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Qualitative Analysis

Analytical chemistry is one of the most important branches of chemistry that mainly deals with the analysis of unknown sample with the help of different techniques such as volumetric analysis (titration), gravimetric analysis, chromatography etc. Gravimetric analysis is used to determine the quantity of unknown compounds formed during any chemical reaction. With the help of this technique we can determine the yield of any chemical reaction.

On the contrary, qualitative analysis involves the detection of elements that are found in an organic compound. Qualitative analysis can only detect the presence of elements in the compound but cannot determine the percentage or amount of them in the compound. For example; carbon and hydrogen are detected with the help of Liebig’s method. Some other common elements which can be determined are oxygen, halogen, sulphur, nitrogen, halogens, sulphur and phosphorus.

The elemental analysis further helps in the quantitative analysis of them with the help of other techniques. Once we know the elemental composition of any organic compound, we can predict the chemical and physical properties of it with the help of all possible molecular structures. Let’s discuss some common methods to detect the presence of various elements in any organic compound.

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Qualitative Analysis Definition

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It’s a type of chemical analysis which is used to separate and detect cation and anions in a sample substance. The chemical properties of an unknown substance are determined in the qualitative analysis procedure by the reaction between the unknown substance and suitable reagents. The qualitative analysis is used for the identification of a simple salt or the identification of various cations present in a solution.

Qualitative Chemical Analysis

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The chemical analysis involves the study of chemical composition of the substance. Two types of chemical analysis are:
  1. Qualitative Analysis
  2. Quantitative Analysis
Qualitative analysis is used for determination of those compounds that are present in unknown sample. In the quantitative analysis, the amount of each element is determined by weight. Various techniques are used in both the analysis.

Qualitative Data Analysis of Cation

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Cations are divided into six groups. A specific reagent for each group is used for their separation from the solution. This separation is done in a particular sequence because some ions of prior groups can also react with the reagent of later groups. 

This can be cause of difficulty in their separation. The qualitative analysis of cations depends on the solubility products of the ions. The cations get precipitated on the optimum needed concentration and easily detected. 

1st group of cations


The group (I) cations form the insoluble chlorides. The dilute hydrochloric acid (generally 1-2 M concentration) is used as the group reagent. The Concentrated HCl is not used because it forms a soluble complex ion ([PbCl4]2-) with Pb2+ and would not be detected. The most important cations of 1st group are Ag+, Hg22+, and Pb2+. Color detection test is not suitable for their chlorides because all are white solid compounds. The analyzing of group is done by adding the respective salt into water and follow it with dilute HCl. This results in a white precipitate in which NH4OH is added. The insoluble precipitate shows the presence of Plumbous (Pb2+) and if precipitate is soluble, then it shows the presence of Argentous (Ag+). If the white precipitate turns black, it shows the presence of Hg22+

The obtained filtrate after first group analysis is used for the test of second group. Usually Pb2+ is also included in the 2nd analytical group because the solubility of PbCl2 is more than the chlorides of other two ions.

Confirmation Test


For lead

Pb2+ + 2KI → PbI2 + 2 K+
Pb2+ + K2CrO4 → PbCrO4 + 2K+

For Silver

Ag+ + KI → AgI + K+
2Ag+ + K2CrO4 → Ag2CrO4 + 2K+

For mercury ion

Hg22++ 2KI → Hg2I2 + 2K+
2Hg22+ + 2NaOH → 2Hg2O + 2Na+ + H2O

2nd analytical group of cations


The ions of this group form acid-insoluble sulfides. Cations of the 2nd group are Cd2+, Bi3+, Cu2+, As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ and Hg2+. The group reagent is H2S (at 0.2 - 0.3 M) which gives S2− ions in solutions. The tests are conducted in presence of dilute HCl to keep the S2- ion concentration at a minimum level. This round of tests helps only in second 2nd group cations precipitation. The cations precipitation of the 4th group if present in the solution might occur in the absence of dilute HCl which can give wrong results. The use of sulfuric acid and nitric acid can cause to the precipitation of the 4th group cations and formation of colloidal sulfur respectively. This group is determined by the addition of the respective salt in water and then adding dilute HCl and then follow it by adding H2S. The precipitate color and the presence of the respective ions are shown in the table below. 

