microfluidic titration

Titration Of Vinegar Using NaOHTable of Contents:
Learning Objectives:
1
Research Questions:
1
Methods:
1
Helpful Videos and Tutorials:
2
Procedure:
Materials:
Performing the Titration:
Trial and Error:
Questions to Investigate in Lab:
3
3
3
4
4
Post-Lab Questions:
4
Learn more!
4
References:
5
Learning Objectives:
1. Use surface tension based micro paper analytical devices to make accurate measurements
of vinegar concentrations
2. Use acid-base concepts to predict and test endpoint pH
3. Use stoichiometry concepts to measure the acetic acid concentration of vinegar
4. Use scientific data to develop experimental procedures
5. Determine sources of experimental failure and iterate on experimental procedure
6. Evaluate the significance of experimental data using appropriate statistics
Research Questions:
1. How do we work within the limitations of this measurement tool in order to produce an
accurate and precise experimental measurement?
2. What is the acetic acid concentration of commercial vinegar?
Methods:
In this experiment, you will be using a microfluidic chip to conduct a titration analysis. This
device is the same kind of technology that could be used by a scientist or a citizen to
measure the effect of acid rain or acid mine drainage on their local water system. 5μL of
sodium hydroxide of the concentration below has been deposited into each cell. This is
where the titration reaction will take place. Please note that these are NOT the final
concentrations once you have added your sample.
0mM 100
200 300
400 500
600 700
800 900 1M
1.5M 3M
Figure 6. Depiction of a titration device included with the lab. 5μL of sodium hydroxide of
the concentration below has been deposited into each cell.
In order to measure the resulting pH, pH paper will be soaked in your vinegar solution to
release the indicator. The indicator will change color based on the resulting pH of the
solution in each square. Figure 7 below gives you a guide for the color at various pH
values.
Figure 7. pH Indicator Paper Scale
Helpful Videos and Tutorials:
1. Bubble Chip Micropad Tutorial
2. Reagent Concentration Calculations Tutorial
Pre-lab Questions:
1. What is the chemical equation for the reaction between NaOH and acetic acid?
2. What color should your bubble turn at the equivalence point? Include your reasoning in
determining the color. (Hint: look at section 11-2 (Region 3: at the equivalence point on
page 245-246 of your textbook.)
3. Look at the NaOH solution concentrations on your chip. Your goal is to measure the
endpoint as accurately as possible. Which spot on the chip will allow you to see a color
change most clearly? Justify your choice. (Hint: Consider the percent difference between
adjacent points.)
4. Why shouldn’t you use the microfluidic chip to titrate the vinegar straight out of the
bottle?
5. What do you think would be a good dilution factor to start with? Search the label of the
vinegar that you’re working with for an acidity concentration of vinegar (it will be ~5%).
The acidity content will likely be under the product name or near the ingredients list. It
might be helpful to review the Reagent Concentration Calculations Tutorial. (Note you
will add the vinegar with a 20 L pipette, and it is recommended to use 40-80 L total.)
6. Given the materials available, develop 2 different ways to make the appropriate dilution
and explain the strengths and weaknesses of each method. (Assume you will have 20L
and 100L pipettes with 1 L increments, and 1 mL transfer pipettes.) Think about how
much solution you will need to make.
Procedure:
Materials:
● Microfluidic titration devices with preloaded NaOH spots
● Food-grade white vinegar
● Volume delivery equipment (one of the following)
○ Micropipette and pipette tips
○ Plastic pipette
○ Weigh boat and mass balance
● pH paper
● Container for making dilutions
● Smartphone or access to image capture technology
● Water
Performing the Titration:
1. Dilute the vinegar to your determined dilution factor. Ensure that there is enough solution
for all the spots; the total amount of solution should be no less than 1000μL. Make sure
your diluted mixture is homogeneous by mixing it with your pipette.
2. Add 1 strip of pH paper into your diluted vinegar solution and let it sit for 10 minutes.
Stir occasionally. The pH paper may change color, this is normal.
3. It may be helpful to review the Bubble Chip Micropad Tutorial before continuing.
4. Add up to 80μL of your mixed vinegar solution to the center of each square. The drops
should form a dome inside the bounds of the square. Be careful not to jostle it too much
or the surface tension bubble will pop and your solutions will escape their squares. Mix
the fluid in each square using the pipette making sure to go from lowest to highest
concentration to minimize contamination.
5. If the bubble pops, leaks, or escapes from the micropad, you will need to start over. You
may want to adjust the total volume added.
6. Allow the reaction to take place for a couple of minutes. Measure the color of the
indicator paper in each bubble by eye or using ColorAssist or ImageJ. Note the pHs
indicated by the colors based on the pH scale in Figure 7.
Data Collection:
1. Compare the end point to where you anticipated it to be? Adjust the dilution factor and
repeat as necessary.
