Quantitative Analysis

What is Quantitative Analysis?

The quantitative determination of a substance by precipitation, followed by isolation and weighing of the precipitate is called gravimetric analysis. Quantitative analysis is used to determine the percentage of a particular element or ion in a sample.

In a typical gravimetric analysis the percentage of an ion of interest in a solid compound is determined. This procedure involves dissolving a substance of unknown composition in water and allowing the ion of interest to react with a counter ion to form a precipitate. The precipitate is then isolated, dried and weighed.

Knowing the mass and the chemical formula of the precipitate the mass of the ion of interest can be determined from the percent mass composition of the precipitate. From the masses of the ion of interest and the original compound, the percentage of the ion of interest in the original compound can be calculated.

Quantitative Analysis of Organic Compounds

The estimation of percentage composition of the various elements present in a compound is the next step in determining the formula of the compound. The different methods employed for the estimation of various elements are described below.

1. Estimation of Carbon and hydrogen

These two elements are always estimated together by Liebig’s combustion method. A weighted amount of the compound is heated strongly with excess copper oxide in an atmosphere of air or oxygen. The constituents hydrogen and carbon are thus oxidised to water and carbon dioxide, which are collected separately and weighed. The percentage of carbon and hydrogen in the compound can be calculated as given below.

% C = 12/44 x Mass of CO2 formed/ Mass of the substance x 100

% H = 1/18 x Mass of H2 formed / Mass of the substance x 100

2. Estimation of nitrogen

Dumas Method

This method can be applied to all organic compounds containing nitrogen. The method is based on the principle that if an organic compound containing nitrogen is heated with copper oxide, free nitrogen and oxides of nitrogen are formed along with other products (carbon dioxide, water vapour etc). The oxides of nitrogen are reduced to free nitrogen on passing over heated copper and the whole of nitrogen is collected over KOH solution. The volume of nitrogen collected is measured and from this the percentage present in the compound is calculated.

% N = 28/22400 x Volume of nitrogen at NTP/ Mass of compound x 100

Kjeldahl’s Method

This method is commonly employed for estimating nitrogen in food materials and fertilizers. Though this method is simpler than Dumas method, it is not applicable to all nitrogenous organic compounds.

3. Estimation of Halogen

Carius Method

A known mass of the organic substance containing halogen is heated with fuming nitric acid along with few crystals of silver nitrate in a sealed tube. The silver halide is formed which o=is separated, washed dried and weighed. From the mass of silver halide obtained the percentage of halogen is calculated.

% Halogen = Atomic mass of halogen x Mass of silver halide obtained x 100 / Molar mass of silver halide x Mass of compound taken.

Carius method does not give satisfactory results with iodine as silver iodide is slightly soluble in nitric acid and some iodine is also produced even in the presence of excess silver nitrate. Moreover, for highly halogenated aromatic compounds, the results are not accurate.

4. Estimation of Sulphur

Sulphur is also estimated by cartius method. In this case, the organic compound is heated only with nitric acid. Sulphur present in the compound is thus oxidized to sulphuric acid which is treated with barium chloride to precipitate barium sulphate. The precipitate of barium sulphate is washed, dried and weighed. From the weighted mass of barium sulphate obtained, the percentage of sulphur is calculated.

% S = 32/133 x Mass of BaSO4/ Mass of the compound x 100

5. Estimation of Phosphorus

Phosphorus is estimated like sulphur, an organic compound is heated with fuming nitric acid. Phosphorus in the compound is thus oxidized to phosphoric acid which is precipitated by adding magnesia mixture. The precipitate of magnesium ammonium phosphate is ignited to obtain magnesium pyrophosphate.

2MgNH4PO4 (Magnesium ammonium phosphate) → Mg2P2O7 + 2NH3 + H2O

Magnesium pyrophosphate is weighed and the percentage of phosphorus calculation is

%P = 62/222 x Mass of Mg2P2O7/Mass of the compound x 100

Determination of Molecular Formula of the Compound

The molecular formula of a compound expresses the actual number of atoms of various elements present in a molecule of the compound.

Molecular formula may be either the same as the empirical formula or a simple multiple of it.

Molecular formula = (Empirical formula) x n

Where n is a whole number. The value of n is obtained by dividing the molecular mass by empirical formula mass.

n = Molecular mass/ Empirical formula mass.

Solved Example

Question:

In a Dumns experiment, 0.1877g of an organic substance yielded 31.7cc of nitrogen gas at 14oC and 758mm Hg pressure. Calculate the percentage of the nitrogen in the compound. (Aqueous tension at 14oC = 12mm)

Solution:

Volume of N2 at STP = V1 + (P-p) 273/(t+273) x 760

Substituting the values V1

= 31.7 x (758 – 12) x 273/(14+ 273) x 760 = 29.6mL

Mass of 29.6mL of nitrogen

= 28/22400 = 29.6g

% of nitrogen = 28/22400 x 29.6 x 100/0.1877 = 19.72

Frequently Asked Questions on Quantitative Analysis

What is qualitative analysis in chemistry?

Qualitative research is a component of chemistry that deals with recognizing the elements or ingredients that make up a compound or mixture. The determination of ingredients of a certain salt solution is an example of qualitative research. Description of yourDictionary and example of usage.

What is quantitative measurement in chemistry?

Quantitative means calculating a quantity-setting it to a value. For instance, you might calculate the rate of a reaction by seeing how many seconds it takes for a change to occur, like a piece of magnesium ribbon to dissolve in acids of different concentrations. Qualitative means, without a meaning being calculated.

What are the methods of quantitative analysis?

Quantitative approaches emphasize objective measurements and statistical, analytical, or numerical analysis of data obtained by interviews, questionnaires, and surveys or by using computational techniques to manipulate pre-existing statistical data.

What are the advantages of quantitative research?

At the end, guided, unbiased research and analysis supports or rejects the conclusions. When collecting and analyzing data, each move is systematic to reduce the bias. A major advantage of this method is that tests for a wider population are true, accurate, and generalizable.

What is an example of quantitative?

Quantitative is an adjective that actually means something tangible. For example, we can count the number of sheep on a farm, or we can calculate the gallons of a cow’s milk. It’s important to be able to quantify what we can in a world of abstract results that can’t be quantified, like rage or memories.

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