Lab 7 Natural pigments as Acid-Base papr of 4 Indicators Prelab Assignment Read

Question

Lab 7 Natural pigments as Acid-Base papr of 4 Indicators Prelab Assignment Read the lab thoroughly, and then complete in your lab notebook the following ections of the report for this lab: Title Bar. An introduction and procedure are not needed before lab, and no prelab questions. sure your table of conte is current. You are encouraged to bring a sample of flower or vegetable material to test a handful of the skin or petals are usually one sample per pa The goal is to have each team test a different plant sample Purpose In this lab you w explore the origin of the colors of many redblue/purple plants and the effect of pH on these pigments Introduction An acid-base indicator is a substance which changes color according to how acidic or basic its environment is. There are numerous natural acid-base indicators that can be obtained from common flowers, fruits and vegetables. Most people are surprised at the color changes in plant juices. How many people would guess that blueberry juice is bright red in acid or that cherry juice is dark green in base? Especially interesting are the extracts from red cabbage, radish skin, rhubarb skin, turnip skin, and various flowers such as dark violet pansies that can act as universal indicators. The plant pigments known as anthocyanins (see Figure 1) are responsible for many of the red, blue and violet colors seen in plants. The red leaves of autumn trees deep purple cabbage and black tulips all owe their distinctive coloration to the presence of anthocyanin pigments. While the yellow and orange carotenoids and the green chlorophylls are contained in plastids within the plant cell, the anthocyanins are found dissolved in the cell sap and, therefore, appear uniformly distributed throughout the cell Since they are water soluble, they are easily extracted for use in the lab OH OH cyanidin (cusHuoe Pelargonidin (C15H11os Quercetin (C15H10O Figure 1. Anthocyanins (Quercetin technically is a sonol rather than an anthocyanin) The color the anthocyanins exhibit depends upon the pH of the solution in which they occur. The same compound (e.g., cyanidin) will appear red or blue (and other colors) depending on the pH (for an explanation of pH see the Appendix.) These properties are the basis for the following exercise.

Explanation / Answer

When changing pH, this means that we are changing [H+] ions concentrations

Note that many substances tend to change color when forming:

A- + H+ --> HA(color)

so; decrease/increase in H+ ions will favour shift in equilibrium, and as products/reactants form and react the color is favoured due to changes in H+ ions

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