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When looking at specific pH applications, we aim to address the necessary protocols required to get the most out of your pH electrode. However, for many applications, your choice of pH electrode is key in attaining the correct levels of performance and durability from your electrode.
Sample type
The most important consideration of your pH application is sample type. Select your sample type from the list of details of the common characteristic problems, application details and pH electrode suggestions.
pH in Dirty/Viscous Samples
pH in Emulsions
Extreme pH or Samples with High Salt Content
Non-aqueous pH Measurements
pH in Pure Water
pH in Solids and Surfaces
pH in Tris, Sulfide and Proteins
pH in Dirty/Viscous Samples
Recommended Electrodes: QP8172, QP8165, QP9172, QP9165
Slow response, drift and pH errors are problems which may arise when measuring the pH of colloids, suspensions, sludge's and slurries. The ceramic or fibre junction contained in most combination pH or reference electrodes shows sensitivity to immersion depth in this type of sample.
Orion have developed the Sure-Flow liquid junction combination electrodes to minimise the problem. Ceramic junctions are easily clogged by the particulates present in this type of sample. The higher flow rate of filling (salt bridge) solution resists clogging and the Sure-Flow design makes cleaning of the junction simple making Sure Flow electrodes the recommended choice for this sample type.
Cleaning the reference junction is accomplished by pressing the cap, rinsing, and then releasing the cap to reset the junction. In the measurement of such samples, it is important if the pH value is to be reproducible, to immerse the electrode(s) to a consistent and defined depth in order to minimise the effect of immersion depth. In all other respects, the procedure for pH measurement in such samples is no different from ordinary samples.
pH in Emulsions
Recommended pH Electrode: QP8172
What do we define an emulsion as?
An emulsion is dispersion of one liquid into another when two liquid phases are not normally miscible. A dispersant, ordinarily a surface-active agent is usually present, but is not strictly needed to form an emulsion. The two liquid phases involved can be classified into aqueous and non-aqueous types. Often the non-aqueous phase is dispersed into the aqueous phase (water in oil emulsion). In this case, the sample must also be considered a non-aqueous sample.
Liquid junction
When measuring the pH of a sample which is an emulsion, several unfavourable phenomena may be observed; slow response, drifting pH, irreproducible pH and pH errors. The first three phenomena are functions of the junction contained in the reference electrode. Typical reference electrodes with ceramic frits often exhibit these faults and changing to a Sure-Flow electrode may greatly reduce these problems.
Double junction reference
If the surfactant concentration is quite high a double junction electrode may be required. This a wide choice of filling solution for the outer chamber so you can choose a solution to suit your sample. Immersion depth is also critical. Exact pH measured is sometimes dependent upon the depth that the electrodes are placed into the sample. To make the answers reproducible, care should be taken to measure all samples in exactly the same way.
Extreme pH or Samples with High Salt Content
Recommended pH Electrode: QP8102, QP8172, QP8165 any other ROSS Electrode
Liquid junction potential
Extreme pH samples pose special problems for the reference portion of the pH measuring system. Drift and slow responses are the result of a liquid junction potential. In samples of normal pH (2-12) or salt content (less than 0.1M), the liquid junction potential is seldom a problem since the filling solution is formulated for best performance under these conditions.
Outside the range of 2-12pH, or over an ionic strength of 0.1M, use of a double-junction electrode is recommended.
For high pH samples, the filling solution in the outer chamber of the double junction reference should be high in pH, i.e. dilute base is added to raise the pH. For low pH, a similar modification of the filling solution with dilute acid is made.
Non-aqueous pH Measurements
Recommended pH Electrode: QP8102, QP8172, any Glass bodied ROSS Electrode
Unstable readings
When measuring pH in non-aqueous solutions, problems such as unstable readings, long response time and errors may be encountered. These symptoms are indicative of high sample resistance, bulb dehydration, contamination and junction potentials. The high resistances (low conductivity) of many organic solvents cause noise and slow response.