2. It is always optimal to make multiple measurements of the same quantity to do a
statistical analysis. Because this measurement is not a continuous measurement (we can’t
measure any concentration as the endpoint, the endpoint will be in one of the discrete
NaOH concentrations), we need to do something to vary the expected endpoint. Propose
2 different ways to do this and perform the experiments until you have 3 measurements
of the end point.
Error Analysis:
3. Compare your results with a student who has the same vinegar solution, and another
student who has a different vinegar solution. What tests will you use to compare results?
Are your results distinguishable from your classmates?
4. Compare your results with those reported by the manufacturer (look online if it is not on
the bottle or consult your TA). What statistical test will you use to determine if your
results are distinguishable from those reported by the manufacturer?
Post-Lab Questions:
1. What factors in your experimental procedure were most critical to achieving
experimental results? Explain how you determined this.
2. List three possible causes of error of the titration experiment and explain the effect of
each one. (Human error is not an acceptable answer).
3. If you could redesign this microfluidic device, how would you make it differently?
Explain your reasoning.
4. In this lab, you were asked to determine many of the parameters of how the experiment
was to be done. Reflect on the things you learned in this process that might be helpful for
future experiments.
5. Read at least one of the references below (either from the reference list or the websites
provided). Propose a use for the paper titration device that would be more difficult with a
traditional titration.
Read More:
● Coal issues: Citizens Coal Council – Home
● For games, animations, and activist information on acid rain: Acid Rain: What
can you do?
● Effects of acid mine drainage on wildlife and ecology:
pebblescience.org/pdfs/Final_Lit_Review_AMD.pdf
● Get involved with the safe drinking water foundation: Acid Rain
● Solutions for reducing acid rain affects: Reducing Acid Rain Or Its Effects |
Environmental Chemistry | Chemistry | FuseSchool
References:
“What Is Acid Rain?” EPA, Environmental Protection Agency, 12 May 2020, www.epa.gov/acidrain/whatacid-rain.
2. “Acid Rain and Water.” USGS Science for a Changing World, U.S. Department of the Interior ,
www.usgs.gov/special-topic/water-science-school/science/acid-rain-and-water?qtscience_center_objects=0#qt-science_center_objects.
3. “Effects of Acid Rain – Human Health.” The Environmental Protection Agency Napa County, California,
Trilogy Integrated Resources , napa.networkofcare.org/ph/library/article.aspx?id=1736.
4. “ENVIRONMENTAL JUSTICE.” Citizens Coal Council, Citizens Coal Council ,
www.citizenscoalcouncil.org/environmental-justice.html.
5. Echolls, Taylor. “What Place in the World Receives the Most Acid Rain?” Sciencing, 2 Mar. 2019,
sciencing.com/place-world-receives-acid-rain-23289.html.
6. Lewicke, A.; Finkelstein, N.; Griswold, A.; Schneps, M.; Cane, M. The habitable planet; [Annenberg
Media]: [S. Burlington, Vt.], 2007.
7. Rastogi, Archi. “Industrialisation Bringing Acid Rain to India.” Down To Earth, Down To Earth, 5 July
2015, www.downtoearth.org.in/news/industrialisation-bringing-acid-rain-to-india-6625.
8. Akcil, A.; Koldas, S. Acid Mine Drainage (AMD): Causes, Treatment and Case Studies. J. Clean. Prod.
2006, 14 (12–13), 1139–1145. https://doi.org/10.1016/j.jclepro.2004.09.006.
9. Kumari, S.; Udayabhanu, G.; Prasad, B. Studies on Environmental Impact of Acid Mine Drain. Indian J.
Environ. Prot. 2010, 30.
10. Mine Drainage. https://www.usgs.gov/mission-areas/water-resources/science/mine-drainage?qtscience_center_objects=0#qt-science_center_objects (accessed Jul 2, 2020).
11. Environmental Justice http://www.citizenscoalcouncil.org/environmental-justice.html (accessed Jun 22,
2020).
12. SkyTruth – US Abandoned Coal Mines (eAMLIS Database). https://skytruth-org.carto.com/viz/743a74d46e94-11e5-9f65-0ecfd53eb7d3/embed_map (accessed Jul 2, 2020).
1.
CHEM 222
Exp. 8: Spectrophotometric Determination of Iron in Vitamin Tablets
Name: ______________________________________________ Date: ___________________
TA ́s Name: _________________________________________
STOCK CONCENTRATION: [Fe] = ___________
CALIBRATION CURVE
Standard Fe Solutions
standard 1
standard 2
standard 3
volume of stock used (mL)
[Fe] in standard (mg/mL)
Absorbance
Value of ε
Average ε
*if your TA had you use M instead of mg/mL, you may use either set of units.
standard 4
Prepare and include a graph of absorbance vs. mg/mL of Fe in the standard solutions. Do a linear
regresssion on the data and report the regression parameters and their errors:
m
sm
b
sb
r2
sy
UNKNOWN ANALYSIS (complete at least one trial, compare with classmates to get st.dev.)
