PREPARING RED CABBAGE LIMUS PAPER

Cut some porous white paper or card sheets into rectangles and soak them in the juice so they well absorb it (figure 10). After about half an hour, remove the cards and put them away to dry (figure 11). To do it quicker, you can also dry them with a hairdryer. Cut the cards in strips (figure 12). Put away the red cabbage cards which are not used immediately: they will last some months. If you store them in a closed envelope to reduce their oxidization, they will last longer. Put the remaining juice in a bottle. After a few days, this juice goes bad and you have to throw it  away. To keep it longer, store it in a refrigerator.

 

   Figure 10 – Soaking cards with red cabbage juice.

 

  Figure 11 – Drying the cards.

 

   Figure 12 – Cutting the strips.

 

    Figure 13 – Red cabbage paper in solution of lemon juice.

 

  Figure 14 – Red cabbage card in a saturated solution of baking soda.

USE RED CABBAGE JUICE AS INDICATOR IN LIQUID STATE

Pour some drops of this liquid on a white surface and observe its changes of color when it is mixed with vinegar or with baking soda. You will see that this liquid becomes red in contact with vinegar or lemon juice, while it becomes green in contact with baking soda (figure 9). This behavior is unusual, and later on we will try to explain it.

 Pour one centimeter of the red cabbage juice indicator liquid into a transparent glass. Add water up to half of the glass. Now, pour vinegar into the glass and observe the color changes of the liquid. Repeat the experiment by adding, this time, a little baking soda instead of the vinegar. Also in this case, you will see color changes.

Figure 9 – Red cabbage juice mixed with baking soda (left) and with vinegar (right). On the top, a drop of unmixed juice.

MAKING RED CABBAGE JUICE

As we have seen, acids and bases have the property of modifying the color of certain substances. This is the case with the juice of the red cabbage. This liquid has a blue-violet color, but when it comes in contact with acidic substances it becomes red, while in contact with basic substances it becomes green and even yellow. Let’s see how it is possible to use the juice of the red cabbage to measure the pH of various substances.

  Figure 5 – Red cabbage.

 Figure 6 – Cut the cabbage into

 Figure 7 – Add water enough to cover the slices and boil for half an hour.
slices and put them in a pot.

  Figure 8 – Pour the juice in a low container.

During winter and spring, it is easy to find a red cabbage at the produce market or greengrocer. It is a cabbage which has a red-violet color (figure 5). Buy one of them and cut it in little slices (figure 6). Put them in a pot and pour enough water to cover them (figure 7). Boil for half an hour, then turn off the heat and let the temperature come down. Pour the blue-violet liquid you have obtained into a large, low container (figure 8). The boiled cabbage slices are edible and you can use them in a recipe

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~SAFETY HANDLING OF ACID AND BASES~

There are a number of proper procedures for the safe handling of acids and bases that you need to know because you will be working with them quite a bit.

	Both acids and bases can be corrosive to human tissue. When concentrated, they can react with tissue and break it down. In general, the more concentrated the acid or base happens to be, the more hazardous it is. Although the more concentrated acids and bases are the most dangerous ones, don’t ignore the dilute ones.
	You must be particularly careful about getting them in your eyes. The best way to avoid this is to wear safety glasses or goggles when handling either acids or bases. If you do get any in your eyes, let the instructor know and flush it out immediately with lots of water, several minutes worth. There are eye washes in the lab. If you have not yet learned where they are, and how to use them, ask for instructions now or the next time you are in the lab.
	Suppose you get some acid or base on you, other than in your eyes. The procedure is the essentially the same: flush that area immediately for several minutes with water and consult the instructor for further advice. If you should be unfortunate enough to spill it all over you, use the safety shower in the lab.
	If you spill acid or base on the lab bench top or on the floor, treat it immediately. If it is an acid, first neutralize it with sufficient sodium bicarbonate, (commonly known as baking soda). We have some readily available in the lab or in the adjoining prep room. When the baking soda no longer bubbles when you work it into the spill, the acid is neutralized and you can clean up the mess. If it’s a base, you can neutralize it with some vinegar, which is dilute acetic acid. In any case, clean up the area thoroughly. We also have spill kits for use with extensive spills. The quantities and concentrations of acids and bases used in the exercises and demonstrations in this lesson are permissible to flush down the drain. However, the quantity involved in a spill should be neutralized before disposal.

