A Pickle of a ‘Dill’emma
Introduction:
The ability to taste depends on the 4 general regions of the tongue: salty, sweet, sour,
bitter. Specifically, food and other substances have characteristic tastes based on the
combination your tongue detects of these four flavors. Two classes of chemicals, acids
and bases, appear in many familiar foods; acids taste sour while bases taste bitter.
The pH scale measures how acidic or basic (alkaline) a substance is. Acids have a pH of
less than 7 and become stronger the lower their pH. Bases have a pH of greater than 7
and become stronger the higher their pH. Adding water to an acid, like gastric juice (pH
1), will make it weaker and the pH will increase. Adding water to a strong base, like
bleach (pH 12.5), will make it weaker and the pH will decrease. At a pH of 7 a substance
is neither an acid or base and is neutral, like pure water.
Acids and bases are opposites: adding together an acid and a base will result in a weaker
solution. One substance that we all know very well is water. If you add an acid and base
in exactly the same amount, they will neutralize each other and the pH will be … 7! This
allows a way to measure the strength of an unknown acid. Adding just enough of the
base of known concentration (to the unknown acid) to reach pH 7 leads to the ability to
calculate the concentration of the unknown acid.
How much base is required to neutralize an unknown acid depends on two factors:
_____________________
_____________________
Formula for calculating unknown acid concentration when exactly neutralized by base:
Pre lab:
1. Name 3 common foods that are acidic (taste ______________)
_______________
_______________________
____________
2. Name 3 common foods that are basic (taste ______________)
_______________
_______________________
____________
Look up on the web ‘Yamada’s universal indicator’.
3. List the pH indicators in Yamada’s.
4. Also note the range in color change and construct a table for the indicators in
Yamada’s (for pH and color change).
Procedure:
A: Testing Household Substances
-Make a table suitable to record pH results of the provided household substances and
indicators:
Indicators
chemicals
Phenolphthalein
bromothymol blue
methyl orange
red cabbage juice extract
Vernier pH probe
Yamada’s universal indicator
vinegar
ammonia
lemon juice
soft drink (clear!)
detergent
Baking soda
aspirin
-Predict the color change for the Yamada’s universal indicator and add to data table.
B: Determination of Concentration of Unknown Acid (Titration Curve):
1. Obtain and wear goggles
2. Obtain the following lab equipment:
 100 ml waste beaker
 250 ml titration beaker,
 either pH meter or pH probe system (Vernier)
 distilled water bottle
 graduated cylinder
3. Observe demonstration of equipment and technique by instructor
4. Fill burette (or reservoir) with 50 ml 0.1 M NaOH
5. Into titration beaker, add:
a. 10 ml of unknown acid
b. 40 ml water
c. 3 drops of your choice of indicator
d. note and record beginning color of titration solution
e. note and record beginning pH
6. Note that the titration directions will depend on whether you use the pH meter
or the pH probe/computer set up
pH meter
Add 1 ml of base from burette, then
record:
pH
Color of indicator
Total volume of base added
Continue procedure until pH
reaches 12
7. Graph results:
pH probe/computer set up
Click ‘collect’ in upper right
Turn stop cock until flow rate is
approximately 1-2 drops per second
Note pH when indicator changes color
Continue procedure until pH reaches 12
Click ‘stop’ in upper right
what is the independent variable _______________
what is the dependent variable _______________
on the graph, indicate:
the neutralization point
the acidic region
the basic region
8. Clean up according to instructor’s directions
C: Forensics File:
Peter Vlasic and Henrietta Heinz both attended a lavish birthday party at Jillian’s,
catered by Wegman’s. Peter Vlasic, a known practical joker, was observed adding an
unknown liquid to Henrietta Heinz’s water during the celebration. Ms. Heinz was found
dead just a few hours after drinking this unknown solution. The medical examiner’s
report lists intestinal bleeding and gastric ulcers, typical of intake of concentrated acid.
Although Ms. Heinz’s drink was emptied and the drink glass cleaned up, Mr. Heinz
admitted to adding a small amount of pickle juice from a flask. liquid used in the Vlasic
factory
The prosecution argues that Vlasic pressured Heinz for access to a family secret recipe
and when Henrietta refused, Vlasic added a strong acid to her drink from the flask. The
defense argues that Henrietta drank too much and was self pickled from the evening’s
alcohol. Unfortunately, a sloppy technician (rumored to be a dismissed Heinz employee)
spilled ketchup into Ms. Heinz’s blood sample and, therefore, the alcohol tests are
unreliable. The only evidence that can clear Vlasic is a sample from his flask. Peter
claims that he added only a small amount of pickle juice from the flask, not enough to
hurt anyone or to make a noticeable change in the volume of Ms. Heinz’s drink.
As a forensics technician, you are to determine:
-
Determine the concentration of acidic solution added from the flask
Make an estimate of the volume that was added to Heinz’s drink
estimate the concentration of the liquid in Heinz’s drink
Analysis:
A.
1. Explain how a universal indicators works.
2. What are the limitations to red cabbage juice extract when used as a pH indicator?
B.
3. What is the concentration of the unknown acid and show your work.
C.
4. What is the concentration of the acid from Vlasic’s flask and show your work.
5. What was the estimate of the volume that was added – explain the method and
reasoning your group chose to determine this value.
6. Estimate the concentration of the acid content of Heinz’s drink after Vlasic
contaminated it.
7. Locate and cite a reference to evaluate the potential harm due to the calculated
acid strength of Henrietta’s drink.
8. Examine your procedure and the case history. Then apply this to your laboratory
results to discuss the range of confidence in your results.
9. Research a real forensics case involving chemical analysis.
a. Provide the source of the case
b. Brief summary of the case, and the
c. Description of chemical analysis involved.
Lab Requirements:
- Title of lab
- Abstract
- table from part A (pH for household chemicals using various indicators)
- table from part B (pH and color of indicators)
- graph of titration curve
- Answer questions from analysis in complete sentences – you must show question
(you can paste these from teacher’s web page)
Teacher Supplement
Red Cabbage Extract pH chart
*Have students suggest other logical samples to test, e.g., other person’s drink, Peter’s
stomach contents, Henrietta’s stomach contents, the bartender’s water source, Henrietta’s
blood sample
Yamada's universal indicator:
Dissolve 0.0025 g of thymol blue, 0.06 g of methyl red, 0.030 g of bromothymol blue,
and 0.05 g of phenolphthalein in 50 mL of 95% ethanol. Add 0.01 mol L–1 aqueous
sodium hydroxide solution until the mixture is green, and dilute the resulting solution to
100 mL with distilled water
Yamada's Universal Indicator exhibits the colors of the visible
spectrum:
pH 10
violet
pH 6
yellow
pH 9
indigo
pH 5
orange
pH 8
blue
pH 4
red
pH 7
green
To prepare 200 mL of stock solution, dissolve
0.005 g Thymol Blue
0.012 g Methyl Red
0.060 g Bromothymol Blue
0.100 g Phenolphthalein
in 100 mL of ethanol. Neutralize the solution (to green) with 0.05 M
NaOH and dilute to 200 mL with water.