentials measured according to standards with her. You can use it in sulfuric or hydrochloric acid concentration of 1 mol/l Hydroflex in a beaker and have directly present a normal hydrogen electrode . Connect this vessel via a salt bridge , in the simplest case with KNO3 soaked filter paper with the system to be measured and your students can directly read on multimeter the standard potentials and compare with the table works.

Experimentel and theoretical standard potentials of simple systems, which can be measured at school.

Experimentel and theoretical standard potentials of simple systems, which can be measured at school.

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Potential – voltage – electromotive force

All three terms refer to a voltage difference between two systems. Is the reference point for the measurement is known, it is called potential, in other cases it is called voltage. Electromotive force refers to the potential difference of a cell, which is measured when the cell works reversibly and no current flows. By convention of the IUPAC1, a standard potential (or the standard voltage) E is determined, in which the standard electrode potential of the right cell is calculated minus the standard electrode potential of the left cell: E =E(rechts) – E(links)
1: Cohen et al: Quantities, units and symbols in physical chemistry, IUPAC Green Book, 3rd edn, 2nd printing, IUPAC & RSC Publishing, Cambridge, 2008, S. 71
IUPAC2 defines: „The standard potential of an electrochemical reaction, abbreviated as standard potential, is defined as the standard potential of a hypothetical cell, in which the electrode (half cell) at the left of the cell diagramm is the standard hydrogen electrode (SHE) and the electrode at the right is the electrode in question.“
2: Cohen et al: Quantities, units and symbols in physical chemistry, IUPAC Green Book, 3rd edn, 2nd printing, IUPAC & RSC Publishing, Cambridge, 2008, S. 74
If, therefore, the standard hydrogen electrode is always considered to be the „left“ cell automatically the standard potentials of the electrochemical series result, specified as standard reduction potentials. In older tables one can often still find standard oxidation potentials specified, then the leading signs have to be reversed. Whether oxidation or reduction potentials are given in a table, can be recognized by the leading sign for the reaction Cu/Cu2+ because the standard reduction potential is +0,34 V.
This always raises the question which electrode is connected to which input of the measuring device, because depending on the connection of the electrodes changes the leading sign of the measured potential difference. If the standard hydrogen electrode is connected to the negative input (COM), the values shown in the electrochemical series will result. If the measurement is performed against a different reference electrode, it is connected to the negative input (COM). The measuring electrode is thus always connected to the positive pole: E =E(electrode) – E(reference electrode)

Examples (satturated electrolytes):

Caculation of standard potentials for common reference electrodes

Caculation of standard potentials for common reference electrodes

The leading sign of the measured potentials reflects which electrode is more positive, the direction in which current flows and which reactions happen spontaneously, because the potential is directly connected to the equilibrium constants of the reaction participants. E > 0 for E(right) > E(left) means, the right electrode is positively charged relative to the left electrode. Therefore the right electrode has an electron deficiency, since the particles there are reduced. This is also referred to as the cathode electrode (reduction). Therefore electrons flow from left to right. The electric current, unfortunately defined as a flow of positive particles, thus flows in the opposite direction. For the left electrode, there is a surplus of electrons. There, particles are oxidized. This electrode is referred to as the anode (oxidation). The reaction therefore tends to proceed spontaneously from left (oxidation) to right (reduction). E < 0 for E(right) < E(left) then means, conversely, that the reaction proceeds spontaneously from right to left.
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Reference electrodes with salt bridge (second-kind reference electrode)

In addition to the reversible hydrogen electrode, in previous years, other systems have been established as reference electrodes, all of which can be used only with an internal electrolyte and corresponding electrolyte bridge. This are second-kind reference electrodes. These reference electrodes should be selected according to the measurement solution, to avoid contamination and unnecessary diffusion voltages. In addition, the diaphragm does not have to have such fine pores, which reduces the contact resistance between the measurement solution and reference electrode.
In aqueous, acidic solutions follwing reference electrodes 2. kind are etablished.

Silver-Silverchloride Electrode

Standard reference potential and reference potentials vs. NHE, 25°C (Hamann, Vielstich: Elektrochemie; Wiley-VCH, 1998)

Silver-silver chloride electrode - dependence of the potential of temperature and concentration of the internal electrolyte.