Never use dangerous substances. Do not use strong acids or bases. Do not put dangerous chemicals in containers for food use (i.e: cups, glasses, bottles) because they could be mistaken for beverages or foods. Never leave dangerous chemicals around the house; work instead in a room like a laboratory, a cellar, or a garage. At the end of the experiments, empty the beakers and wash them. Place the containers of the remaining substances which you want to preserve in suitable places, out of reach of children. Write on the containers their contents and dangerousness. While using ammonia, work in the open air or in an airy room with open windows. As soon as you have taken the amount of ammonia you need, close its container. Boys and girls should keep at least 2 m distant, and upwind. An adult must always be present during these experiments. In any case, we do not assume any liability.

 

TERMS IN CHEMISTRY

• Acid  –  A substance that has the potential to donate a proton (H⁺) or accept an electron pair.
• Acidic  –  Having a pH less than 7.
• Arrhenius Model  –  Arrhenius proposed that acids are substances that produce protons, H⁺, in aqueous solution, whereas bases produce hydroxide ions, OH^–, in aqueous solution. Compare his model with the Bronsted-Lowry definition and the Lewis definition.
• Base  –  A substance that can accept a proton, release OH^–, or donate an electron pair.
• Bronsted-Lowry Definition  –  Bronsted and Lowry define an acid as a proton (H⁺) donor and a base as a proton acceptor. Compare this model with the Arrhenius Model and the Lewis definition.
• Buffer  –  A solution composed of an acid and its conjugate base that serves to moderate the pH of the solution.
• Conjugate Acid  –  A molecule that can be described as a base that has gained one proton.
• Conjugate Base  –  A molecule that can be described as an acid that has lost one proton.
• Indicator  –  A molecule whose conjugate acid or conjugate base has a different color. Indicators are used to measure the pH of a solution.
• Lewis Definition  –  Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. Compare his model with the Arrhenius model and the Bronsted-Lowry definition.
• pH  –  A measure of the hydrogen ion concentration, it is equal to – log [H⁺], where [H⁺] is the concentration of protons.
• Redox  –  Short for “reduction-oxidation”
• Titration  –  An experiment that neutralizes an unknown amount of acid or base with a known volume and concentration of acid or base to determine the amount of unknown acid or base.

~ WELCOME WORLD ~

Acids and bases play a central role in chemistry because, with the exception of redox reactions, every chemical reaction can be classified as an acid-base reaction. Our understanding of chemical reactions as acid-base interactions comes from the wide acceptance of the Lewis definition of acids and bases, which supplanted both the earlier Bronsted-Lowry concept and the first definition–the Arrhenius model. Arrhenius first defined acids as proton (H⁺) producers in aqueous solution and bases as hydroxide (OH^–) producers.  Although this model is intuitively correct, it is limited to substances that include proton and hydroxide groups. Bronsted and Lowry proposed the more general definitions of acids and bases as proton donors and acceptors, respectively. Unlike the Arrhenius conception, the Bronsted-Lowry model accounts for acids in solvents other than water, where the proton transfers do not necessarily involve hydroxide ions. But the Bronsted-Lowry model fails to explain the observation that metal ions make water more acidic . Finally, Lewis gave us the more general definition of acids and bases that we use today. According to Lewis, acids are electron pair acceptors and bases are electron pair donors. Any chemical reaction that can be represented as a simple exchange of valence electron pairs to break and form bonds is therefore an acid-base reaction.

Acid-base chemistry is important to us on a practical level as well, outside of laboratory chemical reactions. Our bodily functions, ranging from the microscopic transport of ions across nerve cell membranes to the macroscopic acidic digestion of food in the stomach, are all ruled by the principles of acid-base chemistry. Homeostasis, the temperature and chemical balances in our bodies, is maintained by acid-base reactions. For example, fluctuations in the pH, or concentration of hydrogen ions, of our blood is moderated at a comfortable level through use of buffers. Learning how buffers work and what their limitations are can help us to better understand our physiology. We will start by introducing fundamentals of acid-base chemistry and the calculation of pH, and then we will cover techniques for measuring pH. We learn about buffers and see how they are applied to measure the acidic content of solutions through titration.