Why: Voltammetry Reference Frit

Application Guide

During 40 years of good chemistry and sensor manufacture, Ionode have developed and accumulated a well-founded compilation of sensor application notes, technical information, trouble shooting guides and other helpful information. Chemistry students through to seasoned technicians and academics alike can find assistance and advice at Ionode. You will find additional information within the Dealer Resources section.

General Information

• What is pH?
• Combination Electrode
• Junction Types
• Membrane Types
• Internal Reference Types
• Reference Construction
• Electrode Construction
• Maintenance
• Good Measurement Practices

What is pH?

The pH is a measurement for the acidity or alkalinity of a solution. In pure water the hydrogen ion (H+) and hydroxyl ion (OH-) concentrations are equal at 10-7 M (25°C). To provide a convenient and effective means of defining acidity and alkalinity, the pH is defined as the negative logarithm of hydrogen activity: pH = -log [H+]

The heart of a pH measuring system is a membrane made from special pH-selective glass on which a very thin layer of hydrogen ions is formed when dipped in water. At high pH values, this layer will have a low hydrogen concentration. However, at low pH values a large number of H+ ions diffuse in the layer. By measuring the generated electrical potential (E) in the layer the corresponding pH can be computed.

Solution-1: sample to be measured
Solution-2 : known buffer solution (7 pH) Reference-1: silver wire in a salt-bridge (KCI) Reference-2: silver wire in a salt-bridge (KCI) Membrane layer-1: H+ ions generated by the sample Membrane layer-2: H+ ions generated by the buffer

The potential (E) between both wires will vary with the pH difference between sample and known buffer according the Nernst-equation (-59.2 mV/pH at 25°C). A salt-bridge around each wire prevents direct metal contact with the solutions by using a wet junction for a stable electrical behaviour.

Combination Electrode

A combination electrode is an indicating and a reference electrode combined into a single body that is easy to use and popular because of its compactness. A minimum amount of sample is required due to the close proximity of the pH responsive membrane and the liquid junction.

Junction Types

General electrodes have annular ceramic junctions. The annular junction is formulated with a special ceramic which is located near the base of the pH sensing glass. Numerous pores in the ceramic provide lower resistance and more stable pH readings.

Sleeve junctions provide the highest flow rate for difficult samples. A double junction reference is constructed with an Ag/AgCl inner chamber and a chemically compatible reference solution in the outer chamber. It is recommended for samples containing organic compounds, proteins, heavy metals, and other compounds that interact with silver, such as bromides, iodides, cyanides, and sulfides.

Membrane Types

• General and low temperature glass is especially suited for low temperature, non aqueous solutions and pH measurements under 12 pH.
• Universal glass is best suited for most pH measurements where minimal or no Na+ is present. It is a low-resistant glass with a very fast and stable response and is designed for pH ranges of 0 to 13 and temperatures of 0° to 135°C.
• High pH, low sodium ion error glass is especially formulated for continuous, long-term use at high temperatures, particularly in strong alkaline solutions above pH 11. It experiences negligible Na+ ion error above 13 pH. The impedance of the glass is much higher than other glasses, and a slower response will be experienced at room temperatures and below. Response time will increase as the temperature is elevated.

Internal Reference Types

Calomel reference electrodes (Hg/Hg2Cl2) can give very accurate potentials. Both its reproducibility and potential stability are superior to those of the Ag/AgCl electrode, although only at a constant and relatively low temperature. Calomel is subject to a constant and relatively low temperature fluctuation with a temperature limitation of 80°C.

Silver/silver chloride reference (Ag/AgCl) electrodes are largely hysteresis-free and can be used at higher temperatures with lower temperature coefficients. Ag/AgCl is the best general purpose reference with a wide temperature range (-5° to 110°C).

Reference Construction

Refillable Reference Cell: selected for high accuracy, stability, and longer electrode life. Refillable types sacrifice convenience and ease of maintenance.
Sealed Reference Cell: sealed gel-filled reference electrodes are designed for convenience where minimal maintenance is required. Slightly lower accuracy and shorter life must be taken in account.

