3 Week Chloride Lab

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 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.

Lab2: 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 La2O3, 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 0.5 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

pH meters

Procedure:

1.) Prepare 500 mL of 0.05M Na-EDTA solution using primary standard grade sodium-EDTA

2.) Weigh 0.5 g of unknown and quantitatively transfer to an Erlenmeyer flask. Add ~50 mL of water to the flask. If needed, warm to dissolve.

3.) Quantitatively transfer the solution from the Erlenmeyer to a 250 mL volumetric flask and dilute to the mark.

2.) Perform as follows a preliminary titration to find the approximate volume of EDTA solution needed to react with the calcium. To 50 mL of the sample solution add 50% NaOH dropwise to reach a pH of 12. (some solid may precipitate, its ok at this point).

3.) Heat with mixing to 60-70o. Add ~0.5 g hydroxy naphthol powder indicator and titrate with EDTA to a clear blue. Titrate rapidly without trying to get an exact end point.Record the amount of EDTA used.

4.) To another aliquot of the sample solution add ~0.5 g hydroxy naphthol powder indicator, and accurately add (carefully measure) ~90% of the volume of EDTA used in the preliminary titration (step 3).

5.) Heat with mixing to 60-70o, add 50% NaOH to a pH of 12via pH paper, mix, and continue the titration to a clear blue end point. Repeat with two additional portions of sample.

Lab 4:Spectrophotometric determination of Calcium

Read Chapter 16 for background information. This spectrophotometric method for the determination of calcium in a sample is used in clinical labs for the determination of calcium in blood serum. Arsenazo III binds with calcium in an alkaline medium to form a purple colored complex which absorbs maximally at 650 nm. The intensity of the color is proportional to the calcium concentration. We will be using the diode array UV-VIS spectrometers in the Instrumental laboratory. The instructor will show you how to use them.

Supplies needed:

Arsenazo III (0.15 mM solution, purchased)

Buffer, prepare a pH 6 sodium phosphate or MES buffer, 0.25 M, ~100 mL
500 mg/L calcium standard from Atomic Absorption lab.

Initial Unknown solution from EDTA lab (step 3) or prepare a new unknown containing ~0.1 g of unknown dissolved 100 mL,of 0.05 M HCl.

Procedure:

1.) Prepare, in 100 mL volumetric flasks, standards for a calcium calibration curve. The 5 calcium standards should cover the range of 1-100 mg/L. In each flask mix the required amount of calcium, 10 mL of buffer solution and 10 mL of arsenazo reagent. (Instructor should make the highest concentration sample here first and test his/her solution on the UV-VIS to check experimental scale…amount of calcium and/or arsenazo reagent can be adjusted to be on the scale of the instrument.)

2.) Prepare a reagent blank containing the arsenazo reagent, buffer and water, but no calcium.

3.) Prepare 2 dilutions of your unknown. Make the first sample containing 5 mL of unknown, 10 mL of buffer and 10 mL of arsenazo reagent in 100 mL. Visually compare the color of this solution to your calibration curve samples. Add more or less unknown to a second sample to assure that you are near the middle of the calibration curve.

4. Collect a blank spectrum using the reagent blank.

5. 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 %CaCO3 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.