Analytical chemistry

Chemistry deals with a number of chemical compounds, some of which are highly useful for man. Thus it is necessary to know not only their uses but also the process of development, identification, etc. In the process of identification of a chemical compound, chemical analysis needs to be done. The relevant subject that deals with systematic study in identification of chemical substance is called analytical chemistry.

Analysis can be of two types-

1. Quantitative analysis
2. Qualitative analysis

In quantitative analysis, the compound with respect to its quantity in a substasnce is analyzed. There are basically two methods of doing this-

• Volumetric analysis
• Gravimetric analysis

Volumetric Analysis:
This method is used to find the concentration (in terms of molarity) of an unknown substance called analyte by studying its reaction with another whose concentration and volume is previously known.

You are given a sample of $\ce{HCl}$ of a certain volume. By the method of volumetric analysis, find its concentration.

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First we will choose a substance which readily reacts with $\ce{HCl}$; for this example let's choose $\ce{NaOH}$. We assume that we are familiar with the concentration and volume of $\ce{NaOH}$ beforehand. The steps of volumetric analysis are given below:

$(1)$ Take the given $\ce{HCl}$ solution in a clean beaker and measure its volume.

$(2)$ Add an indicator to the given $\ce{HCl}$ in the beaker; this indicator will display a colour change during the reaction of $\ce{HCl}$ with $\ce{NaOH}$ to mark its completion. Litmus solution can be used in our case.

$(3)$ Now place the $\ce{NaOH}$ solution in a buret and measure the initial volume reading of the solution in the buret carefully.

$(4)$ Slowly open the stopcock and allow the solution of $\ce{NaOH}$ to fall into the beaker filled with $\ce{HCl}$.

$(5)$ Check the indicator regularly for completion of reaction.

$(6)$ As the end of reaction approaches, add the $\ce{NaOH}$ solution to the beaker very carefully and slowly, i.e. drop by drop, for more accuracy.

$(7)$ As the reaction completes, close the stopcock and measure the final reading of the volume of solution of $\ce{NaOH}$ in the buret. Calculate the volume of solution required to complete the reaction.

Now we can easily figure out the concentration of $\ce{HCl}$ solution.

We know the reaction can be represented as

$$\ce{HCl + NaOH \rightarrow NaCl + H_2O}.$$

Observe that

$$\begin{aligned} \text{Molarity (M)} &=\frac{\text{No. of moles of substance }(n)}{\text{Volume }(V) \text{ of substance (in litres)}}\\ \\ M_{\ce{NaOH}} &= \frac{n_{\ce{NaOH}}}{V_{\text{Final}}-V_{\text{Initial}}}\\ \\ n_{\text{NaOH}} &= M_{\ce{NaOH}}(V_{\text{Final}}-V_{\text{Initial}}). \end{aligned}$$

From the reaction we know that one mole of $\ce{NaOH}$ reacts with one mole of $\ce{HCl}$. So $n$ moles of $\ce{NaOH}$ will react with $n$ moles of $\ce{HCl}$. We already know the volume of $\ce{HCl}$ solution, so

$$M_{\ce{HCl}}= \frac{n_{\ce{HCl}}}{V_{\ce{HCl}}}.\ _\square$$

Gravimetric Analysis :