Silver bromide crystal surface ions

Iron (II) sulphide never has the precise composition FeS—the sulphur is always present in excess. This could be due either to the inclusion in the lattice of extra, interstitial S atoms or to the omission from it of some of the Fe atoms. The second explanation is correct (Hagg and Sucksdorff, 1933), the phenomcon being an example of lattice defect (p. 152). There are two types of lattice defect. In Schottky defects, found in iron(Il) sulphide, holes are left at random through the crystal because of migration of ions to the surface. In Frenkel defects, holes are left at random by atoms which have moved to interstitial positions. Silver bromide has a perfect face-centred cubic arrangement of Br ions but the Ag+ ions are partly in interstitial positions. The effect is even more marked in silver iodide (p. 153). 

High concentrations of bromide, iodide, cyanide, or sulfide ions interfere with the determination. They can damage the electrode. Free ammonia would also damage the measuring surface of the electrode dissolving the silver chloride crystals. 

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