Question

Calibration Method 1: Regular Calibration

Starting with solid CaCO₃, describe how you will prepare Ca²⁺ standards in 0.1 M nitric acid in the concentration range 5-60 ppm (MW CaCO₃ = 100.08 and MW Ca = 40.08).

Remember you do not want to weigh out very small amounts because small fluctuations in the balance can create large errors. You should not weigh out an amount of CaCO₃ less than 0.1 g. Therefore, you may need to prepare a stock solution with a higher concentration than the desired range (such as 150 ppm) and then dilute the stock solution to make standards in the appropriate range.

Include the mass of the CaCO₃, the steps for preparing the solution, and the types of pipets and volumetric glassware you will use.

Answer 1

The concentration of nitric acid = 0.1 M

The reaction will be

CaCO3 + HNO3 --> Ca(NO3)2 + CO2 + H2O

Parts per million concentration required = 150 ppm = 150 mg / L = 0.0015 moles / L

As per stoichiometry, moles of CaCO3 reacted = moles of HNO3 used

so for 0.0015 moles of CaCO3 we have to calculate the mass

Mass = Moles X molecular weight = 0.0015 X 100 = 1.5 grams

the concentration of HNO3 = 0.1 M

so for 0.0015 moles the volume required will be

Volume = Moles / Molarity = 0.0015 / 0.1 = 0.015 L = 15mL

Now we have all the calculations so, Now let us see how to prepare the solution

(i) We will take 15mL of nitric acid (with the help of pipette) and will pour it in 1L of volumetric flask.

Now we will add 1.5 g of CaCO3 in the volumetric flask, will start adding water in it.

We will make up the solution to the mark of volumetric flask.

so the concentration of Ca+2 will be 150 ppm

(ii) now we can dilute the solution

a) 60ppm (M2) ,V2 = 100mL

M1 = 150 ppm V1 = ?

150 X V1 = 60 X 100

V1 = 40mL

so we will take 40mL of solution and will dilute it to 100mL in a measuring flask.

b) 30ppm (M2) ,V2 = 100mL

M1 = 60 ppm V1 = ?

60 X V1 = 30 X 100

V1 = 50mL

so we will take 50mL of solution and will dilute it to 100mL in a measuring flask

c) 15ppm (M2) ,V2 = 100mL

M1 = 30 ppm V1 = ?

30 X V1 = 15 X 100

V1 = 50mL

so we will take 50mL of solution and will dilute it to 100mL in a measuring flask