Electrode Construction

Polypropylene Body Electrodes: ideally suited for most routine pH measurements for accuracy, high temperature, and ease of cleaning. Kynar Body Electrodes: a good choice for applications where rough handling and breakage are a major problem.

Maintenance

A pH electrode is active and stable only after wetting! For this purpose it must be immersed for at least ten hours in a 3...4 M KCl solution. During short interruptions (e.g. storage) the electrode should be immersed in a 3...4 M KCl solution. In doing this it is always kept ready for use. When the interruption is longer than a month, refill the closing cap with 3...4 M KCl and plug it on the electrode tip in order to protect the glass bulb. Before use, ensure that the reference part of the electrode is topped up with a 3...4 M KCl solution.

Avoid a low pressure inside a refillable electrode! Therefore always remove the closure from the refilling aperture during the measurements as well as during the calibration. This allows the saltbridge solution to flow through the ceramic liquid junction and prevents contamination of the electrolyte. For the same reason, the inside level should always be higher than the outside level of the measuring solution. Close the refilling aperture again when storing the electrode. A polluted electrode may be cleaned with a soft detergent or 0.1 M HCl. Greasy substances may be removed with acetone or alcohol (never do this with plastic electrodes!).

If the electrode is polluted by proteinaceous materials (such as blood), it should stand in a cleaning solution overnight and then be cleaned with distilled water before use. The pH electrode wears away by being used. If the electrode tends to respond slower and calibration becomes difficult, even after cleaning, it should be replaced by a new one.

Good Measurement Practices

• While calibrating or measuring all solutions should be stirred gently (e.g. with a magnetic stirrer) to ensure the electrode gives a true representation of the beaker contents.
• Calibration solutions should be chosen which have values near the expected sample value.
• Only fresh calibration solutions should be used! Changing all solutions daily is a good practice.
• All solutions should be maintained at equal temperature.
• Rinse the electrode twice between measurements: first thoroughly in distilled water and then with a small amount of the next sample to be measured.
• Allow the electrodes sufficient time to stabilize while calibrating or measuring. A stability indicator on most of our meters prompts the user when readings should be taken.

Application Advice:

Applications advice and testing methods are an ever-growing resources. As a guide, you'll find Ionode sensors suit most applications, including the following:

• Acidity
• ALKALINITY
• CHLORIDE by titration
• CHLORIDE by ISE
• REDOX TITRATIONS
• #pH and Lubricating Oils
• Acid Numbers and Lubricating Oils
• Acidity in Cheese
• Wine
• Yogurt

ACIDITY

Method –acid/base titration
Sensor – IJ-44A
Applications include:-
acidity in wine, juice, soft drinks, mustard, mayonnaise, milk, edible oils, vinegar, cheese, plating baths & TAN (total acid number)of new and used oils

ALKALINITY

Method –acid/base titration
Sensor – IJ-44A
Applications  include:
alkalinity in potable water, cleaning baths & TBN (Total base Number) of new & used oils

CHLORIDE by titration

Method –precipitation titration (argentimetry) using silver nitrate
Sensor – IJ-Ag
Applications include:-
chlorides in mineral water, food products,  water, and butter

CHLORIDE by ISE

Method –direct potentiometry using Ion selective electrodes
Sensor – IJ-Cl
Applications include:-
chlorides in mineral water, food products & water

REDOX TITRATIONS

Method – Redox titration using titrants such as Iodine, sodium thiosulphate & permanganate
Sensor – IJ-64
Applications include:-
determination of copper in electroplating baths, determination of hydrogen peroxide, Vitamin C in juice, Iodine value of animal & vegetable Fats and Oils, and Peroxide value of edible oils
 
Other Applications include:
 
Carbon dioxide in wine using IJ-44A
Cyanide in water using either IJ-Ag (titration) or direct with IJ-CN ISE
Iodide in milk using IJ-I
Bromide in water using IJ-Br
Fluoride in drinking water, mouthwash, environmental samples, vegetation and toothpaste using IJ-F

Other applications include:

