Preparation of Solutions and different type of concentrations in a Biochemistry Lab

Mr. Arvind Kumar

Introduction

laboratory, there are five main methods used for calculating and preparing solutions.

  1. Concentration in moles per liter, molar concentration, or molarity (mol/L or mol L-1 or M)
  2. Concentration by percentage (either %w/v or % v/v or sometimes %w/w)
  3. Concentration in grams per liter (g/L or g L-1)
  4. Preparing solutions by dilution
  5. Preparing saturated solutions

 

Concentration in moles per liter, molar concentration, or molarity

Concentration in moles per liter, also known as molar concentration or simply molarity (M), is a measure of the amount of a substance (solute) dissolved in a given volume of solution. It is expressed as:

Molarity (M) = moles of solute/liters of solution

Key Points:

    1. Unit: The unit of molarity is moles per liter (mol/L).
    2. Formula:                                 

                                                                   M = n/V

Where:

      • M = molarity (mol/L),
      • n = number of moles of solute,
      • V = volume of solution in liters.
    1. Steps to Calculate Molarity:
    • Determine the number of moles of solute (n).

                                                   n = mass of solute (g) / molar mass (g/mol)

    • Measure the total volume of the solution in liters (V).
    • Divide n by V

Example Calculation:

What is the molarity if 5 grams of NaCl (sodium chloride) is dissolved in 0.5 liters of water?

    1. Molar mass of NaCl = 58.44 g/mol
    2. Moles of NaCl:

                                                         N = mass / molar mass = 5 / 58.44 = 0.0856 mol.

    1. Volume of solution = 0.5 L
    2. Molarity: M = n / V ​ = 0.0856 / 0.5 ​ = 0.171M.

So, the molarity of the NaCl solution is 0.171 M.

Applications of Molarity:

  • Commonly used in chemistry to prepare solutions.
  • Essential for stoichiometric calculations in chemical reactions.
  • Used in titrations to determine concentrations of unknown solutions.

 

Concentration by percentage (either %w/v or % v/v or sometimes %w/w)

Concentration by percentage expresses the amount of solute relative to the total solution. Depending on the system, it can be measured in different ways:

  1. % Weight/Volume (%w/v)

This is the mass of solute (in grams) dissolved in 100 mL of solution. It is commonly used in biological and chemical preparations.

%w/v = mass of solute (g) / volume of solution (mL) × 100

Example: A 10% w/v NaCl solution contains 10 g of NaCl in 100 mL.

  1. % Volume/Volume (%v/v)

This is the volume of solute (in mL) in 100 mL of solution. It is typically used for liquid-liquid solutions, such as ethanol in water.

%v/v = volume of solute (mL) / volume of solution (mL) × 100

Example: A 70% v/v ethanol solution contains 70 mL of ethanol in 100 mL of solution.

  1. % Weight/Weight (%w/w)

This is the mass of solute (in grams) in 100 grams of solution. It is frequently used in pharmaceuticals and industrial formulations.

%w/w = mass of solute (g) / mass of solution (g) × 100

Example: A 5% w/w glucose solution contains 5 g of glucose in 100 g of solution.

Differences Between These Methods:

Type Solute State Solution State Common Uses
%w/v Solids in liquids Liquid solutions Medical and biological solutions (e.g., saline)
%v/v Liquids in liquids Liquid solutions Alcoholic solutions, liquid mixtures
%w/w Solids or liquids in liquids/solids Solid or liquid solutions Ointments, creams, industrial chemicals

 

Applications:

  • Medical: Saline (0.9% w/v NaCl), glucose solutions.
  • Industrial: Cleaning solutions, cosmetics.
  • Laboratory: Preparing reagents and buffers.

 

Concentration in grams per liter

Concentration in grams per liter (g/L) measures how many grams of solute are dissolved in 1 liter of solution. This straightforward concentration unit is often used in laboratory preparations and chemistry.

Formula:

                                                            Concentration (g/L) = Mass of solute (g) / Volume of solution (L)

 

Steps to Calculate:

    1. Measure the mass of the solute in grams.
    2. Measure the volume of the solution in liters.
    3. Divide the solute’s mass by the solution’s volume.

Example Calculation:

 5 grams of NaCl is dissolved in 0.25 liters of water. What is the concentration in g/L?

