P H with glass electrode PDF

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Explanation of pH measurement with glass electrode...

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PH measurement with a glass electrode Operation Principle pH measurement is important in the especially in the industry application, a convenient way of measuring pH is therefore significant. Instrumentation for measuring pH is required to be ion specific (that is only focusing on the quantity of the hydrogen ions in water). One the ways to measure pH is by use of glass electrode which is ion specific. The glass electrode has a glass indicator electrode with another reference electrode (typically silver-silver chloride or saturated calomel electrode), these electrodes are put inside an analyte or a solution which the pH is required to be measured. The glass electrode has a thin layer of wax which covers a first layer solution of potassium chloride, this is the indicator electrode consist at the tip a heavy walled glass which is pH sensitive, the heavy walled glass is microscopically thin and behaves like gel. Within the heavy walled glass is 0.1 M concentrated hydrochloric acid which is saturated with silver chloride, this is the second layer solution. A long thin Silver wire lies within the glass electrode forming silver and silver chloride electrode, this wire extends to connect with an external pH meter along with the other reference electrode. A common glass membrane is corning 015 glass which can detect pH to about 9. The external solution must be stirred. Below is a schematic representation of a glass electrode. SCE || [H3O+] =α1 |Glass membrane |[H3O+] =α2 , [Cl-]= 0.1 M, AgCl (saturated)|Ag Where SCE is the reference electrode, α1 is the activity of the external analyte solution followed by an ion selective glass membrane and α 2 is the activity of the hydrogen ions within the electrode. The conditions and concentrations of the two electrodes must be known (glass electrode and reference electrode), the reference electrode has calibrated pH, a difference in potential is measured between the two electrodes. When this cell is complete and connected to a pH meter, a difference in pH of the external solution and internal solution between the glass electrode causes an emf at the glass electrodes, this electromotive force (emf) is proportional to the difference in of the pH. By calculation, the pH of the sample can be calculated. The difference in pH depends on the hydrogen ion activity.

The area of the glass electrode must be hydrated before usage, dehydrated causes loss of pH sensitivity, so before use the electrode needs to be soaked in water (The membrane is gel like and thus needs to dissolve in water). The hydration of pH sensitive membrane causes exchange in ions, this causes a high concentration of single ions on the surface of the membrane. If the electrode is left to dehydrate a long time will be required for the membrane to regain its gel like structure and in other cases it may be impossible to return it to its state. The glass structure is made of a molecular structure of silver atoms that are tetravalent and a negatively charge oxygen atom, with each silicon atoms connected to four oxygen atoms and in between lies singly charged mobile Hydrogen and sodium cations, which are responsible for electrical conduction within the membrane, since hydrogen cations are small there is a chance of these ions moving between the glass structure( these ions are not bonded to the glass structure). pH electrodes are selective to pH thus the solution being measured must fall within a specific pH range. For electrical conduction to occur along a glass membrane, ions are must be able to move across the membrane, this is done by the activity of hydrogen cation conduction across the hydrated gel, the sodium ions inside the membrane as well carry charge between the interface of the analyte solution and the reference electrode. The side where dissociation occurs frequently is the most negative surface, this causes a boundary potential to develop between the two sides of the glass membrane that is dependent on the activity of the hydrogen cation between the analyte and the reference ration. This potential difference gives reading of that is translated to pH by the potentiometric pH measurement, there is a lot of activities that take place at the membrane, but the potential difference only depends on the activity ratio. To determine the boundary potential, we get the difference between the internal and external solution, Nerst’s equation can be used to determine the potential on each side. The two sides have an equal number of negatively charged sites where hydrogen cations can dissociate, this results in the activities on either side of the membrane being equal. From this, the pH of the analyte or the external solution can be determined. The sizes of the electrodes are small and require careful use as they are expensive. These glass electrodes are also affected by temperature, the pH sensor also require cleaning to remove deposit so that accurate results are given. In pH conditions less than 0.5 glass electrodes tend to give error values, this is a phenomenon called acid error, this does not only depend on one factor but multiple factors in which some

are not fully understood. One common cause of acid error is the buildup of hydrogen ions on one side of the surface membrane where the membrane no longer responds to a further buildup of hydrogen ions on one side, this causes pH readings to be too high. This means that one must be careful when calibrating a glass electrode to get accurate results. In basic solutions glass electrodes respond to both hydrogen ion and alkali metals, pH readings can be less than actual readings. It is expected that when the same solutions is put on both sides of the membrane, the boundary potential should be zero, but in practice there glass electrode gives reading of a significant pH or potential, this is due to asymmetry potential. This is due to strain on both surfaces created during production, mechanical abrasion, and chemical etching. To counteract this effect, the membranes need to be calibrated with one or more analyte solution at least daily....