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Gravimetric, Volumetric/Spectrophotometric Analysis Calcium Exercise Accuracy, Precision/ANOVA | CHEM 2300, Lab Reports of Chemistry

Georgia Southern University–Armstrong CampusChemistry
Prof. Joshua E. Smith

Material Type: Lab; Professor: Smith; Class: PRINCIPLES CHEMICAL ANALYSIS; Subject: Chemistry; University: Armstrong Atlantic State University; Term: Unknown 1989;

Typology: Lab Reports

Pre 2010

Uploaded on 08/04/2009

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CHEM 2300 Labs
Gravimetric, Volumetric and Spectrophotometric Analyses of Calcium
An exercise in Accuracy, Precision and ANOVA
This laboratory experiment is scheduled to last for 4 lab periods and serve as a capstone
experience in Chemical Analysis. Plan your time wisely. You may perform the experiments in
any order. Throughout the experiment, you will analyze the same unknown. DO NOT FORGET
TO RECORD YOUR UNKNOWN NUMBER. ACCURACY AND PRECISION ARE
EXTREMELY IMPORTANT! At the end of the three experiments, you should be able to use
ANOVA to determine if there is a difference in the precision of the four methods.
Unknown (RECORD THE NUMBER OR –20 points since accuracy and precision will not be
able to be determined from my list of known concentrations!!!)
During the course of the 4 weeks, SAVE all calcium standards and all acid/base reagents as they
may be used in multiple experiments.
Lab 1 Gravimetric Method
Background: Read Chapter 27 in Harris for the background information. This method
is for the general application/technique of gravimetric methods. Calcium is chosen as
the sample because it is environmentally friendly and safe to work with.
Go to this web-site http://bcs.whfreeman.com/qca7e/content/cat_070/Harris
%207e.Experiments%2017May06.pdf to obtain the second lab (page 3) entitled
“Gravimetric Determination of Calcium as Calcium Oxalate Hydrate”
Note:
YOU ONLY need to make 100 mL of the ammonium oxalate reagent, not 1 Liter
You will be supplied with SOLID unknowns from Thorn Smith. You should dissolve
~2.5-3 g of unknown in 250 mL of 0.50 M HCl.
Calculate the average % Ca in your sample. Students should take care to make sure that they
calculate the % Ca in the original solid sample! Calculate the standard deviation and 95%
confidence level for this concentration.
Lab 2: AA method; Atomic Absorption Analysis of Calcium.
Read Chapter 20 and 21 in the Harris text for the background information. Most inorganic
chemical analysis of calcium is done via flame atomic absorption analysis.
Supplies needed:
Lanthanum Oxide
Calcium carbonate, primary standard grade
1 L volumetric flask
5 mL volumetric pipettes
Other volumetric pipettes (0.5, 1, 2, 3, 10 mL)
0.50 M HCl (at least 2 L)
Concentrated HCl
10 100 mL volumetric flasks
Procedure:
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CHEM 2300 Labs Gravimetric, Volumetric and Spectrophotometric Analyses of Calcium An exercise in Accuracy, Precision and ANOVA This laboratory experiment is scheduled to last for 4 lab periods and serve as a capstone experience in Chemical Analysis. Plan your time wisely. You may perform the experiments in any order. Throughout the experiment, you will analyze the same unknown. DO NOT FORGET TO RECORD YOUR UNKNOWN NUMBER. ACCURACY AND PRECISION ARE EXTREMELY IMPORTANT! At the end of the three experiments, you should be able to use ANOVA to determine if there is a difference in the precision of the four methods. Unknown (RECORD THE NUMBER OR –20 points since accuracy and precision will not be able to be determined from my list of known concentrations!!!) During the course of the 4 weeks, SAVE all calcium standards and all acid/base reagents as they may be used in multiple experiments. Lab 1 Gravimetric Method Background: Read Chapter 27 in Harris for the background information. This method is for the general application/technique of gravimetric methods. Calcium is chosen as the sample because it is environmentally friendly and safe to work with. Go to this web-site http://bcs.whfreeman.com/qca7e/content/cat_070/Harris %207e.Experiments%2017May06.pdf to obtain the second lab (page 3) entitled “Gravimetric Determination of Calcium as Calcium Oxalate Hydrate” Note : YOU ONLY need to make 100 mL of the ammonium oxalate reagent, not 1 Liter You will be supplied with SOLID unknowns from Thorn Smith. You should dissolve ~2.5-3 g of unknown in 250 mL of 0.50 M HCl. Calculate the average % Ca in your sample. Students should take care to make sure that they calculate the % Ca in the original solid sample! Calculate the standard deviation and 95% confidence level for this concentration. Lab 2: AA method; Atomic Absorption Analysis of Calcium. Read Chapter 20 and 21 in the Harris text for the background information. Most inorganic chemical analysis of calcium is done via flame atomic absorption analysis. Supplies needed : Lanthanum Oxide Calcium carbonate, primary standard grade 1 L volumetric flask 5 mL volumetric pipettes Other volumetric pipettes (0.5, 1, 2, 3, 10 mL) 0.50 M HCl (at least 2 L) Concentrated HCl 10 100 mL volumetric flasks Procedure :

