In the Fajans method, typically dichlorofluorescein is used as an indicator; the end-point is marked by the green suspension turning pink. Prior to the end-point of the titration, chloride ions remain in excess. They adsorb on the AgCl surface, imparting a negative charge to the particles. Past the end-point, excess silver(I) ions adsorb on the AgCl surface, imparting a positive charge. Anionic dyes such as dichlorofluorescein are attracted to the particles, and undergo a color change upon adsorption, representing the end-point. Eosin (tetrabromofluorescein) is suitable for titrating against bromide, iodide, and thiocyanate anions, giving a sharper end-point than dichlorofluorescein. It is not suitable for titrating against chloride anions because it binds to AgCl more strongly than chloride does.
Adsorption indicators can be used in many precipitation titrations, not just argentometric methods. Let’s imagine that we wish to analyte Cl− in a sample solution by titrating with Ag+; the titration reaction would be Ag+ + Cl− <-> AgCl(s)
Silver chloride forms colloidal particles. Before the equivalence point, the surface of the precipitant particles will be negatively charged due to the adsorption of excess Cl− to the surface of the particles. A diffuse positive counter-ion layer will surround the particles. When the equivalence point is reached, there is no longer an excess of analyte Cl−, and the surface of the colloidal particles are largely neutral. After the equivalence point, there will be an excess of titrant Ag+, some of these will adsorb to the solid AgCl particles, which will now be surrounded by a diffuse negative counterion layer. Adsorption indicators are dyes, such as dichlorofluorescein, that usually exist as anions in the titration solution. The doubly charged dichlorofluoroscein anion is attracted into the counterion layer immediately
following the equivalence point, when the surface charge of the particles changes from negative to positive. For reasons that are not fully understood, the closer proximity of the dye to the particles changes the color of the molecule, providing a visual indication of the titration endpoint. In the case of dichlorofluorescein, the indicator changes to a pinkish color.
Fajan’s Method
หลักการคือ ที่จุดยุติ ตะกอนจะดูดซับอินดิเคเตอร์ไว้เกิดการเปลี่ยนแปลงสีของตะกอน โดยใช้Fluorescein เป็นอินดิเคเตอร์
ปัจจัยที่มีผลต่อการไทเทรต
- Indicator ต้องแตกตัวที่ pH ที่ทำการทดลอง
- พื้นที่ผิวของตะกอน ควรเติม dextrin, gelatin agar, PEG 400
- แสงสว่าง ทำให้สารสลายตัวง่าย
- การคนหรือการเขย่า ไม่ควรเขย่าแรงเกินไป
คำศัพท์ที่เกี่ยวข้อง
Adsorption indicator
A substance which is adsorbed or desorbed with concomitant colour change at or near the equivalence point of a titration.
Source: wiki cyberclass IUPAC Compendium of Chemical Terminology Skoog, West and Holler
กำลังเพิ่มเติม (อยู่ระหว่างหาข้อมูล): สารเคมีที่เกี่ยวข้องและคุณสมบัติของสาร, แก้ไขข้อความภาษาต่างประเทศเป็นภาษาไทย
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