Aluminium Ions Using a Fluoride ISE, Bromide in Water, Cadmium using a Silver ISE, Calcium in Animal Foodstuffs, Calcium in Fruit Juice, Calcium in Beer, Calcium in Meat Products, Calcium in Seawater, Calcium in Serum, Calcium in Skimmed Milk, Calcium in Soil, Calcium in Urine, Chloride in Batter Mix and Sausage Rusk, Chloride in Butter, Chloride in Potato Starch Suspension, Chloride in Fruit Juice, Chloride in Mayonnaise, Chloride in Serum, Chloride in Urine, Chloride in Water, Cyanide (General), Cyanide in the Presence of Heavy Metal Ions, Cyanide in Water, Fluoride in Glass, Fluoride in Urine, Fluoride in Drinking Water, Fluoride in Water, Fluoride in Welding Ash Solution, Fluoride in Vegetation, Iodide in Water, Kjeldahl Nitrogen in Water, Nitrate in Liquor used for Beer Preparation, Nitrate in Plant Tissue, Nitrate in Soils, Nitrate in Water, Phosphate Using a Fluoride ISE, Potassium in Fruit Juice, Potassium in Glass, Potassium in Oral Rehydration Salts, Potassium in Pharmaceutical Syrup, Potassium in Soils, Potassium in Water, Potassium in Wine, Silver in Fixer Solutions, Silver in Fixer Solutions II, Sodium in Dietetic Food, Sodium in Glass, Sodium in Oral Rehydration Salts, Sodium in Peanut Butter, Sodium in Pharmaceutical Syrup, Sodium in Serum, Sodium in Water, Sulphate by Sample Subtraction, Sulphide in Water ...

pH and Lubricating Oils

pH is an index of the concentration of Hydrogen Ion (H +) in water. Since oil is not an ionizing solvent, it has no free hydrogen ions and therefore, it does not have a pH per se. If the oil contains materials which when mixed with water supply hydrogen ions to the water phase, then these will register when the pH of the water phase is measured.

Due to dissociation pure water has a pH of 7. The hydrogen ion (H +) concentration in pure water is 1E-7 (pH 7) and the hydroxide ion (OH -) concentration is also 1E-7. Each molecule of H 2O that dissociates produces one of each ion, (HOH H + + OH -). The H + is the acid ion and the OH - is the base ion. Since they are present in pure water in equal concentrations, then the water is “neutral” pH 7.
The fraction ionized is about 0.0000001 (=1E-7) at 22°C; i.e., 10,000,000 liters of water supplies 1 gram- ion of hydrogen. [DEF: The pH value is the logarithm of the number of liters of a solution which must be taken in order to contain one gram ion of hydrogen].
Since this is a reciprocal relationship, raising the hydrogen ion concentration lowers the pH value and vice versa.
1/10,000,000 = 1E-7

pH does not tell us how much total acidic hydrogen is present in a combined of un-ionized form. To determine the concentration of acidic hydrogen we refer to the acid number test. If either ion (H +/ OH -) is present in excess of the other, the excess amount can be found by measuring how much of the other ion is required to bring the system back to neutral.

But what is “neutral” in lubrication oil? As discussed, pure water is neutral at pH 7. Equivalent amounts of “strong” acids and “strong” bases mixed together are neutral at pH 7.
This is because strong acids and strong bases release essentially all (over 90%) of their H + and OH - ions respectively when diluted with water. However, in most lubricating oil systems we are dealing with “weak” acids and bases. Weak acids or bases ionize or release their H + and OH - reluctantly, on the order of 1%, 0.01% or less, at equilibrium.

All of these systems are in dynamic equilibria. Systems at equilibrium with pH 7 are “neutral” in that the concentration of the hydrogen ions (H +) and the hydroxide ions (OH -) are equal. If the equilibrium is shifted either of both ions may be available depending on what other materials may be present. You could say, “pH is characteristic of a particular oil,” but remember that the pH is measured in and refers to what is in the water phase only. It is generally accepted that new unused turbine oil will have a pH of about 7. Slightly higher or lower pH values may be encountered depending on those materials (additives), which are present.