Low resistance electrode
To avoid this, an electrode with a low resistance glass bulb should be selected. It is often helpful to add a small amount of an inert salt (e.g. a quaternary ammonium salt), thereby increasing the conductance of the solution, resulting in a stable response. Addition of a salt will affect the activity of the hydrogen ion, causing a small shift in the pH. However, the error will be small when compared with the problems associated with drift.
Electrode bulb dehydration
Slow response and drift can also be attributed to the dehydration of the pH glass bulb. In order for the bulb to function, the surface of the glass bulb must be hydrated (properly wetted). Hydrogen ion is absorbed onto the surface of the glass, causing it to swell. In a non-aqueous solution, the bulb dehydrates resulting in a slow response. Frequent soaking in water, 0.1M HCl or a pH buffer will re-hydrate the bulb, providing a faster, more stable response.
Contamination
To prevent contamination of samples due to carryover on the electrode bulb, the electrode should be rinsed between measurements with a solvent that will dissolve the sample material from the electrode, and then rinsed with a volatile solvent like acetone to remove the solution, followed by re-soaking in pH buffer.
Non-aqueous pH scale
The pH value of a non-aqueous solution cannot be accurately compared to an aqueous buffer. The activity of hydrogen varies depending on the background medium because of difference in dielectric constants, solvent acidities and ion mobilitys. The pH values obtained can only be used as relative measurements. In other words, the measurement can be used to compare acid-base qualities of similar solvents and to indicate when adjustments in pH are needed. It is essential that solvent backgrounds of samples and buffers be as similar as possible. The development of pH scales for non-aqueous solutions is a formidable task. Some work has been done on methanol and ethanol mixtures.
Changing the fill solution
Large liquid junction potentials causing slow, unstable responses are a problem that is often underestimated when measuring in non-aqueous samples. When a reference electrode with an aqueous filling solution comes into contact with a non-aqueous sample, unequal diffusion of ion occurs, causing continuous drift.
To minimise this problem, the filling solution should be adjusted to provide compatibility with the sample. The ability to alter the filling solution is dependent upon the type of electrode utilised. An electrode with a double junction reference design allows alteration of the filling solution more easily than a conventional single junction design.
- Acetone saturated with Potassium Iodide (KI)
- Methanol saturated with Lithium Chloride (LiCI)
When selecting a reference electrode, the internal reference element and the material of the electrode body should also be considered. Epoxy bodied electrodes should not be used in certain solvents, particularly with polar organic solvents. The best selection for a pH electrode is an all glass electrode with a low resistance glass bulb. The lower resistance glass bulb provides a faster, more stable response. When adjusting the filling solution to better match the characteristics of the sample, remember to allow the electrode to reach equilibrium overnight before use. Several alternatives can be used in the outer chamber of a double junction reference electrode.
- A mixture of methanol and water saturated with Potassium Chloride (KCI)
- Glacial acetic saturated with Potassium Iodide (KI)
When using a solvent in the filling solution, it is important to remember to add a salt to improve electrical conductance. KI is often more soluble than KCI and can be substituted as required. Solvent-based filling solutions should not be used in epoxy body electrodes. Other solutions than those listed above may be suitable. There are some solutions which may prove to be extremely difficult when measuring pH, due to severe junction potentials, drift and noise; typically benzene, heptane and other similar components.
Your non-aqueous pH checklist
Remember, when measuring pH in non-aqueous samples:
- Match the filling solution of the reference electrode to the sample to minimise junction potential problems
- Select a double-junction design electrode for easy alteration of the filling solution
- Select a pH electrode with lower resistance glass or add a quaternary ammonium salt to the sample to improve conductivity
- Re-hydrate the pH electrode by frequent soaking in pH buffer or water
- Prevent sample carryover by properly rinsing the electrode with appropriate solvents, then soaking in pH buffer
- Keep the results in perspective by comparing to other data based on the same system, instead of aqueous standards.
- Use as an indicator of relative acidity/basicity instead of an actual pH.
pH in Pure Water
Recommended pH Electrode: QP8102
Recommended Solutions QS7001
pH measurements are typically made in solutions which contain relatively large amounts of acid or base, or which contain substantial amounts of dissolved salts. Under these conditions, conventional pH electrodes make measurements quickly and precisely.
