The preparation of standard solutions is a fundamental technique in analytical chemistry, essential for accurately determining the concentration of solutions and conducting various chemical analyses. This experiment aims to familiarize students with the techniques involved in preparing standard base solutions and understanding the concept of dilution and its calculation.
In titration, a common analytical technique, an acid in a burette reacts with a base in a conical flask of known concentration to form water and a salt. This neutralization reaction involves the combination of H+ ions and OH- ions to generate water, resulting in the formation of a standard solution.
Standard solutions can be prepared using various methods, including the weighed method, where the mass of the solute is calculated and weighed before dissolving it in a solvent to achieve a specific concentration.
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Alternatively, standard solutions can be prepared by the dilution method, which involves mixing a concentrated solution with additional solvent to achieve the desired concentration.
The dilution process entails mixing a concentrated solution with additional solvent to increase the final volume while maintaining a specific concentration. This experiment explores the relationship between the initial and final molarity of a solution before and after dilution, as described by the dilution equation, M1V1 = M2V2.
Materials
Analytical balance: An analytical balance is a precise weighing instrument used to measure the mass of solid chemicals. Unlike conventional scales, analytical balances can measure mass to the nearest milligram or even microgram, ensuring accurate measurements for preparing standard solutions.
Beaker 100 mL: Beakers are cylindrical containers with a flat bottom used for holding and mixing liquids.
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The 100 mL beaker provides ample volume for dissolving solid chemicals and preparing solutions. Its wide mouth facilitates easy pouring and transfer of liquids.
Burette 50 mL: A burette is a long, graduated glass tube with a stopcock at the bottom, used for precisely dispensing measured volumes of liquid, especially during titration experiments. The 50 mL burette allows for the accurate delivery of hydrochloric acid during the standardization of solutions.
Conical flask 250 mL: Also known as an Erlenmeyer flask, a conical flask is a flat-bottomed, conical-shaped glass container used for holding liquids. The 250 mL capacity of the conical flask accommodates larger volumes of solutions, making it suitable for mixing and titration experiments.
Dropper: A dropper, also known as a Pasteur pipette, is a thin glass tube with a rubber bulb at one end used for transferring small volumes of liquids. In this experiment, a dropper may be employed for adding indicator solutions or small quantities of reagents with precision.
Funnel: A funnel is a conical-shaped utensil with a narrow stem, designed to channel liquids or fine-grained substances into containers with small openings. It facilitates the transfer of liquids into volumetric flasks and prevents spillage or waste.
Glass rod: A glass rod is a cylindrical piece of glassware used for stirring liquids or mixing solutions. Its smooth surface ensures uniform mixing without introducing contaminants or interfering with chemical reactions.
Pipette 25 mL: A pipette is a calibrated glass tube used to measure and transfer precise volumes of liquid. The 25 mL pipette enables the accurate dispensing of sodium hydroxide solution during dilution and titration procedures.
Volumetric flask 100 mL: A volumetric flask is a glass container with a precise volume mark etched on its neck, allowing the preparation of solutions with a specific concentration. The 100 mL volumetric flask is used to dilute solutions to a desired volume, ensuring accuracy in concentration measurements.
Distilled water: Distilled water is water that has been purified through the process of distillation, removing impurities and minerals. It is used as a solvent in preparing solutions to prevent contamination and ensure the accuracy of chemical reactions.
Hydrochloric acid (HCl) 0.1M: Hydrochloric acid is a strong acid commonly used in chemistry laboratories. The 0.1M concentration of hydrochloric acid indicates its molarity, or the number of moles of HCl dissolved in one liter of solution. It is used as a titrant in the standardization of sodium hydroxide solutions.
Sodium hydroxide (NaOH): Sodium hydroxide, also known as caustic soda, is a strong base used in various chemical processes. It is employed in this experiment to prepare standard solutions and undergo titration with hydrochloric acid to determine its concentration.
Thymol blue indicator: Thymol blue is a pH indicator that changes color in response to changes in acidity or alkalinity of a solution. In this experiment, thymol blue is used as an indicator during the titration process to signal the endpoint of the reaction between sodium hydroxide and hydrochloric acid.
Experimental Procedures
Preparation of Standard Solutions
Weighing Solid Sodium Hydroxide:
Begin by accurately weighing between 2.00 to 2.50 grams of solid sodium hydroxide using an analytical balance. Ensure that the weighing process takes place in a dry 100 mL beaker. Record the precise weight obtained.
Dissolving Sodium Hydroxide:
Add 30 mL of distilled water to the beaker containing the solid sodium hydroxide. Stir the mixture thoroughly using a glass rod until the sodium hydroxide completely dissolves in the water. This step ensures the formation of a homogeneous solution.
Transfer to Volumetric Flask:
Using a cone-shaped funnel and a glass rod, carefully transfer the sodium hydroxide solution from the beaker to a clean 100 mL volumetric flask. Rinse the beaker and the glass rod with additional distilled water to ensure that all the solution is transferred without loss.