Acid Numbers and Lubricating Oils

As discussed, used lubricating oils may contain a combination of strong and weak acid formations.
Determining the concentration of strong acids: Titration with a strong base (specifically KOH) will begin at a pH of less than 4.2 and produce a Strong Acid Number (SAN) at an end point of about pH 4.2.
Determining the concentration of weak acids: Titration with a strong base will begin at a pH above 4.2 and produce an Acid Number at an end point of about pH 11.
In the case of the strong acid titration we add only enough base (OH -) to shift the equilibrium up to pH 4.2. In the case of a weak acid titration we add just enough base to shift the equilibrium from some point above 4.2 to a pH of about 11.

Total Acid Numbers (TAN): It follows then, that if both strong and weak acids are present, the Acid Number (commonly referred to as Total Acid Number, TAN) for the system is obtained by titrating to pH 11. The amounts of each type of acid can be obtained by noting the amount of KOH used to reach pH 4.2 for the strong acids and the incremental amount of KOH added between pH 4.2 a pH 11 for the weak acids. These may be recorded as Strong Acid Number and Weak Acid Number respectively with the TAN being the sum of the two.
 

Acidity in Cheese

As the acidity of cheese has a major influence on the taste of the product, this parameter is used to test the quality.
The acidity of cheese is determined by end point titration using 0.1 eq/l NaOH. The end point value is generally fixed at pH 8.4 and the result is expressed in % of lactic acid, which has a MW of 90.08 g/mol.

Electrode and reagents
IJ-44 pH electrode
NaOH 0.1 eq/l solution in distilled water
pH 4.00 buffer or pH 7.00 buffer and pH 10.0 buffer

End Point titration settings
Burette volume: 10 ml
Stirring speed: 400 rpm
Working mode: pH
Number of end points: 1
End point: 8.40 pH
Stirring delay: 30 seconds
Minimum speed: 0.2 ml/min
Maximum speed: 10 ml/min
Proportional band: 4.00 pH
End point delay: 5 seconds
Titration: Increasing pH
Sample unit: g
Sample amount: see below
Result: %

Procedure

Sample preparation Place a known amount of cheese (generally between 10 and 20 g) in a 250 ml beaker, add
100 ml of distilled water at 40°C and homogenize with a high speed homogenizer. Filter or centrifuge according to particular recommendations and dilute to 250 ml using a volumetric flask. Titrate an aliquot of 25 or 50 ml for example.

Titration

1/ Calibrate the combined pH electrode using the 2 IUPAC standards above.
2/ Pipette 25 or 50 ml of sample.
3/ Dip electrode and delivery tip in the solution.
4/ Start method.

Results

Expressed as % of lactic acid (CH3?CHOH?COOH with a MW of 90.08 g/mol)
As in this case 1 molecule of titrant reacts with 1 molecule of lactic acid

R = V(titr) * C(titr) * 90.08 * 100 * F /1000 * W(smp)
?V(titr) = total volume of titrant to reach the end point in ml ?C(titr) = Titrant concentration in eq/l (currently 0.1) ?W(smp = sample amount in g
90.08 = Molecular weight of lactic acid
F = Dilution factor between total volume and aliquot 100 = Factor needed for a result expressed in %.

Wine

pH is a fundamental element of the wine-making industry and strongly influences wine properties such as color, oxidation, biological and chemical stability. The IJ Koala series electrodes are accurate and reliable tools for measuring wine parameters.

pH measures the quantity of acids present, the strength of the acids, and the effects of minerals and other ingredients in the wine. Wine pH depends on three main factors: the total amount of acid present, the ratio of malic acid to tartaric acid, and the amount of potassium present. Wines that contain little acid and excess potassium show high pH values. Wine with more tartaric acid, less malic acid, less potassium and more titratable acid has lower pH values.

pH values range from 2.9 to 4.2 in wine. Wine’s chemical and biological stability are very dependent on pH value. Lower pH values are known to improve the stability, so winemakers usually prefer a pH range of 3.0 to 3.5. The wine is so stable in this range that many winemakers believe pH is a crucial guideline in wine making.

