Preparation of Standard Chemical Solutions and Buffers

Objectives

1. To be familiar with standard solution preparation.
2. To be familiar with buffers and making buffer solutions.
3. To be familiar with using different laboratory glassware.
4. To be familiar with operating the chemical balance.

Introduction

Standard Solutions

Standard solutions are chemical solutions with a precisely known concentration of a substance or an element. The concepts of molarity and molar concentration (mol/L) are used to determine the concentration of standard solutions. These solutions are commonly used to identify the unknown concentration of a substance in a solution.

When preparing a standard solution, the solute should meet the following criteria:

• Pure
• Very stable
• Readily available/inexpensive
• Having a higher molecular weight for convenience.

Upon preparing standard solutions, a known weight of the substance is dissolved in distilled water or purified water. Solutes can be categorized as liquid, powder, pellets, etc.

Example: Trizma acetate (pellets)

Type-wise, standard solutions are divided into two types as primary and secondary standard solutions. Primary solutions are pure and stable, and they do not change their properties upon reacting with other chemicals.

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Examples: NaHCO₃ (Sodium bicarbonate), KHP (Potassium hydrogen phthalate)

Secondary standard solutions are specifically made for a particular task, such as titration, by dissolving solute in small quantities and solvent in large quantities. Secondary standard solutions are solutions with concentrations that we need to compare with a solution of a primary standard.

Example: HCl (Hydrochloric acid)

Buffers

A buffer solution is an aqueous solution that maintains its pH value fairly constant even upon the addition of small amounts of acids or bases.

This feature is crucial for reactions or processes that require a specific and stable pH range. Buffer solutions have a working pH range and a capacity that dictate how much acid or base can be neutralized before pH changes.

There are acidic buffers and basic buffers in general:

Acidic buffers:

A weak acid with its conjugate base.

Example: CH₃COOH (Acetic acid) and CH₃COO^- (Acetate ion)

Basic buffers:

A weak base and its conjugate acid.

Example: NH₄OH (ammonium hydroxide) and NH₄Cl (ammonium chloride)

How buffers work:

Buffers neutralize any added OH^- and H⁺, making it a weaker acid or a weaker base, to maintain a moderate pH level.

When H⁺ is added, the base component of the buffer neutralizes the added H⁺, hence maintaining the pH level.

CH₃COO^- + H⁺ → CH₃COOH + H₂O

Similarly, when OH^- is added, the acid component of the buffer neutralizes the OH^- and maintains the pH level.

CH₃COOH + OH^- → CH₃COO^- + H₂O

Materials

Equipment

• Analytical balance
• Wash bottle
• Spatula
• pH meter

Glassware

• Funnel
• 100 cm³ beaker
• 100 cm³ volumetric flask
• Pipette
• Glass rod
• Watch glass

Consumables

• Distilled water
• Tris base

Methodology

As the initial step, the mass of tris base needed was calculated. Then, the required mass was determined using the analytical balance. The procedure was as follows:

1. A clean watch glass was placed inside the analytical balance, and tris base was scooped onto the watch glass using a spatula.
2. The measured solid was transferred to a beaker using distilled water, creating a 50 ml solution.
3. The solution in the beaker was stirred well using a glass rod.
4. The solution was then transferred to the 100 ml volumetric flask through a funnel.
5. Distilled water was added until the solution reached the mark of 100 cm³.
6. The lid of the volumetric flask was closed, and it was shaken well.
7. Finally, the standard solution was labeled and stored.

Calculations

Mass of the tris base required to prepare a 100 ml solution of 0.001M tris buffer solution (Molecular Weight of Tris base: 121.14 g/mol)

Concentration (C) = Number of moles (n) ÷ Volume (V)

Number of moles (n) = Concentration (C) × Volume (V) / 1000

Number of moles (n) = 0.001 × 100 / 1000 = 0.0001 moles

Mass of tris base = 0.0001 moles × 121.14 g/mol = 0.01214 g

Approximately 0.0121 g of tris buffer was used for the experiment.

Observations and Results

1. Initial Reading of the Analytical Balance:
   The analytical balance displayed an initial reading when we started the experiment.
2. Mass Measurement of Tris Base:
   We measured the mass of tris base using the analytical balance, which was found to be approximately 0.0121 grams.
3. Dissolving Tris Base in a Beaker:
   • We placed a clean watch glass on the analytical balance and added tris base to it using a spatula.
   • The measured tris base in the watch glass was then dissolved in a beaker by adding distilled water.
   • The solution in the beaker was mixed thoroughly using a glass rod.
4. Transferring the Solution to the Volumetric Flask:
   The solution from the beaker was transferred to a 100 ml volumetric flask through a funnel.
5. Adding Distilled Water to Reach the Mark:
   Distilled water was added carefully to the volumetric flask until the water level reached the meniscus, ensuring a total volume of 100 ml.
6. Final Labeled Solution:
   • After ensuring the proper volume and mixing, we closed the lid of the volumetric flask.
   • Finally, we labeled the flask to identify the standard solution.

Discussion and Conclusion

Discussion

To prevent the mass of the watch glass from being calculated, the analytical balance's screen was reset to zero after placing the watch glass.

For a more accurate reading, the watch glass was placed inside the balance, and then, using both hands (spatula in one hand and tris bottle in the opposite hand), tris base was added.

After adding tris base, when it was time to get the final reading, the shutters of the analytical balance were closed to minimize errors caused by wind.

When transferring the watch glass full of tris out of the analytical balance, the watch glass was covered to prevent loss due to wind.

Conclusion

In conclusion, this experiment helped students become familiar with operating the analytical balance and preparing standard/buffer solutions. Specifically, a standard solution was prepared as a buffer solution.