Precipitate color
Presence of cation
Reddish brown or black precipitate Bi3+, Cu2+, Hg2+ or Pb2+
Yellow precipitate Cd2+ or Sn4
Brown precipitate Sn2+
Red orange
Sb3+

  • If the black or reddish brown precipitate comes, then it is boiled in diluted HNO3. If the precipitate is insoluble in diluted HNO3 then Hg2+ is present.
  • If it is soluble, then Cu2+, Bi3+ or Pb2+ can be present- I this solution sulfuric acid is then added. A white precipitate shows that Pb2+ may be present;
  • If no any precipitate forms, then an excess of ammonium hydroxide in the original salt solution is added. A blue color indicates the presence of Cu2+ while a white precipitate indicates bismuth.
  • For the yellow precipitate, an excess of NaOH is added to form a white precipitate. If the white precipitate dissolves, then Sn4+ is present; if it’s not then Cd2+ is present.

Confirmation Test


For Lead

Pb2+ + 2 KI → PbI2 + 2K+
Pb2+ + K2CrO4 → PbCrO4 + 2K+

For Copper

2Cu2+ + K4[Fe(CN)6] + CH3COOH → Cu2[Fe(CN)6] + 4K+
Cu2+ + 2NaOH → Cu(OH)2 + 2Na+
Cu(OH)2 → CuO + H2O (endothermic)

For Bismuth

Bi3+ + 3KI (in excess) → BiI3 + 3K+
BiI3 + KI → K[BiI4]
Bi3+ + H2O (in excess) → BiO++ 2H+

Confirmation test for Mercury

Hg2+ + 2KI (excess) → HgI2 + 2K+ HgI2 + 2KI → K2[HgI4] (red precipitate)
2Hg2+ + SnCl2 → 2Hg + SnCl4 (gray white precipitate)

3rd analytical group of cations


The ions of this group form insoluble hydroxides. Cations in the 3rd group are Fe2+, Fe3+, Al3+, and Cr3+. The group detection is carried out by preparing a respective salt solution in water and then adding NH4Cl and NH4OH. If a reddish brown precipitate forms then Fe3+ is present, a gelatinous white precipitate shows Al3+, and a green precipitate is for Cr3+ or Fe2+. The Cr3+ or Fe2+ is identified by adding excess of sodium hydroxide to the green precipitate. The soluble precipitate indicates Fe2+, otherwise Cr3+ is present.

4th analytical group of cations


This includes Zn2+, Ni2+, Co2+, and Mn2+. For determination of this group, the solution of the salt is mixed with NH4Cl, NH4OH, and H2S gas. A green color of the original solution is for Co2+

The color and the presence of the other respective ions are shown in the table below.

Precipitate color
Presence of cation
Colored precipitate Mn2+
White precipitate
Zn2+
Black precipitate
             Ni2+

5th analytical group of cations


Ions of 5th group of cations forms the carbonates which are insoluble in water. The (NH4)2CO3 (Approx 0.2 M concentration) is group reagent with a neutral or slightly basic pH. The ions of the 5th group are Ba2+, Ca2+, and Sr2+. The most common way to detect them is flame test. The color of flame for particular ion is shown in table. 

Ions
Color of flame
Barium Yellow-green flame
Calcium
orange red
Strontium
deep red

In the flame test, the solutions of ions mixed with concentrated HCl and heated on a nickel/chromium wire in a flame. Due to the presence of particular atom, the flame gives a characteristic color. 

6th analytical group of cations


The ions of 6th group are Mg2+, Li+, Na+, K+ and NH4+. The NH4+ ion after reacting with Nessler's reagent gives a brown colored precipitate. The rest of the ions are identified by flame color which is shown in table.

