Spektrofotometrijsko određivanje konstante disocijacije

undergraduate thesis

• Title (english): Spectrophotometric determination of dissociation constant
• Author: Patrik Orešković
• Mentor: Sandra Babić (mentor)
• Committee member: Sandra Babić (predsjednik povjerenstva)
• Committee member: Dragana Mutavdžić Pavlović (član povjerenstva)
• Committee member: Irena Škorić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Chemical Engineering and Technology; Zagreb
• Defense date and country: 2021-09-07, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Chemistry

Abstract

Vrijednost konstante disocijacije kiseline nam govori o stupnju disocijacije kiseline, odnosno o odnosu ravnotežnih koncentracija disociranog i nedisociranog oblika molekule. Kako fizikalno-kemijska svojstva disociranog i nedisociranog oblika mogu biti bitno drugačija, poznavanje vrijednosti pKa je značajano za istraživanje u raznim znanstvenim područjima. Poznavanje konstante disocijacije kiseline važno je i kod kiselo-baznih indikatora koji su široko korišteni u kemijskoj analizi za praćenje kemijskih reakcija. Cilj ovoga rada bio je razmotriti spektrofotometrijsko određivanje konstante disocijacije te odrediti pouzdanost određivanja ovom metodom. Pri tome su korištene dvije računske i dvije grafičke metode za određivanja konstante disocijacije na temelju spektrofotometrijskih mjerenja. Spektrofotometrijski su određene konstante disocijacije tri kiselo-bazna indikatora: bromfenol plavog, bromtimol plavog te fenolftaleina. Otopine indikatora pripremljene su u puferima tako da je pokriven široki raspon pH-vrijednosti. Praćena je ovisnost apsorbancije o pH-vrijednosti pri konstantnoj valnoj duljini te su iz dobivenih rezultata, primjenom Henderson-Hasselbachove jednadžbe, određene konstante disocijacije ispitivanih indikatora. Za indikator bromfenol plavo određene su pKa vrijednosti u rasponu od 3.21 – 3.77 , za indikator bromtimol plavo pKa vrijednosti su u rasponu od 6.61 – 7.37 te za indikator fenolftalein pKa vrijednosti su u rasponu od 7.85 – 9.70. Nakon određivanja apsolutnih i relativnih pogrešaka izračunatih pKa vrijednosti, utvrđeno je da prva grafička metoda daje slične p-vrijednosti konstante disocijacije kao i obje računske metode, dok druga grafička metoda pokazuje najveća odstupanja, dajući najmanje pKa vrijednosti.

Abstract (english)

The acid dissociation constant gives us information about the acid dissociation degree, but also information about the concentration ratio between dissociated and undissociated species of the molecule. Since numerous physicochemical properties of dissociated and undissociated species may vary, knowing the pKa value has a great significance in various scientific fields. Knowing the value of the acid dissociation constant is also very important in the use of acid-base indicators, which are widely used in chemical analysis to observe chemical reactions. The aim of this research was to consider the spectrophotometric determination of the dissociation constant value and to determine the reliability of this method. Two computational and two graphical methods were used to determine the dissociation constant based on spectrophotometric measurements. Three acid-base indicators were analyzed spectrophotometrically, and those are: bromophenol blue, bromothymol blue and phenolphthalein. Samples of indicators were prepared in buffer solutions that covered a wide pH range. The dependence of the absorption on the pH value at a constant maximum wavelength of absorption was monitored, and the dissociation constant was determined from the obtained results using the Henderson-Hasselbach equation. For indicator bromophenol blue determined pKa values were in the range of 3.21 – 3.77, for indicator bromothymol blue pKa values were in the range of 6.61 – 7.37, and for the indicator phenolphthalein pKa values were in the range of 7.85 – 9.70. After determining the absolute and relative errors for the calculated pKa values, it was found that the first graphical method gives similar p-values of the dissociation constant as the both computational methods, while the second graphical method shows the largest deviation giving the smallest pKa values.

• Keywords: konstanta disocijacije; spektrofotometrija; puferi; indikator; bromfenol plavo; bromtimol plavo; fenolftalein
• Keywords (english): the dissociation constant; spectrophotometry; buffers; indicator; bromophenol blue; bromothymol blue; phenolphthalein
• Language: croatian
• URN:NBN: urn:nbn:hr:149:264173
• Study programme: Title: Materials Science and Engineering - Undergraduate study; Study programme type: university; Study level: undergraduate; Academic / professional title: sveučilišni/a prvostupnik/ prvostupnica (baccalaureus/baccalaurea) inženjer/inženjerka kemijskog inženjerstva (sveučilišni/a prvostupnik/ prvostupnica (baccalaureus/baccalaurea) inženjer/inženjerka kemijskog inženjerstva)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access; Embargo expiration date: 2022-09-07
• Created on: 2023-04-12 11:17:24