The errors in the estimation of the Ka and pKa values might be caused by the following reasons. First, there might be some carbon dioxide gas and carbonic acid present in the cola

sample. The carbonic acid could consume the added NaOH. So more NaOH was added to the solution and pKa might have been underestimated and the Ka might have been overestimated. Moreover, the titration curve was only plotted with non-connective data points. The derivative that shows the biggest change in pH might have been in between the data points. Therefore, the equivalence point might have been inaccurate using the graphical way of determination. Using the.The literature values for the Ka1 of phosphoric acid are 6.9 X 10-3 and 6.3 X 10-8, The values that were obtained for the first two Ka1and ka2values were 1.3 X 10-3 and 1.0 X 10-7. There is a noticeable issue when using graphical analysis to determine the Kaand pKavalues. In order to determine these values, the location of the inflections points on the titration curve should be first determined. Although the inflections points were clearly visible on the graph of the derivative pH (the local maximums and minimumsof pH), this only represents the general region around which the actual equivalence point and halfway point. The titration curve was based on a set of non-connective of data points, and the curve was not always completely smooth. Therefore, the maximums and minimums on the derivative function can only represent the inflection points of the given data. Using the inflectionpoints determined on the graph might not accurately represent the true value of the Ka for the phosphoric acid. It is very important to degas the cola prior to the titration. because degassing eliminated the risk of a side neutralization reaction between carbonic acid and. If carbon dioxide was left in the cola, then the NaOH wouldhave reacted with the gas based upon the following equation: H2CO3(aq)+ 2NaOH (aq)→ Na2CO3(aq)+ 2H2O(aq). Then hydronium that determined the pH of the solution does not entirely come from H3PO4. The initial concentration of the Ka and pKa of phosphoric acid would have been inaccurate if the cola was not degassed prior to titration. To eliminate the reaction of carbonic acid with NaOH during the titration, it is necessary to degas the cola sample prior to titration.

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- Spring '11
- Dr.JohnZhang
- pH, Sodium hydroxide, Phosphoric acid, NaOH solution