Ions Color of flame
lithium
Red flame
Sodium
bright yellow
Potassium
Violet
Magnesium
Bright white

Qualitative Data Analysis

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Qualitative Data Analysis (QDA) is the series of processes and procedures which based on an interpretative philosophy.
In QDA, all the meaningful and symbolic content of collected qualitative data are investigated.

Qualitative Data Analysis Methods

Interpretive Techniques

This is the simplest analysis of qualitative data. This is based on an observer's impression. In this technique, expert or observers examine the data and interpret all data by forming an impression. After interpreting the data, data impressions are reported in a structured and sometimes in quantitative form.

Coding

It is an interpretive technique. This is used to organize the data and provide means to introduce the interpretations of it into certain quantitative methods. Generally,  coding requires the analyst to read the data and convert it into special segments. Each segment is coded with a special code which can be a word or short phrase. These codes give the relationship of data segments with the research objectives.

After completing the coding, the analyst prepares reports by mixing of summarizing the prevalence of codes; evaluating similarities and differences in related codes across distinct original sources or contexts, comparing the relationship between codes.

Some of the highly structured qualitative data like open-end responses from surveys and interview questions, etc., are coded without additional segmenting of the content. The quantitative analysis of these codes is the capstone analytical step for highly structured qualitative data.

Contemporary qualitative data analyses

Computer programs like Computer Assisted Qualitative Data Analysis Software are used in this data analysis method. These programs increase the efficiency of analyst in data storage or retrieval process and for applying the codes. The limit of coding method is that it seeks to transform qualitative data into quasi-quantitative data which reduces the variety, richness, and individual character of data.

Recursive abstraction

In this method, qualitative data sets are analyzed without coding. The sets of data are summarized again and again two or three times. Thus the end result is a more compact summary. The main disadvantage of this method is that many times the final conclusions are removed from the underlying data. Thus the poor initial summaries give the inaccurate final report.

Mechanical techniques

The mechanical techniques are based on counting words, phrases, or content analysis. The outputs are used in statistical analyses. These techniques are especially used for too large data sets to effectively analyze or when analysis of large databases would cost prohibitive relative to the value of information or if the chief value of a data set is the extent to which it contains "red flags or green flags".

Noticing, Collecting and Thinking model of data analysis

This model is given by Seidel (1998). This is useful model for explaining the basic process of qualitative data analysis. The noticing, collecting, and thinking about interesting things are three parts of this model which are interlinked and cyclical to each other.

For example, when we think about things, we notice all the related things and collect them. Thus for analyzing the data, first notice interesting things in the data and the codes assigned to them according to topic or theme. This breaks the data into fragments. These Codes of data act as sorting and collection devices.

data analysis model

Quantitative Chemical Analysis

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This branch of chemistry is especially for the determination of the amount or percentage of constituents of a sample. The methods used for quantitative analyses are classified as chemical or physical. The classification depends upon the utilized properties.

Chemical methods depend on chemical reactions like precipitation, neutralization, oxidation etc. The chemical methods are known as gravimetric analysis and volumetric analysis etc. The physical property like density, absorption of light, electromotive force, magnetic properties, etc., are measured in physical methods. A quantitative analysis generally requires a combination of methods like combination of qualitative and quantitative for separating particular constituents from a sample and for measuring the amounts present in a sample respectively.
The analytical balance is the basic tool for all quantitative analyses. This balance is used for the accurate weighing of samples and precipitates. Generally, in analytical work the balance should be able to measure the differences in mass of 0.1 milligram (about 0.000004 ounce). The balance should be 1,000 times more sensitive in microanalysis. The titration is the bases of volumetric analysis in a neutralization reaction, precipitation or redox reaction or complex formation reactions etc. For example, the chemical reaction between an acid and a base giving salt and water is the neutralization reaction.

In the precipitation reactions, the standard solution of silver nitrate is used as a reagent which reacts with the ions present in the sample and forms an insoluble precipitate. Precipitation methods are known as argentometry. The titration with formation of Complex is a reaction between metal ions and a standard solution.
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