Qualitative Chemical Analysis

The rate of formation of a percentage of the various elements present is a combination of the next step in determining the formula of the Compound. The various methods used to measure the various elements are described below.

1. Measurement of Carbon and Hydrogen

These two elements are always measured together in Liebig's flammable form. The weight of the compound is heated by excess copper oxide in the atmosphere or oxygen. Therefore, the Hydrogen and Carbon components are dissolved in water and Carbon dioxide, which are collected separately and measured. The percentage of Carbon and Hydrogen in the compound can be calculated as given below.

% C =\[\frac{12}{44} \times \frac{\text{Mass of }CO_2 \text{ formation}}{\text{Mass of object}} \times 100\]

% H =\[\frac{1}{18} \times \frac{\text{Mass of }H_2O \text{ formation}}{\text{Mass of object}} \times 100\]

2. Nitrogen Estimation

The way of Dumas

This method can be applied to all organic compounds that contain Nitrogen. The method is based on the principle that when an organic compound containing Nitrogen is burned with copper oxide, free Nitrogen and Nitrogen oxides are formed along with other products (carbon dioxide, water vapour etc.). Nitrogen oxides are reduced to free Nitrogen when they pass over burnt copper and all Nitrogen is collected over KOH solution. The amount of Nitrogen collected is estimated and from this, the percentage present in the compound is calculated.

% N =\[\frac{28}{22400} \times \frac{ \text{Nitrogen volume in NTP}}{ \text{Bulk Compound}} \times 100\]

The way of Kjeldahl

This method is often used to measure nitrogen in nutrients and fertilizers. Although this method is simpler than the Dumas method, it does not apply to all nitrogenous organic compounds.

3. Halogen Estimation

The way of Carius

The known weight of an organism containing Halogen is heated by flammable nitric acid and a few crystals of silver nitrate in a closed tube. The silver halide is made up of o = separated, washed, dried and weighed. From the weight of the silver halide obtained the percentage of Halogen is calculated.

% Halogen = \[\frac{\text{Atomic weight of Halogen} \times \text{ Bulk halide of silver obtained}}{ \text{Molar mass of silver halide} \times \text{Weight of composite mixture}} \times 100 \].

The Carius method does not provide satisfactory results with iodine as silver iodide dissolves slightly in nitric acid and some iodine is also produced even when there is excess silver nitrate. Moreover, in Halogenated aromatics compounds, the results are inaccurate.

4. Sulfur Estimation

Sulfur is measured in terms of caution. In this case, the organic compound heats up only with nitric acid. The Sulfur present in the compound is released by oxide to Sulfuric acid-treated with barium chloride to absorb barium sulfate. The precipitate of barium sulfate is washed, dried and weighed. From the heavyweight of the barium sulfate obtained, the percentage of Sulfur is calculated.

5. Phosphorus Estimation

Phosphorus is classified as sulfur, a naturally occurring component of combustible nitric acid. Phosphorus in the compound is therefore oxidized into a Phosphoric acid formed by adding a mixture of magnesia. The precipitate of magnesium ammonium phosphate burns to form magnesium pyrophosphate.

\[2MgNH_4PO_4 \text{(Magnesium ammonium phosphate)} \rightarrow Mg_2P_2O_7 + 2NH_3 + H_2O \]

Magnesium pyrophosphate is measured along with the percentage of Phosphorus count

% P = \[\frac{62}{222} \times \frac{\text{Weight of }Mg_2P_2O_7}{\text{Weight of combination}} \times 100 \]

More About the Topic

Qualitative and quantitative analysis is analytical techniques in Chemistry that are used for giving details about the components in an unknown sample.

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Qualitative analysis in Chemistry gives details of the presence or nonappearance of different chemical components in an unknown sample, while quantitative analysis gives the measure of various chemical components present in a given sample.

Most often, both the techniques are used together, i.e., the use of qualitative analysis followed by quantitative analysis.

On this page, we will focus on qualitative chemical analysis, qualitative inorganic analysis, and understand what qualitative analysis is in chemistry in detail.

Qualitative Analysis Definition

Now, let us understand what qualitative chemistry is. Qualitative analysis is also called subjective investigation. In the field of Chemistry, it is a part of science that examines the substance piece or chemical composition of a sample. 

It demonstrates various elements, or gatherings of elements like functional groups, and so forth present in the sample. Thus, a subjective investigation of any sample can be utilized to decide if a specific component is available in a sample or not. 

Notwithstanding, this examination doesn't give any data about the amount of that chemical component. 

The characteristics in the sample that are frequently thought to be in the investigation are colour, smell, melting point, limit, reactivity, precipitation, and so forth.

