Determination of pH and Precipitation Reactions

Chemical analysis is divided into two broad categories of qualitative and quantitative analysis. Quantitative analysis involves the determination of the amount of a known substance in a chemical sample, while qualitative analysis is the determination of the identity of an unknown chemical in a given sample. Both categories of analysis are vital since they inform Chemists on the composition and identity of vital chemical materials. In this experiment, a qualitative analysis is used to determine the identity of five unknown samples. The three sets of provided samples have salts, acids, bases, and water. By determining the pH of the solutions in the assigned set, we can identify the set by correlating the number of acidic, neutral, or basic outcomes with the number of salts and water, and bases in the set. This is possible since salts and water are expected to give a neutral pH and the acids and bases are expected to show acidity and basicity respectively. Thereafter, precipitation reactions are deployed to determine the identity of each solution. The reactions follow the solubility rules where chlorides and sulfates of specific salts are expected to either be soluble or insoluble, with all nitrates being soluble.   

Qualitative analysis has several applications such as the medical field. Medical laboratory analysts carry out qualitative analysis in cases of poisoning to determine the ingested chemical before a diagnosis and prescription are made. 

Materials 

Unknown solutions A, B, C, D, and E 

Pipettes labeled A, B, C, D, and E 

A spot plate 

Litmus paper 

  pH Determinations 

Label spots A-E on the spot plate and cut five pieces of litmus paper, placing each in the labeled spots. 

Using the labeled pipettes, add 2-3 drops of the unknown solutions to the litmus paper, each labeled solution added to its corresponding marked spot with a corresponding marked pipette. 

Note and record color changes 

Reactions 

Label six spots on the spot plate as B+A, D+A, E+A, B+C, D+C, and E+C. 

With the use of the labeled pipettes, transfer 5-6 drops of the unknown solutions into the spots as labeled, allowing each pair to react. 

Observe and record the observation.   

pH Determinations 

Reactions 

pH analysis indicates the assigned set has just one basic solution with two acidic and two neutral solutions. This implies the assigned group is Set B which has two acids (HCl and H 2 SO 4 ), two neutral solutions (AgNO 3 and Ba(NO 3 ) 2 ), and one base (NaOH). 

From the pH analysis, the salts are A and C, the base is B, and the acids are D and E. Since we only have one base, the identity of B is NaOH. NaOH reacts with A to give a brown precipitate. The only ions that would form a different color of a precipitate, besides the white, are Silver ions. This is due to the formation of brown AgO solid. Therefore A, which is also neutral, is AgNO 3 . 

AgNO 3 gives a white precipitate of AgCl when it reacts with HCl. The reaction of AgNO 3 with D gives white precipitate, thus D is HCl. 

Since A is identifies as AgNO 3 , this implies the other neutral salt (C) is Ba(NO 3 ) 2 . To confirm this identity, Barium ions are expected to form white precipitate (BaSO 4 ) on reaction with H 2 SO 4 but remain clear and colorless on reaction with HCl. The reaction of HCl with C to give a clear colorless solution and E with C gives white precipitate confirms C is BaSO 4 and E is H 2 SO 4 acid.

Conclusion 

These results mean that use of pH and solubility of salts can be deployed to carry out a qualitative analysis of common elements. The identity of the solutions from the results was expected, thus a successful determination. Therefore, the purpose of the experiment was achieved.