Matching Pin A matching pin is a differential measurement technique used to eliminate ground loops and common mode perturbations for a measurement system. In a system without a matching pin, electrical currents in the sample can affect the reference half cell voltage that is connected via the liquid junction with the sample. In this case, the reference electrode picks up the electromagnetic fields and the measurement of the pH is altered. The matching pin isolates these current/magnetic fields from the reference electrode. Hanna manufactures a number of models with the matching pin design for safe precise pH measurements. Types of Connectors Most Hanna meters accept pH electrodes with one of the connectors listed below. The BNC connector is the most versatile since it can be used with any meter that utilizes BNC, regardless of brand. DIN, 3.5 mm, Screw, and T-type connections are generally proprietary to the meters they are supplied with. Screw and T-Type connectors attach directly to the meter. Even though both Screw and T-type connectors attach directly to the meter, they can also be made interchangeable with other meters by using Hanna BNC extension cables. Water Conductivity and pH Measurement pH is the measurement of hydrogen ion activity. Ultrapure water is the perfect solvent and readily dissolves many things. The pH glass surface can actually become dehydrated if stored or used in deionized or distilled water as ions are leached from the sensing surface. pH electrodes require ions in a solution, preferably with a conductivity of or exceeding 200 μS/cm to function properly. In the case of low conductivity samples that are below 200 μS/cm, we suggest the use of specific electrodes, such as the HI1053 which has LT glass suitable for low temperatures. This pH electrode has a triple ceramic junction that allows a higher flow rate of reference electrolyte to help provide electrical conductivity. Alkaline Error Alkaline error exists in high pH solutions when the hydrogen ions in the gel layer are partially or completely substituted with alkali ions; the resulting pH displayed is lower than it actually should be. The difference between the theoretical and measured pH is called the alkaline error. Sodium ions are typically the ions that are responsible, but potassium and lithium ions can also contribute to this error. In earlier glass compositions, the alkaline error was seen to start at 9 pH. Newer glass formulations and ones especially formulated to minimize this error now exhibit an error starting at 12 or 13 pH. To solve the problem of alkaline error, Hanna’s high temperature (HT) glass minimizes alkali error in highly alkaline solutions. The tables below show the alkaline error that exists with Hanna glass types at ambient temperatures: pH Electrodes Designed and Manufactured by Hanna Alkaline Error with 0.1 M Sodium pH GP HT LT HF 10.0 10.5 0.06 11.0 0.15 11.5 0.05 0.22 12.0 0.01 0.18 0.30 12.5 0.11 0.05 0.28 13.0 0.23 0.11 0.35 13.5 0.35 0.16 0.45 14.0 0.48 0.20 0.54 Alkaline Error with 1.0 M Sodium pH GP HT LT HF 10.0 0.01 0.25 10.5 0.14 0.25 11.0 0.02 0.30 0.48 11.5 0.11 0.01 0.46 0.71 12.0 0.21 0.06 0.62 12.5 0.32 0.11 0.79 13.0 0.43 0.15 13.5 0.45 0.21 14.0 0.65 0.27 BNC BNC + PIN Y-Type BNC + RCA Quick Connect DIN DIN T-Type Screw 3.5 mm digital 2 pH 2.128 | www.hannainst.com electrodes
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