Silver-silver chloride electrode – dependence of the potential of temperature and concentration of the internal electrolyte.

Standard potentials of silver/silverchloride electrodes

Standard potentials of silver/silverchloride electrodes

 

 

 

 

 

 

 

 

 

Mercury-mercury chloride electrode (calomel electrode)

Standard reference potential and reference potentials vs. NHE, 25°C (Hamann, Vielstich: Elektrochemie; Wiley-VCH, 1998

Calomel electrode - dependence of the potential of temperature and concentration of the internal electrolyte

Calomel electrode – dependence of the potential of temperature and concentration of the internal electrolyte.

Standard potentials of calomel electrodes

Standard potentials of calomel electrodes

 

 

 

 

 

 

 

 

 

Mercury mercury sulfate electrode

Standard reference potential and reference potentials vs. NHE, 25°C (Hamann, Vielstich: Elektrochemie; Wiley-VCH, 1998

Mercury-mercury sulfate - dependence of the potential of temperature and concentration of the internal electrolyte.

Mercury-mercury sulfate – dependence of the potential of temperature and concentration of the internal electrolyte.

Standard potentials of mercury/mercury sulfate electrodes

Standard potentials of mercury/mercury sulfate electrodes

 

 

 

 

 

 

 

 

 

In alkaline solutions:

Mercury mercury oxide electrode

Standard reference potential and reference potentials vs. NHE, 25°C (Hamann, Vielstich: Elektrochemie; Wiley-VCH, 1998

Mercury-mercury oxide - dependence of the potential of temperature and concentration of the internal electrolyte.

Mercury-mercury oxide – dependence of the potential of temperature and concentration of the internal electrolyte.

Standard potentials of mercury/mercury oxide electrodes

Standard potentials of mercury/mercury oxide electrodes

 

 

 

 

 

 

 

 

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Electrolyte bridge – Salt bridge – Electrolyte key

Different salt bridges in conventional second-kind electrodes.

Different salt bridges in conventional second-kind electrodes.

An electrolyte bridge, also known as salt bridge or electrolyte key, is used for contacting different electrolytes. The diaphragm is the ion-conducting contact point. In case of conventional reference electrodes and salt bridges, diaphragms consisting of the following materials like ceramics, platinum wire, joints, smallest drilled holes, capillary (Haber-Luggin-Capillary), fibers, tissues (pellon) or robust Teflon (PTFE) can be found.
The different materials have different flow rates, depending on the porosity of the material and the size of the diaphragm. Processing during the reference electrode’s manufacturing, damages, and impurities in the diaphragm can affect flow rates.

Typical materials used as diaphragm.

Typical materials used as diaphragm.

Leakrates of different types of diaphragms.

Leakrates of different types of diaphragms.

When using electrolytic bridges it should be noted in general that depending on the type of the diaphragm different amounts of internal electrolyte will leak into the measurement solution. The reference electrode selection is aimed, therefore, according to the measurement solution, in which it will be used. To avoid contamination of the inner electrolyte in the reference electrode with diffusing measurement solution, it is necessary that there is a leakage of the inner electrolyte into the measurement solution. So the filler opening should be open during the measurement or at least not be closed airtight. The level of the inner electrolyte should be well above the level of the measuring solution. It is important that the reference electrodes always be refilled again!
In difficult measuring media such as dirty or highly viscous samples, joint diaphragms are preferred due to their high effluent flow rates.
Reference electrodes with solid internal electrolytes have no electrolyte output flow. However, here the measuring solution can enter through the diaphragm into the solidified inner electrolyte and contaminate it and eventually dissolve and dilute it.

Inner electrolytes of reference electrodes are poising your solution you like to investigate. Reference electrodes must be refilled with inner electrolyte. the liquid level in the reference electrode must be higher than the level of your solution. Reference electrodes with solid electrolytes become worse by time, because ions of your solution will penetrate into the solid electrolyte and change it.
At any Diaphragm develops a diffusion potential, leading to deviations of the original potential.
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Diffusion potentials

The diaphragm is the contact point between the electrolyte bridge and electrolyte to be measured. At this point so-called diffusion voltages occur because the anions and cations of different electrolytes have different migration rates. they can be minimized by choice of the electrolyte in the saltbridge.

diffusion potentials

diffusion potentials

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