    1. Mass of solute (NaCl): 5 g,
    2. Volume of solution: 0.25 L,
    3. Concentration:

Concentration (g/L) = 5 / 0.25 = 20 g/L.

Answer: The concentration is 20 g/L

Applications:

    • Industrial Chemistry
    • Biology and Medicine
    • Environmental Science

 

Preparing solutions by dilution

Preparing solutions by dilution involves reducing the concentration of a stock solution by adding a solvent (usually water). This process maintains the total amount of solute but increases the total volume, lowering the concentration.

Dilution Formula

                                                                                                   C1V1 = C2V2

Where:

    • C1: Initial concentration (stock solution),
    • V1: Volume of the stock solution to be used,
    • C2: Final concentration (diluted solution),
    • V2: Final volume of the diluted solution.

Steps for Preparing a Diluted Solution

    1. Determine the desired final concentration (C2​) and volume (V2​).
      For example, prepare 500 mL of 0.1 M HCl from a 1 M stock solution.
    2. Rearrange the formula to find the volume of stock solution (V1​):

                                                                          V1 = C2V2 / C1 ​​

Substituting:

                                                                   V1 = 0.1 × 500 / 1 = 50 mL

    1. Measure V1​ (50 mL of stock solution) using a pipette or measuring cylinder.
    2. Dilute the stock solution to the final volume (V2​) with solvent:
      Transfer the 50 mL of stock solution into a volumetric flask and add distilled water until the total volume is 500 mL.

Example Calculations

    • Prepare 250 mL of 0.5 M sulfuric acid (H2SO4​) from a 2 M stock solution.
      • C1 = 2 M,
      • V2 = 250 mL = 0.25 L,
      • C2 = 0.5 M.

                V1 = C2V2 / C1 = 0.5 × 0.25 / 2 = 0.0625 L = 62.5 mL.

Procedure: Measure 62.5 mL of 2 M H2SO4​, then dilute to 250 mL with water.

Applications

    • Laboratory Reagents: Diluting stock solutions for experiments.
    • Medical Preparations: Preparing saline or IV fluids.
    • Industrial Processes: Adjusting concentrations of cleaning agents or chemicals.

Preparing Saturated Solutions

A saturated solution contains the maximum amount of solute that can dissolve in a solvent at a specific temperature and pressure. Any additional solute added will not dissolve and will remain as a solid (precipitate).

Steps for Preparing a Saturated Solution

    1. Determine the Solubility of the Solute:
      • Look up the solubility of the solute in the solvent at the desired temperature (usually in grams per 100 mL of solvent).
        Example: The solubility of table salt (NaCl) in water at 25∘C is 36 g per 100 mL.
    2. Weigh the Solute:
      • Measure an amount of solute exceeding the solubility limit for the given solvent volume.
        Example: To make a saturated solution of NaCl in 100 mL of water, measure at least 36 g (slightly more to ensure saturation).
    3. Add the Solvent:
      • Pour the required amount of solvent (e.g., 100 mL of water) into a beaker or container.
    4. Mix Thoroughly:
      • Stir the solution continuously to dissolve as much solute as possible. Heating the mixture can increase the dissolution rate (if the solubility increases with temperature).
    5. Achieve Saturation:
      • Stop stirring when no more solute dissolves and excess solute settles at the bottom of the container. This indicates that the solution is saturated.
    6. Filter (Optional):
      • If you want to use only the saturated solution without the excess solute, filter the solution to remove undissolved particles.

Example: Saturated NaCl Solution at Room Temperature

    1. Solubility: NaCl has a solubility of 36 g/100 mL at 25∘C.
    2. Procedure:
      • Add 40 g of NaCl to 100 mL of water (slightly more than the solubility limit).
      • Stir until no more salt dissolves and a small amount remains undissolved.
      • The solution is now saturated.

Factors Affecting Saturated Solutions

    1. Temperature:
      • Solubility usually increases with temperature for most solids (e.g., sugar dissolves better in hot water).
      • For gases, solubility decreases with increasing temperature (e.g., less oxygen dissolves in warm water).
    2. Pressure (for gases):
      • Higher pressure increases the solubility of gases in liquids (e.g., carbon dioxide in soda).

