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To be able to prepare standard solutions. To determine the strength of a given solution of Hydrochloric acid (HCI) To analyze errors that occur during standardization experiments. Introduction: A standard solution can be defined as a solution that contains a precisely known concentration of a substance. Concentration refers to the abundance of a substance in its mixture or solution. Standard solutions are used to determine the concentration of other substances with which they react in known ratios/ proportions.
Standardization, therefore, refers the process in which the value of a potential standard is fixed by a measurement made with respect to a standard whose value is known; or simply the act of accurately determining he concentration of a substance by titrating it with a solution of accurately known concentration (standard solution). Standardization, therefore, basically involves titration, especially acid-base titration.
Common reagents used for standardization of acids include: Pure sodium carbonate Pure borax (sodium metabolite) Pure calcium carbonate (Iceland spar) Common reagents used for standardization of alkalis comprise of solid crystalline acids like: Succinct acid Oxalic acid Good standardizing reagents should have the following characteristics: They should have a high degree of purity They should be stable and unaffected by the atmosphere, i. E they should not be efflorescent or deliquescent, for easy weighing.
They should have a relative molecular mass such that a solution of about 0. 01 M to 0. MM can be prepared easily. A standard solution of Noah cannot be made by direct weighing. This is because Noah is hygroscopic and will therefore absorb moisture from the air. Therefore, a standard solution of some stable acid (e. G Oxalic acid) has to be prepared to standardize the given Sodium Hydroxide solution. This can then be used to determine the unknown concentration of the Hydrochloric acid solution.
In the experiment, two titration were therefore performed: Titration A: Standardization of Sodium Hydroxide solution using standard solution of Oxalic acid. Titration B: Determination of the concentration of Hydrochloric acid solution using the standardized Sodium Hydroxide solution. The terms used in titration are therefore used here too. Therefore, in titration A, the analyze was the Sodium Hydroxide solution; and the iterant was the Oxalic acid. In titration B, the analyze was the Hydrochloric acid solution; while the iterant was the Sodium hydroxide solution.
Precaution: Oxalic acid is very poisonous. Apparatus: Electronic balance Volumetric flask Pipette Burette Conical flask Funnel Reagents: Oxalic acid (Ethnocentric acid) Sodium hydroxide solution Hydrochloric acid solution (of unknown strength) Phenolphthalein indicator Water Procedure: Titration A: Standardization of Noah Solution A standard solution of Oxalic was made by measuring 0. Egg of Oxalic acid and adding it to distilled water in a volumetric flask. The mixture is shaken till the crystals dissolve and the solution is made up to mall. Ml of this standard solution is pipettes into a conical flask and two to three drops of the indicator are deed to it. The contents of the flask are then titrated against the Noah solution from the burette till a permanent light pink color is obtained. The titration were repeated till concordant results were obtained and then recorded. Titration B: Determination of the Concentration of the given HCI solution ml of the given Hydrochloric acid was pipettes into a conical flask. Two to three drops of the indicator were then added.
These errors include: Personal errors One example of this is whereby the solution taking part in the reaction, say the Oxalic acid, is not homogeneous. This happens when the student does not shake the crystals well after adding water when preparing the solution. This error is minimized by thorough shaking of the crystals with water when preparing the elution so as to ensure that it is homogeneous. Another personal error is Parallax error. This error occurs when the line of sight of the analyst/student when taking readings is not at right angles to the scale.
It may have occurred in three instances in the experiment: 1. When filling the Oxalic acid in the volumetric flask to the mall mark. 2. When petting the Oxalic acid during titration A and the Hydrochloric acid during titration B. 3. When making the readings of the Sodium hydroxide solution from the burette before or after each titration. Parallax error is indeterminate/uncertain It is minimized by proper positioning of the eyes by the student when making the readings or measurements. Instrumental error This error is also indeterminate. It is caused by imperfect calibration of the measurement instruments, e. The burette, pipette and volumetric flask. This is because these instruments are manufactured by mass production methods, which pose high chance for certain inaccuracies. This error is minimized by use of instruments of higher precision. Averaging of the titers obtained also minimizes the error. Contamination/limpidity of reagents This error may have risen due to the use of unclean apparatus, like the limerick flask, pipette, conical flasks and the burette. It arose in the experiment especially due to improper cleaning of the conical flask after a titration and before the next.
It is minimized by thorough cleaning of the experimental apparatus before use. Contamination also arises from presence of impurities in the various reagents provided. Oxalic acid usually contains impurities of calcium and potassium oxalate’s which may have interfered with its reaction with sodium hydroxide. The oxalic acid also may have contained some amount of moisture before weighing leading to an error in mass. Such errors are minimized by storing the substance in a desiccators for a few hours before use so as to remove the moisture.
Contamination errors may also have arisen from action of atmospheric carbon (IV) oxide, oxygen and dust particles on the standard solutions. This is minimized by use of colored glass bottles, tightly fitting stoppers and soda lime tubes to absorb carbon (V) oxide. Indicator error The pink color of the indicator used (phenolphthalein) which was obtained after each titration faded away when the solution was vigorously shaken till it became colorless. This gave the impression that the color change witnessed was not permanent, hence prompting us to continue with the titration.
This led to obtaining of a larger volume than necessary. The fading away is due to the reaction of the phenolphthalein with atmospheric carbon dioxide. The indicator, being a weak acid, may have taken part in the reaction, hence affecting the readings. There also may be an inaccuracy in the end-point recorded. This is caused by adding of too much indicator to the solution being titrated. This is minimized by adding very little but sufficient indicator, and also having flask intonating water for comparison so that any alteration in color can be easily observed.
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