Chem 111H/113 Laboratory Experiment 14	Get Chemscape Chime to view this molecule Chime Commands: [Press and hold left mouse while roaming mouse to rotate] [Press and hold SHIFT and left mouse button to zoom] [Press and right mouse button for more Instructions and Options]       Carbon Hydrogen Nitrogen Oxygen Calcium II	Complexometric Titration:  Determination of Water  Hardness or Calcium in a Calcium Supplement
EDTA4- molecule		Ca(EDTA)2- complex Notice the two coordinate bonds formed by the lone pairs of the nitrogens and the four coordinate bonds formed by the deprotonated oxygens of the carboxylic acid groups.

All experiments this semester: Weigh and record to  ± 0.0001 g.

Cross out the section marked "Preparation of 0.01M Disodium EDTA Solution".  It has already been prepared.

Cross out the section marked "Preparation of Ammonia/Ammonium Chloride Buffer Solution". It has already been prepared.

Obtain three 10 ml or 25 ml Erlenmeyer flasks, three magnetic stir bars, one 100 ml volumetric flask, one 2 ml microburet, one 1 ml volumetric pipette, one 2 ml graduated pipette, and one pipette pump from the sideboard.

Obtain 20 to 25 ml EDTA solution, 5 ml NH₃/NH₄Cl solution, an unknown, 1 to 2 ml 6M HCl. There will be one dropper bottle of Eriochrome Black T (EBT) per group of four students on your bench.  Use just one drop of EBT, you may also wish to add some distilled water (depends on the size of the Erlenmeyer flask used) to make the color change easier to observe.

Calculate the standard deviation and the %RSD for all three runs.

Report the average and standard deviation properly and the %RSD in your conclusion.  Also include a statement as to the reasonableness of your result (accuracy and precision).

All liquid wastes may be poured down the sink.  Save yourself some time and collect wastes in a large beaker and then dump the beaker when you are finished with all runs.

Heads-up

The microburet will be used to dispense the EDTA solution only.

Your graduated microburet will read to three decimal places.  The first two decimal places are determined by the markings, the third you estimate.  Make sure you record all three decimal places in your notebook.  If it is a zero, and you know it to in fact be zero, it is a significant figure right of the decimal point.  You must record and report all significant figures.

Use a volumetric pipette to dispense the CaCl₂ solution.

Use a graduated pipette to dispense the unknown solution.

Preparation of 0.01 M calcium chloride solution:

Quantitative transfers involve washing the weight boat, paper, or funnel and the powder funnel used to transfer the dry reagent with solvent into the volumetric flask.  Alternatively you may reweigh the weigh boat, paper, or funnel and subtract the amount of reagent not transferred.

Do not completely fill the volumetric until after all the CaCO₃ is dissolved.  Use as little water as necessary to transfer the CaCO₃ and perform the quantitative transfer. Use a wash bottle to fill to the beginning of the neck and a Pasteur pipette to add the remaining solvent needed to bring the lower edge of the meniscus to the mark, or etched line on the volumetric flask.

Perform a "run 4 - keep 3 titration".

Add the EDTA in .5 mL increments for the first run. Determine between which two .5 mL additions the endpoint was reached.  If more than 2 mL of titrant are required reduce the amount of CaCl₂ or "hard water".  Accurately record the amount transferred.

For the 3 keeper runs, add EDTA up to the .5 mL just before the endpoint, allow the color change to equilibrate, then add dropwise to the accurate volume.

        This is actually stated elsewhere, but is good to think about; the purpose of a standardization is to account for any contamination that may alter the value of a quantitatively transferred chemical, such as EDTA.  It is very expensive (cost in time and money) to acquire nanopure water that would not contain multivalent cations that would complex with the EDTA.  It is much less time consuming to prepare the EDTA, allow it to complex with any magnesium II or calcium II and then use a primary standard to determine the amount of uncomplexed EDTA that will be useful in the experiment.  Once a titrant such as EDTA is standardized the fact that the solution was prepared with an impurity becomes moot and will have no effect on the outcome of the titration.  Please see pages 211-213 of your laboratory text for a suitable discussion of titration and the properties of a primary standard.

Techniques that will increase the efficient use of the lab time .
This is important.  It will be very important in labs you will take in later semesters as you
receive less and less supervision due to your expected increase in skills and knowledge.

• If the experiment calls for 1 ml instead of 1.000 ml estimate the volume using marks on beakers or a single full squeeze of the bulb on a pasteur pipette instead of graduated cylinders.  Beaker marks are very effective for volumes of 5 ml or larger.  Graduated cylinders are very good for obtaining and storing the bulk reagents at your bench in a total amount you would use during the entire experiment.
• Perform other tasks while waiting for a task to complete that does not require your constant attention.
• Keep a large beaker under your hood to collect liquid waste and glassware washes generated during the experiment in.  Then you can make one trip to the waste hood.