Transcript Measuring the pH

Measuring the pH of Orange Juice
pH of Orange Juice 1
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
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pH of Common Substances
pH of Orange Juice 3
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
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What does the pH scale look like?
grams/Liter
grams/Liter
The mathematical equation for pH is:
pH = - log (concentration of H+)
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How do we measure pH?
 There are several ways to measure pH
1. 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
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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
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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
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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
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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.
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pH vs. OJ concentration
Data Table
OJ Concentration
0.0% (tap water)
0.5%
1.0%
2.0%
5.0%
10.0%
pH
_______
_______
_______
_______
_______
_______
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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.
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Semi-log Paper for Graphing Results
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Questions
1. Which solution is the most acidic?
2. What is the pH of tap water?
3. Using the graph, predict the pH value
of a 50% OJ solution.
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