Indicators Lab

Indicators lab

Lab partners:

Abstract: According to the plot and analysis of the data collected using a mixture of buffer solutions made using .2 NH4Cl and .2 M NaOH, the Ka value for Thymol Blue is 6.41x10^-5.

Purpose: To precisely determine the Ka value for a particular indicator using the light absorbvity collected using a spectrometer.

Procedure: The pre-lab for this experiment required the formation of a buffer solution based upon various available ingredients and the appropriate Ka values. Ideally, the provided reagents could make a buffer solution by partial neutralization. It was determined that the best buffer solution was made from a mixture of .2 M NaOH and .2 NH4Cl which would yield a pKa value of 9.25. This reaction was determined to be the optimal buffer solution because it contains a pKa value that is close to a pKa value at which the selected indicator (in this case Thymol Blue), will change color. In order to determine the pKa range for Thymol Blue, the following steps were taken: a well plate with twelve compartments was obtained, and each compartment was filled about half way with a solution of a certain pH. The pH’s used ranged from 1-12 (one solution of a certain pH per compartment). After filling the well plate with the various solutions, 2 drops of Thymol Blue indicator were added to each compartment. From this coarse survey, the pKa values were found to be approximately around 2, and again between the 8 and 9 range. According to the buffer solutions calculated in the pre-lab portion of the experiment, 9.25 is the only pKa value that was found to correspond appropriately a pKa value found for the Thymol Blue (ph 8-9 range). After identifying the appropriate buffer chemistry, a recipe that will yield four buffer solutions with the following volumes of indicator, NaOH and NH4Cl were obtained and made:

Solution # | Indicator | NH4Cl | NaOH |

1 | 2 mL | 25 mL | 1 mL |

2 | 2 mL | 25 mL | 2 mL |

3 | 2 mL | 25 mL | 5 mL |

4...