In this video, you will learn the mechanisms behind the determination of calcium and magnesium content in milk sample by back titration and how to perform the calculations accordingly in Session 15.
The actual experiment can be roughly divided into three parts, including the standardization of EDTA, back titration until the first endpoint, and back titration until the second endpoint.
Please note the data shown in this video are just examples.
The first step is to determine how much calcium chloride standard solution is equivalent to 1 mL of EDTA solution through standardization.
To the 5 mL of EDTA solution, add appropriate amount of potassium hydroxide to increase the pH between the range of 12 to 12.5
then add a small amount of calcein indicator. The color of the solution should turn pink at this point
To the pink solution, add the calcium chloride solution.
In this process, additional calcium ions bind to EDTA until all of them are bound.
Keep adding the calcium chloride solution slowly, and a small amount of excess calcium ions will bind to the calcein indicator,
which causes the color to turn into a permanent green. Stop here and record the volume of calcium chloride used
To perform the calculation, first you will need to determine the exact molarity of Calcium chloride given the weight of CaCl2 solids, volume of diluent, and purity of the compound.
Please note, if you’re using CaCl2*2H2O also know as calcium chloride dihydrate instead, make sure to account for the additional molecular weight.
Then you can determine the molarity of EDTA used.
Recall that the volume of EDTA used is 5 mL and the titrated volume of calcium chloride solution is 4.25 mL, while its exact molarity was calculated to be 0.033 M. Thus the molarity of EDTA is 0.028 M
Now you just need to calculate how much calcium chloride solution is equivalent to 1 mL of EDTA,
and the result is 0.85 mL
Don’t forget to use the exact molarity of calcium chloride in this step as well.
The next step is to determine the amount of calcium and magnesium in your milk sample using the data obtained from the back titration until the first endpoint
After you add EDTA solution and the calcein indicator to the milk sample, the sample should turn pink.
At this point, all calcium and magnesium ions are bound to EDTA, with some excess EDTA and free calcein indicator in the solution.
When you back titrate the sample with calcium chloride solution, the additional calcium ions from the titrant bind to excess EDTA in the sample until all of them are bound.
Because the affinity of EDTA to calcium is higher than that to magnesium
if you keep adding calcium chloride slowly to the sample, the additional calcium ions will start to compete with magnesium in the sample for EDTA.
This leads to a very small amount of magnesium dissociating from EDTA and binding to the calcein indicator, which causes the color of the sample to turn green.
A permanent green colour indicates that the first endpoint is reached,
which means all excess EDTA is bound to additional calcium ions, and you should stop the back titration here.
Record the volume of calcium chloride used.
This volume of calcium chloride corresponds to the excess EDTA in the sample.
Although some additional calcium ions do replace magnesium in the binding with EDTA,
the amount is very small so you do not need to take that into account when performing the calculation.
To start the calculation, you first need to know how much EDTA is bound to calcium and magnesium in the sample.
To do this, first calculate the amount of excess EDTA in the sample solution by dividing the volume of calcium chloride solution used by the amount of calcium chloride solution equivalent to 1 mL of EDTA solution.
Then subtract this value from the total amount of EDTA used.
Now you can use the following equation to calculate the total amount of calcium and magnesium in your sample.
The unit is milliequivalents per litre.
The next step is to determine how much magnesium is in the sample.
By adding an additional amount of potassium hydroxide to increase the pH
that small amount of magnesium bound to calcein indicator in the last step is now dissociated from the indicator and precipitated as magnesium hydroxide
Again because the amount is very small, you do not need to take that into account in the calculation.
Now since all calcein indicator is free, the solution will again turn pink
Perform the back titration with calcium chloride.
In this process, all magnesium ions bound to EDTA are replaced by additional calcium ions, and magnesium hydroxide precipitates are formed as a result.
A small amount of excess calcium ions bind to the calcein indicator and cause the solution to turn into a permanent green.
This is the second endpoint. You should stop here and record the volume of calcium chloride used.
Like in the last step, you need to calculate how much EDTA is bound to magnesium.
This is calculated using the additional amount of calcium chloride added in this step divided by the equivalent value of calcium chloride to 1 mL of EDTA.
Then use this equation to calculate the amount of magnesium. The unit is also milliequivalents per litre.
To calculate the amount of calcium in the sample, you can simply subtract the amount of magnesium from the total content of calcium and magnesium in your sample.
Now you can convert the magnesium and calcium content units to milligrams per 100 g of sample.
Please note, the molarity of magnesium and calcium is equal to their equivalents per litre divided by 2.
Follow the rest of the steps to complete the calculation.
Here we also assume the density of the sample is 1 g/mL,
but if you have the actual density of your sample, remember to take that into account instead.
You can follow the same steps to calculate the calcium content.
Thank you for watching.