1.) Prepare 100 mL of a 5% w/v Lanthanum Oxide solution (in the hood). Weigh out the La 2 O 3 , add ~25 mL of water. Then, CAREFULLY AND SLOWLY, add ~25 mL of concentrated HCL (this will be an exothermic reaction, so allow the flask to cool). After cool, dilute to the mark. 2.) From primary standard grade calcium carbonate, prepare a stock solution that is 500 mg/1L of calcium. Weigh out the calcium carbonate, add it to the flask, quantitatively by rinsing the weigh boat with water. Dilute to the mark with 0.5 M HCl. Swirl to dissolve. Save this solution for the Spectrophotometric lab. 3.) Using 100 mL flasks, prepare a minimum of 5 dilutions of the stock Calcium solution to cover the range of 1-20 mg/L of calcium. Add 5 mL of the lanthanum oxide solution to each flask before diluting to the mark with 0.50 M HCl. Make a blank. 4.) Prepare the unknown solution #1. Weigh out 0.100 g of solid unknown, quantitatively transfer to a 100 mL flask, add 5 mL of lanthanum solution and dilute to the mark with 0.50 M HCl. Dilute this solution 3 times to assure that you obtain a concentration that is on the calibration curve (you may end up diluting more times, depending on the actual concentration of calcium in your unknown…be patient). Dilution #2 should be 5 mL of solution #1 diluted to 100 mL (with 5 mL of lanthanum and the balance of the solution 0.5 M HCl). Dilution #3 should be 5 mL of Dilution #2 diluted to 100 mL (with 5 mL of lanthanum and the balance of the solution 0. M HCl) etc. When all standard and unknown flasks are prepared, take the flasks to the instrumental laboratory and the instructor will show you how to use the AA instrument. The wavelength for Calcium analysis is 422.7 nm and the slit width should be set to 0.7 nm. Take a notebook to record the data (in triplicate). Clean up all flasks. You can save the unknown solution if you have any left for other calcium unknown experiments. Save and left over 0.5 M HCl. Put the lanthanum waste into a waste beaker. After the laboratory, construct a calibration curve for the calcium standards. Fit ONLY the linear region of this curve to a straight line (i.e. if you reject any points as not linear, you must address this in your laboratory report). Calculate the concentration of calcium in the unknown from the graph (using which ever of the unknown dilutions is within the linear range of the curve). Back calculate to the % calcium in the sample. Students should take care to make sure that they calculate the % Ca in the original solid sample! Calculate the standard deviation and 95% confidence level for this concentration. In your report, be sure to answer the following question: WHY is the lanthanum used? Lab 3: EDTA Determination of Calcium Read Chapter 12 for background information. EDTA titration of calcium and magnesium is the standard volumetric method for the determination of water hardness. Supplies needed: Sodium EDTA 500 mL volumetric flask 250 mL volumetric flask pH paper Erlenmeyer flasks, stir bars Hot plate stirrer Hyroxynapthol blue

assure that you are near the middle of the calibration curve.

  1. Collect a blank spectrum using the reagent blank.
  2. Collect the absorbance spectrum for each of the standards and unknowns. Mark the absorbance at 650 nm using the instrument software (instructor will show you). Print the spectra when complete. Plot absorbance versus calcium concentration. Perform a linear regression and use the equation of the line to calculate the % Ca in the unknown sample. Students should take care to make sure that they calculate the % Ca in the original solid sample! Calculate the standard deviation and 95% confidence level for this concentration.

Summary Questions for all 4 sections:

Construct a table of method name and amount of calcium carbonate found in each trial. Use ANOVA to determine if there is significant difference in the precision BETWEEN the 4 methods in the analysis. Your table should list each method at the top and under each method the %CaCO 3 found. You must draw a conclusion about which method seems to be most precise. Why? You must also draw a conclusion about which method should be most accurate. Why? Follow the procedure for a formal laboratory report write-up. You have 1 week from the completion of the last experiment to turn in the report.