Measuring the pH of Orange Juice

What is pH? pH is a measure of the concentration of hydrogen ions (H+) in grams/1000 ml of water The pH scale ranges from 1 to 14  number 7 represents a neutral solution  numbers below 7 show an acidic solution  numbers above 7 show a basic solution

pH of Common Substances

How is the pH scale made? The pH scale is “logarithmic” Each whole value of pH below 7 is ten times MORE ACIDIC than the next lower whole number: - pH 4 is 10 times more acidic than pH 5 - pH 4 is 100 times more acidic than pH 6 - pH 4 is 1000 times more acidic than pH 7 Each whole value of pH above 7 is ten times MORE ALKALINE (basic) than the next higher whole number: - pH 10 is 10 times more basic than pH 9 - pH 10 is 100 times more basic than pH 8 - pH 10 is 1000 times more basic than pH 7

The mathematical equation for pH is: pH = - log (concentration of H+) What does the pH scale look like? grams/Liter grams/Liter The mathematical equation for pH is: pH = - log (concentration of H+)

How do we measure pH? There are several ways to measure pH ph Meter A digital readout gives the pH directly. This meter has a special built-in electrode placed in the solution. A pH meter is the most accurate, but most expensive method

pH Measurements 2. pH Paper Paper strip on a roll shows different colors for each pH unit. pH paper is the least accurate, but cheapest method

pH Measurements 3. pH Test Strips Test strips show different colors for pH in fractions of a pH unit over a narrow range. We will use pH strips because they are more accurate than pH paper

Prepare Orange Juice Solutions 1. Using a thawed can of frozen orange juice, make a batch of juice that you would normally drink – 3 cans of tap water plus 1 can of frozen juice LABEL THIS 100% OJ 2. Measure 50 ml of the 100% OJ into a 500 ml graduated Erlenmeyer flask and fill with tap water to the 500 ml mark. LABEL THIS 10% OJ 3. Repeat step 2 using 25, 10, 5, and 2.5 ml of 100% OJ - LABEL THESE 5%, 2%, 1%, and 0.5% OJ

Measure pH of OJ Solutions 1. Pour about 50 ml of the 100% OJ into a 100 ml beaker and label it. Repeat with the 10%, 5%, 2%, 1% and 0.5% solutions. 2. Hold the white end of a 4070 test strip and dip it into one of the solutions prepared in 1 above to about ½-inch for 2 seconds. 3. Shake off excess liquid and match the color of the strip to the color chart before it dries. Record the pH value on the data table. 4. Repeat the process with all the solutions and the tap water.

pH vs. OJ concentration Data Table OJ Concentration pH 0.0% (tap water) _______ 0.5% _______ 1.0% _______ 2.0% _______ 5.0% _______ 10.0% _______

Graphing the Results Make a graph of the results using “semi-log” paper – this paper has a logarithmic scale on the y-axis and a linear scale on the x-axis. Show the OJ Concentration on the y-axis and the pH value on the x-axis.

Semi-log Paper for Graphing Results

Questions Which solution is the most acidic? What is the pH of tap water? Using the graph, predict the pH value of a 50% OJ solution.