Adjusting Volume:
Gradually add distilled water to the volumetric flask containing the sodium hydroxide solution until the liquid level reaches the calibration mark on the flask. Ensure that the meniscus of the solution aligns precisely with the mark. Once the desired volume is achieved, stopper the flask securely and gently shake it to ensure thorough mixing.
Dilution
Pipetting Sodium Hydroxide Solution:
Utilizing a 25 mL pipette, transfer precisely measured 25 mL of the prepared sodium hydroxide solution from the volumetric flask to a clean 100 mL volumetric flask. This step facilitates dilution of the sodium hydroxide solution to a desired concentration.
Adjusting Volume:
Add distilled water to the flask containing the pipetted sodium hydroxide solution until it reaches the calibration mark on the flask. Securely stopper the flask and gently swirl it to ensure uniform mixing of the solution.
Standardizing the Solutions
Preparing Conical Flask:
Pipette 25 mL of the diluted sodium hydroxide solution into a clean and dry 250 mL conical flask. This step ensures that a precise volume of the sodium hydroxide solution is available for titration.
Adding Thymol Blue Indicator:
Introduce 3 or 4 drops of thymol blue indicator solution into the conical flask containing the sodium hydroxide solution. Thymol blue serves as an indicator to detect the endpoint of the titration by changing color in response to the pH change.
Filling Burette:
Fill the burette with standardized 0.1M hydrochloric acid solution up to the zero mark. This solution serves as the titrant for neutralizing the sodium hydroxide solution in the conical flask.
Titration Process:
Begin the titration process by slowly adding hydrochloric acid from the burette into the sodium hydroxide solution in the conical flask while swirling the flask gently. Titrate until the first noticeable color change occurs, typically from blue to yellow, indicating the endpoint of the reaction.
Results
Preparation of Standard Solutions
Weight:
Weight of beaker + solid NaOH: 58.2268 g
Weight of beaker: 55.7779 g
Weight of solid NaOH: 2.4489 g
Calculate the concentration of the prepared sodium hydroxide solution:
Concentration = 2.4489 g / 0.10 L = 24.489 g/L
Calculate the concentration of the prepared sodium hydroxide in molar:
Average volume of hydrochloric acid used: 40.40 mL
Calculate the molarity of the titrated sodium hydroxide:
Mb = (0.1 M × 40.40 mL) / 25.00 mL = 0.1616 M
Calculate the molarity of the original sodium hydroxide:
M1 = (0.1616 M × 100 mL) / 25 mL = 0.6464 M
Discussion
The experimental results indicate that the concentration of the prepared sodium hydroxide solution is 0.6123 mol/L, while the titrated sodium hydroxide has a concentration of 0.1616 M, and the original sodium hydroxide used has a concentration of 0.6464 M. The percentage difference between the concentration of the prepared and original sodium hydroxide solutions is 5.57%. This discrepancy may be attributed to errors in measuring the weight of the solid NaOH and potential errors during the titration process, such as air currents affecting the analytical balance and human errors.
Standardizing the hydrochloric acid solution ensures its accuracy and known concentration (0.1 M), which is essential for titrating the sodium hydroxide solution of unknown concentration. The titration process involves the neutralization reaction between HCl and NaOH, with the thymol blue indicator changing color from pale blue to pale yellow at the endpoint of the reaction.
To improve the accuracy of the experiment, it is crucial to follow the correct procedures outlined in the laboratory manual and use a conical flask instead of a beaker during titration to prevent chemical spillage. Proper titration techniques, such as swirling the conical flask with the non-dominant hand while adding the titrant with the dominant hand, should be employed.
Precautions should be taken before the experiment, such as ensuring no air bubbles are present in the burette nozzle and maintaining eye-level with the meniscus during volume readings. These precautions minimize errors and contribute to obtaining accurate results.
Conclusion
In conclusion, the experiment demonstrates the preparation of standard solutions using the weighed and dilution methods and the standardization of solutions through titration. The calculated concentrations of the prepared, titrated, and original sodium hydroxide solutions provide valuable insights into the principles of solution preparation, dilution, and titration. Despite the inherent errors in experimental procedures, adherence to proper laboratory techniques and precautions enhances the accuracy and reliability of the results.
References
Standard Solution: Definition & Method
Preparation and Standardization of a 0.1 M NaOH Solutions
Zeynep Eslek and Aysen Tulpar. Solution Preparation and Conductivity Measurements: An Experiment for Introductory Chemistry. Journal of Chemical Education 2013, 90(12), 1665-1667. DOI: 10.1021/ed300593t
Updated: Sep 26, 2024
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Preparation of Standard Solutions. (2024, Feb 27). Retrieved from https://studymoose.com/document/preparation-of-standard-solutions
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