Pure Water
There has been growing interest in making pH measurements in 'pure water' - water in which the total amount of acid or base is very small, and in which there is a low level of dissolved salts. The terms 'pure water' and 'low ionic strength' can be used interchangeably. Samples which may fall into this category include:
- Distilled water
- Deionized water
- Some process water
- Well water
- Some surface water
- Treated effluent
- Boiler feedwater
- Rain water
Unstable Measurements
Measurement in these pure water samples is difficult. Although electrodes respond quickly in buffers, in pure water the electrode response is unsatisfactory - slow, noisy, drifty, non-reproducible and inaccurate. These commonly observed problems can be attributed to the low conductivity of the sample, difference between low ionic strength solutions and normal ionic strength buffers, changes in liquid junction potential and absorption of carbon dioxide.
Because pure water solutions are poor conductors, the solution will tend to act as an 'antenna' and the response can be noisy. Standardisation (calibration) of an electrode in a high ionic strength buffer will increase the time required for stabilisation when going to a low ionic strength solution. In addition, the possibility of sample contamination will be increased.
Accurate Measurements
For most precise measurements, buffers and samples should be of similar ionic strength. When any two solutions come into contact, diffusion occurs until equilibrium is reached.
Since ions have different mobility's and diffuse at different rates, a change imbalance occurs causing a junction potential. A junction potential occurs when the reference electrode fillings solution meets the sample. It is important that the junction potential be constant during the measurement.
If the two solutions are quite different, normal fluctuations in the boundary layer will cause noise. Constant, reproducible junction potentials are achieved by measuring in samples and standards with ionic strengths similar to the filling solutions. Orion has developed a Pure Water Test Kit (link to QS7001) containing ph buffers designed specifically for calibration prior to measurement in ultra pure water.
Since pure waters contain little dissolved material, their buffering capacity is small. Absorption of carbon dioxide from the atmosphere will cause a slow change in the pH, observed as drifting pH reading and a different pH from the original sample. Samples which are not previously saturated with carbon dioxide must be handled with care.
pH of solids and surfaces
Recommended pH Electrode: QP8135, QP8163, QP7120
There are many electrodes available for use with soft or semi-solid samples. Needle shaped electrodes work well in meats and cheeses. If the sample is softer still, such as cottage cheese or cosmetic cream, an electrode with a hardened bulb may prove useful (QP8104).
Flat surfaces
If it is difficult or inconvenient to prepare a sample for pH measurement by any other means, it is possible to measure the pH of a flat surface by using a special flat surface electrode.
The sample must be moist enough for both the sensing bulb of the electrode and the reference junction to make adequate electrical contact with the sample. If necessary, a drop of distilled or deionised water or potassium chloride solution may be used to wet the surface before the electrode is placed on it. For reproducibility, all samples should have the same amount of liquid added before measurement. Response time may be longer than in a conventional sample, usually 1min is sufficient time for electrode response. As with all such samples, the method will make a difference in the actual value of pH measured. As long as measurement technique is uniform, reproducible values are obtained.
pH in Tris, Sulfide and Proteins
Recommended pH Electrodes: Any ROSS Electrode
When using a pH electrode containing Ag/AgCI internal reference, a problem can arise if the sample contains a species which complexes with or precipitates silver (Ag). The ceramic junction of the reference electrode may become blocked with silver precipitates, causing a slow response or no response at all.
Common species causing this problem:
Tris [tris (hydroxymethyl) aminomethane] a compound frequently used as a biological buffer.
Sulfide present in some waste waters and petroleum products.
Proteins which may coat the sensing bulb as well.
Soaking the electrode in sales (1M KCI) or acids (0.1M hydrochloric or nitric) for a period of time may relieve this problem.
pH electrodes that contain no silver in the filling solution suffer from none of the problems affecting the reference junction described here. ROSS electrodes, using KCl instead of AgCl as the reference solution are highly recommended for use in these applications.
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