Purple-cabbage indicator

Tragedy is like strong acid—it dissolves away all but the very gold of truth. 

To see how this works, put on your safety glasses and then add about a tablespoon (15 milliliters) of purple-cabbage indicator (prepared as outlined in the Shopping List and Solutions ) to a cup (240 milliliters) of water and observe the color. It should be light lavender. Now make up a saturated solution of baking soda in water. A saturated solution is one in which no more solid can be made to dissolve. Baking soda is not very sol- 

Using one-cup (240-milliliter) samples of saturated baking soda solution, vinegar, ammonia, and water, you can set up the following banks of solutions and observe the colors that result when you add purple-cabbage, swimming-pool, or fish-tank indicator. 

Now take some of the clear, carbonated soda that was a suggested purchase in the Shopping List and Solutions and add the purple-cabbage indicator. You should observe a pale pink solution. Carbonated soda 

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Pour a half cup (120 milliliters) of vinegar and a half cup (120 milliliters) of water into the bottle and swirl to mix. A two-liter soda bottle may be substituted if you use twice as much vinegar and water. Add enough purple-cabbage indicator so that the resulting solution is bright pink, but do not add more than one tablespoon. 

Gas production is a good indication that a chemical reaction has taken place. Color change is also an indication of a chemical reaction however, in this case the reaction could have proceeded just as well without the purple-cabbage indicator and without changing color. The purple-cab-... 

So what are acids and bases Vinegar is actually a dilute solution of acetic acid in water, about a 5 percent solution, but it rather nicely displays the characteristic properties of acids they are sour, they turn purple-cabbage indicator red or pink, and they react with bases to form water. A solution of sodium bicarbonate nicely displays several of the characteristics of basic solutions it tastes bitter, it turns purple-cabbage indicator blue, and it reacts with acids to form water. The last property, listed for both acid and base, the ability to react with each other, is really the defining property because acid-base reactions, like redox reactions, occur in tandem one substance acts as an acid and one substance acts as a base. Acid neutralizes base and base neutralizes acid. 

In this demonstration, we first isolate the active ingredient of the modem form of aspirin, acetylsalicylic acid, using a time-honored organic chemistry procedure called extraction. The preparation of purple-cabbage indicator is a type of organic extraction the organic purple dye is extracted from the cabbage with water. Here we use alcohol to extract acetylsalicylic acid from aspirin. 

Universal indicators are mixtures of several acid-base indicators that display a continuous range of colors over a wide range of pH values. Figure 18-2 shows concentrated solutions of a universal indicator in flat dishes so that the colors are very intense. The juice of red (purple) cabbage is a universal indicator. Figure 19-2 shows the color of red cabbage juice in solutions within the pH range 1 to 13. 

Figure 19-2 The juice of the red (purple) cabbage is a naturally occurring universal indicator. From left to right are solutions of pH 1, 4, 7, 10, and 13.

Litmus is not the only plant material that turns a different color in response to acidic or basic conditions. For example, when red cabbage or beets are boiled, the solids can be separated from the liquid. The liquid is then cooled for use as an acid-base indicator. Red cabbage juice is red or purple in acidic conditions, while bases cause it to turn blue or yellow. When a solution is neutral, the juice is a bluish-purple. 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

Acid-base indicators can be extracted from different plant materials. An easy way to prepare an indicator is to cut about half of a small red cabbage into small pieces. Place these pieces in a pot, cover with water, and boil until the water turns a deep purple. It should take about 5 to 10 minutes of boiling. Let the solution cool and then either pour off the liquid or strain to separate the liquid from the cabbage leaves. 

Set the beaker on a hot plate, and heat until the water has boiled and become a deep purple color. Remove the beaker from the hot plate using beaker tongs, and allow it to cool. Pour off the cabbage juice indicator liquid into a clean beaker. 

Freshly made red cabbage juice has a blue-purple tint. However, after a while, it may assume more of a bluish tint. This slight color change will not interfere with its indicator properties. 

Many natural substances are acid-base indicators. The most familiar one is litmus, an organic dye extracted from certain lichens. Litmus turns from blue to red in acidic solutions (< pH 7) and from red to blue in basic solutions ( pH 7). Some other natnral indicators include red cabbage extract, blueberry juice, black tea, beet juice, rhubarb, and tomato leaves, and flowers such as the rose, daylily, blue iris, and purple dahlia. Red cabbage extract undergoes sharp changes of color at several pH values. The deep purple color of red cabbage leaves is caused by a mixture of water-soluble... 

Red cabbage contains a material that makes an acid-base indicator. Place two cups of chopped red cabbage in a blender, cover with boiling water, and blend. Use a coffee filter to filter out the plant material. Add the indicator to materials like lemon juice, baking soda, antacids, and seltzer water. The indicator will turn red/purple/violet in the presence of acids and blue/green/yellow for bases. 

Obtain a bottle of cranberry juice or cranberry drink, or use a red cabbage to prepare the pH indicator, as follows Tear up several red cabbage leaves and place them in a saucepan and cover with water. Boil for about 5 minutes. Cool and eoUeet the purple solution. Alternatively, place red cabbage leaves in a blender and cover with... 

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