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Ursprungligen postat av mrrandom14
Ja i samband med en pH-elektrod
Ber om ursäkt för lite sent svar, glömde bort dig...
Det beror på den sk. liquid junction potentialen.
Om två olika elektrolyter med olika jonstyrka kommer i kontakt med varandra kommer jonerna diffundera snabbare åt något håll vilket ger upphov till en liten potentialskillnad (mV).
En pH-elektrod innehåller en elektrolyt så när man mäter sina kalibrerlösningar uppstår en viss liquid junction potential. Mäter man sedan på ett okänt prov med samma jonstyrka kommer liquid junction potentialen vara lika stor som den var för kalibrerlösningarna. Mäter du däremot på ett prov med signifikant annorlunda jonstyrka kommer liquid junction potentialen ändras gentemot dina kalibrerlösningar. Detta gör att du erhåller ett felaktigt pH-värde.
EDIT: Förutom detta fel viss det även många andra saker som kan påverka mätningen och ge ett felaktigt värde. Skriver av en fin lista som finns i "Quantitative chemical analysis" av Harris.
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Ursprungligen postat av Quantitative chemical analysis
1. Standards: A pH measurement cannot be more accurate than our standards, which are typically +-0.01 pH unit.
2. Junction potential: A junction potential exists at the porous plug near the bottom of the electrode. If the ionic composition of the analyte solution is different from that of the standard buffer, the junction potential will change even if the pH of the two solutions is the same. This effect gives an uncertainty of at least ~0.01 pH units.
3. Junction potential drift: Most combinations electrodes have a Ag | AgCl reference electrode containing saturated KCl solution. More than 350 mg Ag per liter dissolve in the KCl, mainly as AgCl4^3- and AgCl3^2-. In the porous plug, KCl is diluted and AgCl can precipitate. If analyte solution contains a reducing agent, Ag(s) also can precipitate in the plug. Both effects change the junction potential, causing a slow drift of the pH reading. You can compensate for this error by recalibrating the electrode every 2h.
4. Sodium error: When [H+] is very low and [Na+] is high, the electrode responds to Na+ and the apparent pH is lower than the true pH. This is called the sodium error or alkaline error.
5. Acid error: In strong acid, the measured pH is higher than the actual pH, because the glass is saturated with H+ and cannot be further protonated.
6. Equilibration time: It takes time for an electrode to equilibrate with a solution. A well-buffered solution requires ~30 s with adequate stirring. A poorly buffered solution needs many minutes.
7. Hydration of glass: A dry electrode requires several hours of soaking before it responds to H+ correctly.
8. Temperature: A pH meter should be calibrated at the same temperature at which the measurements will be made.
9. Cleaning: If an electrode has been exposed to a hydrophobic liquid, such as oil, it should be cleaned with a solvent that will dissolve the liquid and then conditioned well in aqueous solution. The reading of an improperly cleaned electrode can drift for hours while the electrode re-equilibrates with aqueous solution.