UNKNOWN #_______________
Trial 1
absorbance
Concentration of Fe,
final dilution
(mg/mL)
concentration Fe in
vitamin sample
(mg/mL).
std. dev.
rsd (%)
Trial 2
Trial 3
Spectrophotometric Determination of Iron in Vitamin Tablets
Page 1 of 3
Spectrophotometric Determination of Iron in Vitamin Tablets1
In this procedure, iron from a liquid vitamin supplement is reduced to Fe2+
with hydroquinone and complexed with o-phenanthroline to form an
intensely colored complex (Color Plate 15 in the textbook).
Research questions:
•
What is the concentration of iron in a vitamin tablet?
Learning Objectives:
•
•
Use spectrophotmeter
Use a calibration curve to determine the precision and accuracy of your results.
Reagents
Hydroquinone: (20 mL/student) Freshly prepared solution containing 10 g/L in distilled water.
Store in an amber bottle.
Trisodium citrate: (20 mL/student) 25 g/L Na2citrate × 2H2O (FM 294.10) in distilled water.
o-Phenanthroline: (25 mL/student) Dissolve 2.5 g in 100 mL of ethanol and add 900 mL of
distilled water. Store in an amber bottle.
Standard Fe (40 µg Fe/mL): (35 mL/student) Dissolve 0.562 g of reagent-grade
Fe(NH4 )2 (SO4 )2 × 6H2O (FM 392.14) in distilled water in a 1-L volumetric flask containing
1 mL of 98 wt% H2SO4.
Unknown Iron Solution: Prepared by TA by adding 0.5 mL of the stock unknown to a 250 mL
volumetric flask and diluting with DI water.
Procedure
1.
R. C. Atkins, J. Chem. Ed. 1975, 52, 550
Spectrophotometric Determination of Iron in Vitamin Tablets
Page 2 of 3
1. Pipet a 10.00-mL aliquot of Fe standard into a 100-mL volumetric flask and add 2.00 mL of
sodium citrate solution. Add 2.00 mL of hydroquinone solution and 3.00 mL of ophenanthroline solution, dilute to the mark with water, and mix well.
2. Prepare three more solutions from 5.00, 2.00, and 1.00 mL of Fe standard and prepare a
blank containing no Fe. Use sodium citrate solution in proportion to the volume of Fe
solution. (If 10 mL of Fe requires 2.00 mL of citrate solution, 5 mL of Fe requires 1.0 mL of
citrate solution.)
3. Now that the standards are prepared, determine how many drops of citrate solution are
needed to bring 10.00 mL of the unknown solution to pH 3.5. Check the pH of the unknown
to start. The unknown may already be near the correct pH. If so, no sodium citrate needs to
be added. If the pH is too high, you may need to add citric acid.
4. Transfer 10.00 mL of the unknown to a 100-mL volumetric flask. Then add 2.00 mL of
hydroquinone solution and 3.0 mL of o-phenanthroline solution. Dilute to the mark and mix
well.
5. Since the sample contains dye, we also want to run a sample blank. Transfer 10.00 mL of the
unknown to a 100-mL volumetric flask. Dilute to the mark and mix well.
6.
Allow the solutions to stand for at least 10 min. Then measure the absorbance of each
solution at 510 nm in a 1-cm cell. (The color is stable, so all solutions may be prepared and all
the absorbances measured at once.) Use distilled water in the reference cuvette and subtract the
absorbance of the blank from the absorbance of the Fe standards. Make 3 replicate measures of
your unknown.
7. Make a graph of absorbance versus micrograms of Fe in the standards. Find the slope and
intercept (and standard deviations) by the method of least squares. Calculate the molarity of
Fe(o-phenanthroline)32+ in each solution and find the average molar absorptivity (e in Beer’s
law) from the four absorbances. (Remember that all the iron has been converted to the
phenanthroline complex.)
8. Using the calibration curve, find the number of milligrams of Fe in the tablet. Use Equation
4-27 in the textbook to find the uncertainty in the number of milligrams of Fe.
Spectrophotometric Determination of Iron in Vitamin Tablets
Page 3 of 3
*Note the calculations and lab notebook are worth a lot of points. Be sure that these are in good
order.
Lab component:
Sample
Calculations (7
pts)
Table 1
calibration curve
(7 pt)
Calibration curve
chart (7 pt)
Table 2
calibration stats
(5 pt)
Table 3 unknown
analysis (7 pt)
Notebook pages
(7)
Description
Must include units, labels and be clear. For functions done by hand, provide the
equation and sample calculation. For functions in excel, include functions used
and any options selected. Include your excel sheets after your notebook.
Complete, calculations properly performed.
Includes all appropriate labels.
Complete, calculations properly performed.
Complete, calculations properly performed.
Questions appropriately answered
Must include brief procedure, single-line cross-outs, all data collected in
notebook, labels/dates/names on top of the pages. Include Excel pages.