Water Hardness by EDTA Titration
Determination of Calcium
chem 128 September 20, 2002
Web document edited: September 17, 2001
To determine the Calcium content of a sample such as tap water, milk.
Titrate the sample with the chelating agent known as EDTA.
- use an indicator to determine the endpoint.
- EDTA is shorthand for a long name,
- ethylenediamine tetraacetic acid.
- EDTA acta as a powerful ligand, capable of forming six coordinate covalent bonds to most metal cations, including calcium.
Remember how ammonia, NH3, acts as a ligand because it can offer a lone pair of electrons for bond formation.
Ethylenediamine can be though of as two ammonia
molecules bound together with a two atom carbon change. Each nitrogen offers a pair of electrons so each is capable of forming a coordinate covalent bond with metal ions.
Now remove each of the hydrogens and replace with a bound acetate ion, CH3CO2-. One of the
oxygens on the acetate is capable of forming another bond to the metal ion. (Actually there are two oxygen atoms with four electron pairs, but only one can be oriented towards the metal ion. Note that the acetate ion loses one H to form the C-N bond.))
Since there are four such acetate ions, we now have six bonds to the metal ion. The EDTA literally wraps around the metal ion and binds it quite strongly.
We find the end point by supplying a second chelating agent as the indicator.
This one is strongly colored and the color changes when it binds to metal ions.
It's important that the indicator binds metal ions weaker than EDTA does
During the titration the indicator retains a metal ion and stays the same color. After the endpoint, the next drop or two of EDTA will steal the Ca from the indicator, causing it to change colors.
- Several minor points
- .The "A" in EDTA is "acid." In practice we use the compound as the sodium salt (it is more soluble.) We use it in basic solutions where the acid itself won't exist. We always are working with the acetate salt, an ion of charge -4.
- The term "chelate" or "chelation" is intended to conjure up images of grabbing and holding, much like the claws of a lobster. (A lobster's claw is officially called a Chela")
- EDTA and closely related compounds are used in many products. When added to tub and shower cleaners, they trap and remove calcium deposits that otherwise become the residue known as soap scum. Small amounts are added to foods like kidney beans to prevent discoloration, mainly by iron atoms.
- Chelation therapy has become a pseudo medical fad-- the Internet is overrun with promoters for EDTA. Consuming a chelating agent is offered as a way remove harmful metals such as lead from your body, assuming it reaches your bloodstream intact. In fact, intravenous chelation therapy is an important tool in treating extreme cases of poisoning by lead, mercury and cadmium. Unfortunately, EDTA doesn't discriminate and it can deprive your body of critically needed metal ions from iron (hemoglobin) to cobalt (needed for vitamin B12 to work.)
- 1. Clean and assemble your buret and mount it in a buret clamp.
- 2. Collect 200 ml of EDTA reagent in a clean DRY* beaker or flask; take it to your work area.
- Be sure to record the exact concentration; each bottle may have a different concentration
- *If your container is wet, first rinse it with three small sample of the reagent.
- 3. Fill your buret with EDTA (Sodium salt)
- First, rinse your buret with 3 small rinses (10 ml) of the titrant.
- Then fill above the calibrated region and drain until the liquid level can be measured.
- It is not necessary that the level be exactly at 0.00 ml.
- 4. Carefully measure the sample into a clean 250 ml Erlenmeyer flask.
- The appropriate amount varies with the sample you use.
- It's ok. if your flask is wet; rinse with distilled water
- 100 ml of Bowling Green Tap water or other water sample
- __ ml of milk (see label on bottle) or __ mg of powdered milk (+50 ml water)
- 5. Add 20 ml of the pH 10 buffer
- This is a mixture of NH3 and NH4Cl to keep the pH near 10.
- 6. Add 10 drops of the indicator
- 7. Titrate until the indicator changes color from red / violet to pure blue.
- record the volume to the nearest 0.01 ml
- if you are unsure of the endpoint, record the volume and then add another drop of two of titrant to be sure.
- 8. Repeat twice (for a total of three titrations on the same sample)
- If possible, repeat on a related sample (such as treated tap water)
- 9. Also, titrate a sample of distilled water (50 ml) with buffer and indicator to serve as a blank.
- the blank will only take a few drops of titrant
- titrate to the same color change you saw earlier
subtract the blank from each titration volume
multiply by the exact conc. of EDTA
this is the number of moles of EDTA used
the stoichiometry is 1:1 Ca: EDTA
so the last result equals the number of moles of Ca2+ in the original sample
the concentration of Ca2+ is calculated moles divided by your sample volume
- we also usually report mg/liter
- multiply moles per liter by 40.0 (atomic wt of Ca)