How do you determine pH from molarity? What does alkalinity mean? What is a base? What is a neutral pH? How does alkalinity affect pool water? Find an indicator for the titration of a 0.
First, you should estimate the pH at the equivalence point, at which the solution is 0. This is a hydrolysis problem, but the following method employs the general principle of equilibrium. Phenolphthalein in the table above has a p K ai value of 9. A parade of the color intensities is shown below:.
The equivalence point is when the color changes most rapidly, not when the solution has changed color. Improper use of indicators will introduce inaccuracy to titration results. Indicators change color gradually at various pH. Let us assume that the acid form has a blue color and the basic form has red color.
The variation of colors at different pH is shown below. The background color affects their appearance and our perception of them. If a solution has a color matching this, the pH would be the same as the p K ai of the indicator, provided that the conjugate forms of the indicator have the BLUE and RED colors. Because of their chemical properties, reactions involving acids and bases are different from the chemical reactions students have seen so far in Chapter 6.
In the previous lessons, it was always the electrons that were being shared or transferred when atoms interacted. In the next three lessons about acids and bases, things are a little different. With acids and bases, it is a proton from a hydrogen atom that is transferred from one substance to another. The main aspect of acids and bases that students will explore in the next three lessons deals with the influence of acids and bases on water. The reactions of acids and bases with water are measured using the pH scale.
Understanding pH on the molecular level will give students a better appreciation for some of the environmental issues involving acids and bases. The meaning of pH and the way it is affected by acids and bases can be a little tricky, but by using animations, drawings, and some simplifications, students should be able to understand the main ideas. Note : Your local tap water is likely fine for the demonstration and activities in this lesson. If the indicator solution you make is not green, this means that your water is either acidic or basic.
If this happens, use distilled water, which is available in supermarkets and pharmacies. Note : In the activity, students will fill 12 wells with universal indicator solution. Check to make sure that 25 mL of solution is enough. You will need about 50 mL of indicator solution for your demonstration. If mL of solution is not enough, make more using the same proportions. Pour about 50 mL indicator solution into a clear plastic cup for you to use in the demonstration. The citric acid turns the indicator from green to reddish.
The sodium carbonate turns the indicator from green to purple. Tell students that the green solution was made by adding a substance called universal indicator to water. Explain that you put a small amount of a substance, one an acid and one a base, in each cup.
Tell students that when you poured universal indicator solution into the cups, the acid and base each reacted with the indicator and changed its color. Usually, when two substances are mixed and a color change results, that is a clue that a chemical reaction has taken place. The cause of this color change will be discussed later in this lesson when students do their own activity. Tell students that they will use an acid, a base, and universal indicator solution to learn about how acids and bases affect water.
They will also learn how to measure the effect with colors and numbers on the pH scale. Explain that the chart shows the range of color changes for universal indicator when acidic or basic solutions are added to the indicator.
Point out that each color has a number associated with it and that students will learn more about these numbers later in the lesson. As the solution becomes more acidic, the color changes from green toward red. As the solution becomes more basic, the color changes from green toward purple. Explain that before class, you placed a small amount of citric acid in the cup that turned red and a small amount of sodium carbonate in the cup that turned purple.
So citric acid is an acid and sodium carbonate is a base. Explain that sodium carbonate is one of the chemicals commonly used in detergents made for dishwashing machines. Tell students that next they will explore the color changes of universal indicator with small amounts of citric acid and sodium carbonate. Students will record their observations and answer questions about the activity on the activity sheet. To find the answers to the activity sheet, go to the downloads area within the online version of this lesson.
Explain to students that they will first make their solutions for the activity. Either go through each step with them or have them follow the procedure described on their activity sheet. Use masking tape and a pen to label one cup citric acid solution and another cup sodium carbonate solution. Use a small piece of masking tape and a pen to label one dropper citric acid solution and the other dropper sodium carbonate solution. Use a flat toothpick to pick up as much citric acid as you can on the end of the toothpick as shown.
Add this citric acid to the water in the citric acid cup. Gently swirl until the citric acid dissolves. Add this sodium carbonate to the water in the sodium carbonate cup. Gently swirl until the sodium carbonate dissolves. Explain to students that in this activity they will fill the wells in each spot plate with universal indicator solution. Then in the first spot plate, they will test how different concentrations of citric acid affect the color of universal indicator solution.
In the other spot plate, they will test how different concentrations of sodium carbonate affect the color of universal indicator solution. Tell students that in each spot plate, they will add nothing to the indicator solution in the first well. This is because the first well will serve as the control. Use your dropper to add 1 drop of citric acid solution to the second well. Gently mix the liquid with a clean toothpick. The color of the indicator should turn yellow-green or yellow. If there is no obvious color change after adding a toothpick of citric acid, have students add a little more citric acid to the solution.
Add water into the pot, making sure the water covers the cabbage entirely. Place the pot on the stove and allow to cook at medium heat for about 30 to 35 minutes. Allow it to cool, then pour contents into the bowl using the strainer. Soak your coffee filters in the cabbage juice for about 25 to 30 minutes. Allow the filters to fully dry, then cut them into strips. Now start your pH testing starts out blue , changes to green [basic], and red [acidic].
Practice Problems 1. References General Chemistry: principles of modern applications. New Jersey: Pearson Education,Inc, Utah State University Extension: Understanding your watershed. Geiger, J and Mesner, N. Department of Agriculture, June Nature: International weekly journal of science , Oceanography: Anthropogenic carbon and ocean pH.
Caldeira, K and Wickett, M.
0コメント