2022 IB Diploma Extended Essays

1. Introduction 1.1 Research question

How do various titration methods compare to the Total Acidity value when titrating tartaric acid (of wine) against NaOH to determine the Titratable Acidity of the wine? This investigation will compare the theoretical value to the results of four different titration methods. These methods are a titration with phenolphthalein pink indicator, with bromothymol blue indicator, a back titration with bromothymol blue and titration with a pH logger. 1.2 Purpose This experiment aims to determine which TA-determining method is the most accurate. Four titration methods; titration with phenolphthalein pink indicator, bromothymol blue indicator, back titration with bromothymol blue and titration with a pH data logger, were done and compared to the theoretical value (the theoretical Total Acidity). Engagement My knowledge of the Acids and Bases topic in IB Chemistry enabled me to decide the variable and parameters of this investigation. For the independent variables, I researched previously done TA-determining experiments and gathered an initial list of methods I could use in my own investigation. The Australian Wine Research Institution suggested two methods to measure the titratable acidity of wine: titration by colour indicator and by pH meter (Australian Wine Research Institute, 2022). The resource gave me the idea to do titrations with different indicators, as well as a titration with a pH data logger. Additionally, I used my chemistry experience to come up with another titration method, a back titration. Through my knowledge on Acids and Bases, I knew that in this method stopping the titration at the endpoint is more manageable and harder to miss (this will be explained later). Once I checked if the methods’ materials were accessible, and after a discussion with my supervisor, some methods were excluded. For example, I initially thought of doing a potentiometric titration since it was explored in an article in the Agricultural Sciences Belgrade journal (Petrović, 2007) . However, I found that I did not have access to the specific electrodes essential to the experiment. Therefore, it was excluded from this investigation. After refining the list of methods to the four ultimately chosen, I tested each method to determine all aspects of the methods: the volume of wine, concentration of Sodium Hydroxide (NaOH), the setup, which indicator to use for the back titration, which pH is the exact endpoint (required to know for the pH logger method) and to check that I did not miss any steps in my research/preparation. I used the titration (with an indicator) method outlined by the Royal Australian Chemical Institute to have a base idea of the volumes and concentrations of the chemicals involved in the experiment. Through the pre-trials, these details were adjusted to suit my investigation better. In the first trial, I used 0.5M of NaOH against 20ml of the white wine and found that 0.5M was too strong since the colour would significantly change after one millilitre of NaOH, making it difficult to visually evaluate the endpoint. Because of this I later trialled the titration with 0.1M of NaOH and concluded that it was effective; it was weak enough to gradually head towards the endpoint, which also had a more visually significant change of colour. During this trial, I also established the endpoint to be when the wine turned into a muddy orange/pink colour when titrated with phenolphthalein and a muddy orange colour when titrated with bromothymol (this will be evaluated later in this investigation). For the back titration method, I did several trials with phenolphthalein and bromothymol. As a result, I decided to use the bromothymol indicator since it had more consistent and thus reliable