l �́�79t�)�ot��a`0p��x�!YFAO���!�]����� J ?P Indicators are acids or bases whose various protonated forms have different colors. Give one example (different from the use in lab today), of why you would need to use a solvent-solvent extraction At the half EP, pH=pKa, so the pH corresponding to the volume at the half EP is the pKa of bromothymol blue. The determination of pKa' is performed for various ionic strengths, which reveals the thermodynamic acid constant (pKa=7.5) and the number of charge on each chemical species; the yellow form is negatively charged species of -1 and the blue form that of -2. pH = pKa - log ( [HIn] / [In-] ) … (2) Spectrophotometric determination of the pKa of Bromothymol Blue. Chloroform showed the highest affinity for bromothymol blue, followed by methylene chloride, ethylene dichloride, benzene, toluene, and carbon tetrachloride. In this experiment the individuals will experimentally determine the pKa of bromothymol blue and the concentrations of the base form of the analyte at different absorbance using spectrophotometric processes and knowledge of acid- When compared to the literature pK c value of 7.1, the percent difference is 1.4%. At an intermediate pH, the blue and yellow combine to yield a green solution. An aqueous solution of BTB exhibits a color of yellow in a weak acid solution, and it turns to blue via green on raising pH. The indicator (HIn) is a monoprotic acid and we can represent its dissociation as follows: HIn → H+ + In- … (1) The equilibrium expression for such dissociation can be written as. 0000001831 00000 n startxref The experiment correlates well with the lecture topic of equilibrium even though the subject of the study is an acid-base indicator, bromothymol blue. The pKa and the isosbestic point of the universal pH indicator Carlo Ebra 1-11 (catalog number 45712) were determined using UV-Vis spectrophotometry. Bromothymol blue (BTB) is a widely used acid-base indicator , which is a member of sulfonphthalein dyes . Aqueous buffer solutions with pHs ranging from 3.83 to 10.85 were mixed. 0000000773 00000 n [1] http://www.sigmaaldrich.com/catalog/product/sial/114413?lang=en®ion=US, [1]http://ehsrms.uaa.alaska.edu/CMS/Laboratory/ChemList/Acid%20Base%20Indicators.pdf, Determining the pKa of Bromothymol Blue by spectrophotometry, http://www.sigmaaldrich.com/catalog/product/sial/114413?lang=en®ion=US, http://ehsrms.uaa.alaska.edu/CMS/Laboratory/ChemList/Acid%20Base%20Indicators.pdf. Uranium isotopes in groundwater occurring at Amazonas State, Brazil.. PubMed. Then 12.5 drops of bromothymol blue were added to 100 mL of water to keep the concentration of bromothymol blue constant. In this experiment, spectrophotometry is employed to measure the pKa of bromothymol blue, an acid-base indicator. 0000019359 00000 n 0000006385 00000 n 0000006001 00000 n Fig. Aqueous buffer solutions with pHs ranging from 3.83 to 10.85 were mixed. xN���@, Qb�c��|P�p�ن����� The volume and concentration of the BB in each flask when starting. %PDF-1.4 %���� Learn vocabulary, terms, and more with flashcards, games, and other study tools. In the experiment the pKa of bromothymol blue (3',3"- dibromo-thymolsulfonephthalein) is determined by the two methods which have been discussed. In this case, the estimated pKa value is ~7.5. Spectrophotometric determination of the pKa of Bromothymol Blue Given the equation: y= 48048x- 0.0105, find the molar absorptivity and use this value in your experiment. 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