There are many advantages to low pH values in wine. Low pH inhibits bacteria, causes sugar fermentation to progress more evenly and makes malolactic fermentation easier to control. Low pH also has a direct influence on the hot stability of wine. When bottled wines are stored in warm areas, protein precipitates out of them, causing serious problems. These wines are then treated with bentonite, which removes excess protein. pH is important to the treatment because bentonite successfully removes more protein when the pH value is low. If wine pH increases, bentonite is less effective, making it necessary to add larger amounts. The danger is adding too much bentonite because it can strip wines of their unique aromas and flavors.

An example of the importance of low pH is displayed with Sauvignon Blanc. Sauvignon Blanc grapes normally have large amounts of protein, so they can be difficult to sufficiently stabilize when pH values are too high.
Low wine pH results in better visual qualities as well. When pH is lower, both red and white wines maintain better color intensity. Red wines have more and better color and white wines do not brown as easily.
When wine has high pH values, bacteria grow rapidly and undesirable bacterial fermentation is more problematic. This condition causes less biological and chemical stability, and poorer color. Wines with a high pH always need more attention and greater care.

Refer to the table below as to the effects of pH levels on wine quality:

Yogurt

Use IJ-44 pH electrode for rugged and easy-clean performance.

Yogurt is a popular dairy product made from concentrated milk fermentation. The quality of the product depends on production control of lactic acid formed by fermentation. Lactic acid provides the tart flavor and the destabilization of milk protein forms the gel structure. pH measurement monitors lactic acid production and aids in the quality control of yogurt’s ingredients.

The production of yogurt starts by selecting and blending the correct ingredients, like milk concentrate and other dairy products, thickening agents, sweeteners and fruit. These ingredients add the correct solids, flavor and viscosity. The blend of ingredients is homogenized at high pressures to prevent fat separation and cause solid dispersion. Next, the temperature is raised to destroy harmful microorganisms and restructure protein to help with the viscosity. After cooling, the smarter culture, which contains a particular lactic fermentation bacteria, is added to the mix. Incubation then takes between 4 and 11 hours.

During fermentation, lactose (milk sugar) converts to lactic acid, decreasing the pH values to a range of 4.25 to 4.5. Bacterial action is stopped by rapid cooling at the right lactic acid level. pH meters are the best instrumentation to determine the completion time of fermentation. Incorrect pH levels can lead to discoloration, excessive free whey and excess or insufficient tartness.

Perform the test on sample by putting the probe in the sample, gently stirring for a few moments to be sure no air bubble is trapped on the sensor surface.
Note: In 0.01 resolution, the endpoint time is about twice that of the 0.1 resolution. If you don’t need 0.01 precision, put the meter in 0.1 resolution to speed response time. This research grade resolution is not required in most dairy applications. Be sure not to use 0.001 resolution or unnecessary time will be spent reaching an endpoint reading.

Rinse probe with a jet of distilled or de-ionized water from a right angle squirt bottle. Alternatively, you can remove the reference sleeve, clean the reference tube and reference area around the glass.  Add a bead of electrolyte at the base of the glass and slide the reference tube back onto the probe.  Ready to go!

Protein is the biggest single problem in pH testing of dairy applications. In glass electrodes it gets in the glass and is very difficult to remove. We recommend HCl acid dips or pepsin solutions.  The best approach is a 2 minute soak in 0.1M HCl. After this soak, use a gentle rub of the sensor surface with a soft cloth. Rinse well, then recalibrate. It is not necessary to use this method unless response time slows.

• Replace the soaker sleeve over the pH electrode tip with a small amount of pH 4 buffer or a storage solution.
• Remove the bottle by reversing the sequence.

The best storage solution is pH 4 buffer solution saturated with KCL. The pH 4 solution is acidic, and will eat away most fouling of the pH electrode. Additional solution, if required, may be made using commercially available buffer capsules, KCL crystals, and distilled water. The sensor will tolerate the periodic absence of the soaker bottle and can be returned to initial performance by soaking for a few hours. However, exposure of the bare sensor to temperature extremes (e.g., strong direct sunlight on a hot day) can cause a loss of internal electrolyte. Subsequent cooling will draw air into the sensor, which will lead to pressure-related problems.