Qualitative Analysis Chemistry

The seven methods to perform the qualitative analysis of a chemical compound lie hereunder:

1. Change in Colour

2. Flame test

3. Distillation

4. Extraction

5. Precipitation

6. Chromatography

7. Spectroscopy

What is Qualitative Analysis in Chemistry?

Qualitative analysis in Chemistry is if two types:

1. Qualitative organic analysis

2. Qualitative inorganic analysis

1. Qualitative Organic Analysis

Qualitative organic analysis determines the chemical bonds and functional groups in a sample.

2. Qualitative Inorganic Analysis

Qualitative inorganic analysis frequently determines the ions in a given sample.

What did we understand so far?

So, in a nutshell, qualitative analysis chemistry uses techniques, like distillation, extraction, and change in colour, chromatography, etc, to determine the composition of a sample. 

In other words, these techniques are helpful in determining the presence of different chemical components in a sample.

Now, let’s focus on examples of qualitative analysis in chemistry:

Examples of Qualitative Analysis in Chemistry

Here, we will consider an of example of qualitative organic analysis:

1. Iodine Test

The iodine test is utilized to show the presence or absence of starch. It is sugar, which is a natural compound. There, liquid iodine is utilized as the marker. 

A spot test can be utilized to test in the following manner:

Take a white tile and put a few drops of the samples that will be examined. At that point add iodine solution for each drop of the test. 

On the off chance that the colour of the sample is changed to brown colour, which shows the presence of starch.

Now, let us consider an example of qualitative inorganic analysis:

2. Flame Test 

In the flame test, the presence of a specific metal or its ions can be ascertained. Diverse metallic atoms give various tones to the fire. As per the flame colour when a bit of the sample is scorched in the Bunsen burner, can decide the presence of some metal ions. 

Ex: Zinc gives a green fire colour, as we can see in the image below:

Zine gives a green flame colour.

Qualitative Analysis

There are a few points to remember on qualitative analysis; let’s understand these:

1. Recognition of chemical species by means of colour, reaction producing a colour, reaction producing a precipitate, a reaction involving a change of a parameter.

2. Coloured ions are Cu2+ Blue, Cr3+ Green, CrO4- Yellow, Cr2O7- Orange, MnO4- Violet, Ni2+ Green, Co2+ Pinkor Blue, Mn2+ Pink, and generally ions of transition elements.

3. Precipitates are slightly soluble compounds: Sulfur of heavy metals, like (As, Sb, Hg, Cu, Pb, Cd, Sn, Bi, Zn, Ni, Co, Mn), BaSO4, Hg2Cl2, AgCl, PbCl2, and many hydroxides of heavy metals.

Steps Involved in Qualitative Analysis of a Sample

On the off chance that the sample is introduced as a solid (salt), it's critical to take note of the form and colour of any crystal. 

Reagents are utilized to isolate cations into groups of related components. 

Ions in a group are isolated from one another. After every division stage, a test is performed to affirm certain ions really were eliminated. The test isn't performed on the original sample.

Divisions depend on various qualities of ions. These may include redox reactions to change oxidation state, differential solvency in acid, base, or water, or precipitating certain particles.

Protocol for Qualitative Analysis of a Sample

To start with, ions are eliminated in groups from the underlying aqueous solution. After each group has been isolated, at that point testing is led for the individual ions in each group. Here is a typical group of cations: 

1. Group -  I:

 Ag+, Hg22+, Pb2+ 

Precipitated in 1 M HCl 

2. Group - II:

Bi3+, Cd2+, Cu2+, Hg2+, (Pb2+), Sb3+, Sb5+, Sn2+ and Sn4+ 

Precipitated in 0.1 M H2S arrangement at pH - 0.5 

3. Group - III: 

Al3+, Co2+, (Cd2+), Cr3+, Fe2+, and Fe3+, Mn2+, Ni2+, Zn2+ 

Precipitated in 0.1 M H2S arrangement at pH  - 9 

4. Group -  IV: 

Ca2+, Ba2+, K+, Mg2+, Na+, NH4+ 

Ba2+, Ca2+, and Mg2+ are precipitated in 0.2 M (NH4)2CO3 arrangement at pH - 10; different ions are solvent.

Point To Note:

Numerous reagents are utilized in the subjective investigation, however, a couple is engaged with virtually every gathering system. The four most usually utilized reagents are 6M HCl, 6M HNO3, 6M NaOH, 6M NH3. 

Understanding the employment of the reagents is useful when arranging an analysis. The common qualitative reagents are:

1. 6M HCl

2. 6M NaOH

3. 6M NH3