Standardization Of NaOH And Preparation Of Standard Solution

Standardization Of Naoh and Preparation of Standard Solutions

This experiment involves the preparation of a standard oxalic acid solution and a sodium hydroxide solution, which will be used in subsequent acid-base titrations. The key objectives are to standardize the NaOH solution using a primary standard acid (potassium hydrogen phthalate, KHP) and to prepare accurate standard solutions. Precise preparation and careful titration techniques are essential to ensure the reliability of the results, as they directly influence the accuracy of subsequent unknown acid analyses.

The standardization process requires the preparation of a known concentration of oxalic acid dihydrate, calculation of the molarity of the sodium hydroxide solution, and titration procedures involving KHP. The overall goal is to produce reliable, high-precision standard solutions by meticulous weighing, solution preparation, and titration practices. These standardized solutions will help determine the molarity of subsequent unknown acids with confidence, emphasizing the importance of precise measurements and adherence to best laboratory practices throughout the experiment.

Paper For Above instruction

The process of standardizing NaOH solutions and preparing standard acids in a laboratory setting is fundamental to analytical chemistry, ensuring that subsequent titrations yield accurate and reproducible results. This experiment highlights critical steps including solution preparation, titration, and data analysis, emphasizing the importance of precision and accuracy.

Preparation of a Standard Oxalic Acid Solution

The first step involves preparing a known concentration of oxalic acid dihydrate (H₂C₂O₄·2H₂O), which is used as a primary standard to calibrate the NaOH solution. Although anhydrous oxalic acid would be ideal, its unavailability necessitates using the dihydrate form, which can be accurately weighed and prepared in solution. The balanced chemical equations for the titration reaction are essential to understand the stoichiometry involved.

The reaction between sodium hydroxide and oxalic acid is as follows:

• Balanced molecular equation:

  H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O

• Complete ionic equation:

  H₂C₂O₄(aq) + 2OH⁻(aq) → 2H₂O(l) + C₂O₄²⁻(aq) + 2Na⁺(aq)

• Net ionic equation:

  H₂C₂O₄(aq) + 2OH⁻(aq) → 2H₂O(l) + C₂O₄²⁻(aq)

The calculation for the required mass of oxalic acid dihydrate uses its molar mass and the desired molarity. Preparing 250 mL of a 0.0300 N oxalic acid solution involves calculating the amount in grams that corresponds to this normality, considering that oxalic acid is diprotic, and normality accounts for the number of reactive hydrogen ions.

The molar mass of oxalic acid dihydrate (H₂C₂O₄·2H₂O) is approximately 126.07 g/mol. Since the molarity and normality are related through the number of reactive hydrogen atoms (two in this molecule), the amount of oxalic acid dihydrate needed is derived from these parameters. Using a test tube, the respective mass is weighed by difference to ensure accuracy, then dissolved in water to produce the standard solution, stored in a labeled container, and used in titrations.

Mass of oxalic acid dihydrate = (Desired normality) × (Volume of solution in L) × (Equivalent weight)

where the equivalent weight of oxalic acid dihydrate is half its molar mass because (H₂C₂O₄) provides two protons per molecule.

Preparation of Sodium Hydroxide Solution

The NaOH solution is prepared by diluting a concentrated stock solution. The calculation involves determining the volume of 6 M NaOH required to prepare 1 L of 0.09000 N NaOH. The dilution equation (C₁V₁ = C₂V₂) governs this calculation, with proper care taken to avoid contamination and ensure mixing.

Once prepared, the NaOH solution is stored in a tightly capped plastic bottle to prevent atmospheric CO₂ from reacting with hydroxide ions and altering concentration. The solution is also thoroughly mixed by inversion and is ready for calibration via titration with KHP.

Standardizing NaOH Solution Using KHP

Potassium hydrogen phthalate (KHP) serves as a primary standard because of its high purity and stability. It is weighed by difference using an analytical balance and transferred carefully to titration flasks. Typical masses range between 0.6 g and 0.8 g, with at least 0.01 g difference between samples to ensure reliable reproducibility.

The titration process involves using a buret filled with the NaOH solution to titrate a known mass of KHP, monitoring it with an appropriate indicator, such as phenolphthalein, until endpoint. Repeated titrations ensure consistency, with results expressed as normality of NaOH calculated from the titration data.

Back-titration with oxalic acid solution is often employed to confirm the standardization, especially if titration does not reach a clear endpoint on the first attempt. The back-titration involves titrating the excess NaOH remaining after reacting with KHP with the standard oxalic acid, thus allowing calculation of the exact NaOH molarity and normality.

Data Analysis and Precision

Measuring the normalities from multiple titrations enables calculation of the mean, in addition to analyzing data for outliers using the 4-d rule. Precise calculations of deviations, averages, and relative deviations (in parts per thousand, ppt) are necessary to assess the consistency of the titrations.

Any outliers significantly deviating from the mean are statistically evaluated to decide whether they should be discarded. Adjustments to the data set improve the reliability of the standard solution. Maintaining precise, consistent titrations and measurements ensures high-quality data suitable for determining unknown acid concentrations.

Concluding Remarks

This experiment emphasizes not only the technical skills required for solution preparation and titration but also underscores the importance of data analysis for ensuring accuracy. The strategic use of primary standards, accuracy in weighing, correct titration techniques, and thorough data evaluation collectively contribute to producing reliable standard solutions essential in quantitative analysis.

References

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