pH Electrodes Designed & Manufactured by Hanna Four Different pH Sensitive Glass Formulations Application driven design has influenced our offering of pH glass formulations. Hanna has selected the best glass compositions possible for each sensor to ensure the most accurate measurements in a given application. The characteristics of the sensitive glass used in the manufacture of pH electrodes are extremely important in determining how the electrode will respond. Characteristics of pH glass include workability (what shapes can be made with a certain glass composition), impedance of the glass (influenced by shape and thickness), pH range, alkaline error, acid error, hydrofluoric acid resistance and abrasion resistance. Hanna utilizes four different types of pH sensitive glass to cover the vast number of applications. For instance, some electrodes with low impedance glass are particularly suited at performing measurements in solutions with low conductivity or cold solutions. As a general rule, the pH of glass impedance doubles for every 10°C (50°F) drop in temperature. Very high impedance results in a very noisy, erratic signal that is prone to errors in measurement. Hanna offers low temperature (LT) glass, a low impedance glass for these applications. At elevated temperatures, glass can dissolve readily, shortening the life and performance of the sensor. Hanna offers high temperature (HT) glass for these applications. GP Glass Hanna’s general purpose (GP) hydrogen sensitive glass provides the best response over the entire pH range and can be used for a wide range of applications. Great results are obtained with a sphere geometry with a diameter of 9.5 mm (0.37’’), achieving a system with 100 MΩ impedance. The GP glass is also used on smaller diameter spheres. As the diameter of the sphere is reduced, the system impedance increases. The response time then increases from the usual 2 seconds for the 9.5 mm (0.37’’) sphere to about 6 seconds with a 3 mm (0.12’’) sphere. The color of the GP glass is green. LT Glass Due to low impedance, LT glass is used on flat and conical shaped membranes, as well as sensors used at cold temperatures. If an electrode has very high impedance, the measurement response will be sluggish, and a voltage drop causing error can occur. At temperatures below -8°C (17°F) the internal buffer may freeze and expand, causing the mechanical destruction of the sensor. This glass has a more limited pH range, and is colored dark green. HT Glass Designed for extended use at elevated temperature, the impedance of HT glass has a temperature coefficient of about 14.3% per degree Celsius. HT sensitive glass has an impedance of 400 MΩ at approximately 25°C (77°F). At extremely high temperatures the impedance drops significantly; HT glass makes it possible to obtain accurate, high temperature pH measurements for extended periods of time at 90°C (194°F) and for several weeks at 100°C (212°F). At room temperature, the response time may increase so additional time for equilibration in buffers should be allowed. The color of HT glass is clear. HF Glass Hydrofluoric acid can dissolve glass rapidly. Hanna uses HF resistant glass for aggressive applications that have fluoride ions. Electrodes manufactured with this glass live ten times longer than electrodes made with standard pH glass formulations (from 10 days to 100 days). The alkaline error is very high for this glass, so it is not suited for pH measurements above pH 10. The recommended pH range with this glass is from 2 to 10 pH and for samples with less than 2 g/L fluoride. Different Shaped Membranes (Tips) The pH membranes used as the sensor on pH electrodes can be fabricated with different shaped membranes; spherical, conical, and flat tips are used in Hanna’s products. For analysis of small samples, microelectrodes are also available. A spherical tip is recommended for general use in aqueous or liquid solutions and provides a wide surface of contact with the sample. A conical tip is recommended for semi-solid products, emulsions, cheese, meat, and food in general. A flat tip is recommended for direct surface measurement on skin, leather, paper, etc. Body Material Combination pH electrodes are often made entirely of glass. The bodies of these electrodes are lead free glass, which is not pH sensitive. All glass electrodes are ideal for routine laboratory work because they respond quickly to temperature changes, are easily cleaned, and are compatible with organic solvents. However, in the hands of some, glass can be very breakable. The electrode body can be made less fragile by incorporating an outer body made from a thermoplastic. Hanna uses PEI resin, PVDF and PP as examples of materials utilized for outer body construction. Some industrial sensors utilize additional materials such as PVC and/or titanium, the space age metal. A titanium body increases immunity to electrostatic and magnetic fields and features strong corrosion resistance, even in seawater. Our titanium bodied electrodes’ outer casing also serves as a matching pin. Spherical Conical Flat 2 pH 2.127 www.hannainst.com | electrodes
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