EXP. NO. 6 Acid Base Titration How To Prepare a Solution from Solid Compound? Example: Prepare 100 mL solution of 0.1 N Borax (Na2B4O7•10H2O)? First calculate the molecular weight of borax which is [(22.99 × 2) + (10.81 × 4) + (15.99 × 7)] = [(45.96) + (43.24) + (111.93)]. [10 (2+16)] = 381.4 g/mol Equivalent weight = molecular weight / valence  Note: For compounds like HCl, NaCl, Kcl, etc.. The valence=1  For compounds like H2SO4, CaCl2, MgSO4, Na2B4O7 ... the valence =2  For AlCl3, Na3PO4, etc.... Valence =3  So, in other words the equivalent weight will be equal to M.wt only if the compound is monovalent, but with di, trivalent compounds,... where the valence is larger than 1,and the equivalent weight will be the division of M.wt by a No. higher than one (ex. 2, 3,...).

So Equivalent weight borax = 381.4 / 2 =  190.7 g/mol Wt. (g) = 0.1 N × 190.7 (g/mol) × 100 mL / 1000 = 1.907 g Weigh out 1.907 g Borax on sensitive balance. Place the Borax in a 100 mL beaker. Add a small volume of distilled, to dissolve the borax (if it is not dissolved stir it with stirrer). Then pour the solution in to a volumetric flask and fill the flask by D.W to the 100 mL line.

How To Prepare a Solution from Liquid Compound Example: Prepare 100 mL solution of 0.1 N HCl (hydrochloric acid)? An approximate 0.1 N HCl solution in any given volume, can be prepared from the information labeled on the concentrated bottle (sp.gr, or density,(w/w)% content, Mol. Wt.).

Acid-Base Titration Titration is an analytical technique where a solution of known concentration is used to determine the concentration of an unknown solution. Typically, the titrant (the know solution) is added from a burette to a known quantity of the analyte (the unknown solution) until the reaction is complete. burette is a laboratory instrument used to add measured volumes of solutions to other containers.

Setup for a Typical Titration:   Setup for a Typical Titration:  In a typical titration, the titrant in the burette is added to the solution in the Conical flask ( titrand ) until the indicator changes color to show that the reaction is complete.

Quantitative analysis is a branch of chemistry that deals with the determination of the amount or percentage of one or more constituents of a sample. (Quantitative analysis is any method used for determining the amount of a chemical in a sample). For example, how much product can be made from a known starting material or how much of a given component is present in a sample. Standardization is the process of determining the exact concentration (molarity) of a solution, by titrating it against exact known volume of another standard solution. Titration is one type of analytical procedure often used in standardization. Standard Solution: A solution of known concentration (molarity) is called a standard solution. Its concentration is determined by a process known as standardization In titration, the completion of reaction is usually shown by a change of color caused by a substance called an indicator. An indicator is added to the solution being titrated. The indicator is a substance that changes color when the reaction is complete; it is often added to the analyte solution to produce an observable physical change near the endpoint. In our example, phenolphthalein, which is a commonly used acid-base indicator, is added to the hydrochloric acid solution in the Conical flask

The above equation showing that an acid reacts with a base to yield water plus an ionic compound called a salt.  This acid-base reaction is called a neutralization reaction. Phenolphthalein has two chemical forms. In acidic conditions, it is in the acid form, which is colorless. In basic conditions, an H+ ion is removed from each phenolphthalein molecule, converting it to its base form, which is red.

H+ (aq) + OH-(aq) → H2O (l) In this example, the NaOH solution which has known concentration is slowly added from the burette until the mixture in the Conical flask changes from colorless to red. The OH- ions in the NaOH solution react with the H+ ions in the HCl solution, which has an unknown concentration. H+ (aq) + OH-(aq) → H2O (l)   As long as there are excess H+ ions in the solution, the solution stays acidic, the phenolphthalein stays mostly in the acid form, and the solution is colorless. When enough NaOH solution is added to react with all of the H+ ions, the reaction is complete. When a small amount of extra NaOH solution is added, perhaps one drop, there will be an excess of hydroxide ions, OH, in solution. These excess react with the phenolphthalein molecules, changing them from the acid form to the base form. Because the base form is red, the solution turns red, assigning for that the reaction is complete (or just slightly beyond complete).

Equivalent point cannot be detected experimentally; we can only determine the endpoint, which is a physical change that we see change in the color near the endpoint (E.P).

Reagents used as standards are divided into: Standardization Background Standardization is the process of determining the exact concentration (molarity) of a solution, by titrating it against exact known volume of another standard solution. Standard Solution: A solution of known concentration (molarity) is called a standard solution.  The accuracy of standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid–base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant’s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant’s concentration is known.  Reagents used as standards are divided into:

1--Primary reagent (primary standard) is a reagent that is: Extremely pure (%99.9 must be of known chemical structure). Highly stable in air and not affected by environment Cheap and easily obtained. Absence of hydrate water, so that the composition of the solid does not change. It’s better to be in high molar mass (molecular weight, equivalent weight) This can be used to prepare a standard containing an accurately known amount of analyte. Example: Sodium Carbonate (Na2CO3), Borax   2-Secondary reagent (secondary standard): is a standard that is prepare in the laboratory for a specific analysis, it is usually standardized against a primary standard to determine its purity.

Borax is chosen for preparation of the standard solution, because it satisfies the requirements of primary standard substance which is, high purity, not hygroscopic, stable, cheap, and it has a high molecular weight. The reaction of borax with hydrochloric acid satisfies the condition of the reaction, which used in volumetric analysis). Na2B4O7 +2HCl +5H2O  H3BO3 +2NaCl So the equivalent weight (Eq.wt) of Borax =Mol.Wt /2, because each mole of borax consumed (2mole) of HCl. Boric acid (H3BO3) is the product of the reaction which is the weak acid so the medium at (end point) of the titration is slightly acidic, so the indicators {Methyl red (4.2 - 6.3), Methyl orange (3.1- 4.4) can be used and also some other pH indicators, which change their colors around the range of pH (4-10) can be used. But the indicators which have ranges (pH > 5 ) are not successful in the presence CO2 (from air or from NaOH soln. used ).

Procedure First standardization 1- Weigh out accurately 1.907 gm of borax on a sensitive balance, transfer it quantitatively after dissolving it in distilled water to a 100ml volumetric flask and complete the volume to the mark. 2- Pipette 10ml 0f the prepared solution and transfer it to a conical flask, add 3 drops of the methyl red indicator (yellow color will appear). 3- Titrate the solution in the conical flask with HCl solution in the burette until the color changes from yellow to red. Repeat the titration twice, and calculate the (N) of HCl solution.

Post Lab Questions 1- What are acid base indicators? How do they work? 2- Which one is more accurate, end point or equivalent point? 3- Why concentrated HCl is not considered as primary standard material?