Applications of Saturated Solutions

    1. Chemical Reactions: Preparing solutions for crystallization experiments.
    2. Industrial Use: Making brines, saturated sugar syrups, or fertilizer solutions.
    3. Supersaturation: A saturated solution can be cooled or manipulated to create a supersaturated solution for crystallization or other processes.

Requirements for Solution Preparation

  1. Basic Lab Equipment:

    • Volumetric Flasks: To prepare solutions with precise volumes.
    • Beakers: For mixing chemicals initially.
    • Graduated Cylinders: For measuring approximate liquid volumes.
    • Pipettes: For accurate measurement of liquids (manual or automatic).
    • Analytical Balance: For accurate weighing of solutes.
    • Magnetic Stirrer and Stirring Bar: To ensure uniform mixing.
    • pH Meter: For adjusting and verifying the pH of the solution.
    • Filter Paper and Funnels: For filtration, if necessary.
    • Storage Containers: Properly labeled bottles or vials for solution storage.

  2. Chemicals:

    • High-purity reagents (analytical or reagent grade).
    • Solvent: Usually deionized water (DI water) or distilled water.

  3. Safety Materials:

    • Gloves, goggles, and lab coats to avoid direct chemical contact.
    • Safety shower and eyewash station in case of emergencies.
    • A well-ventilated workspace or fume hood for volatile or hazardous reagents.

 

Prepare Buffer Solutions

  • Buffer solutions are available as premade solutions and ready-to-mix capsules and envelopes.
  • Buffers are typically mixtures of a weak acid and the salt of the acid or a weak base and its salt.
  • This combination is called a conjugate acid-base pair, and it will resist changes in pH upon adding small amounts of acid or base.
  • Recipes for three common buffer solutions are provided.

pH 4:    

  1. Dissolve 5.10 g of potassium hydrogen phthalate in 250 ml of Distilled water.
  2. Add 0.50 ml of 0.10 M hydrochloric acid.
  3. Then dilute to 500 ml.

pH 7:    

  1. Prepare 0.10 M potassium phosphate monobasic solution by dissolving 3.40 g in 250 ml of Distilled water.
  2. Prepare 0.20 M sodium hydroxide solution by dissolving 0.8 g in 100 ml of Distilled water.
  3. Mix 250 ml of the 0.10 M potassium phosphate solution and 73 ml of 0.2 M sodium hydroxide solution.
  4. Then dilute to 500 ml.

pH 10: 

  1. Prepare 0.025 M sodium borate solution by dissolving 2.38 g in 250 mL of Distilled water.
  2. Prepare 0.20 M sodium hydroxide solution by dissolving 0.8 g in 100 ml of Distilled water.
  3. Mix 250 ml of the 0.025  M sodium borate solution and  27 ml of the 0.2 M    sodium hydroxide solution.
  4. Then dilute to 500 ml.

 

Primary Standard: IN Sodium Carbonate 

Requirements 

1) Sodium carbonate, anhydrous (AR) 

2) 250 mi beaker 

3) 25 ml conical flask 

4) 5.0 ml graduated pipettes 

5) 1.0 ml volumetric pipette 

6) Graduated cylinder 

7) Volumetric flask 

8) Analytical balance 

9) Hot air oven

10) Reagent bottle, 125 ml.

Procedure

1) Keep anhydrous sodium carbonate in the hot air oven at 110°C for one hour. 

2) Cool it in the desiccator to room temperature. 

3) Weight exactly 5.3 g on an analytical balance using butter paper. 

4) Transfer it to the beaker. 

5) Dissolve it by using about 80 ml of distilled water. 

6) Make the final volume 100 ml using a volumetric flask and store it in the reagent bottle at room temperature. (25°C + 5°C) 

 

Preparation of hydrochloric acid 

Requirements 

1) Concentrated hydrochloric acid. AR 

2) Indicator: methyl orange 

3) IN sodium carbonate : (reference, primary standard) 

4) The glassware used is the same as used  

Procedure 

1) By using a measuring cylinder, transfer 100 ml of distilled water in a 250 ml beaker. 

2) Add 10 ml of conc. Hydrochloric acid, mix well.

3) Titration: Pipette 2.0 ml. of diluted hydrochloric acid in a conical flask by using a volumetric pipette. 

4) Add one drop of methyl orange. 

5) Titrate against IN sodium carbonate till the color of the reaction mixture changes from red to yellow. 

6) Note the titration reading. 

7) Take two more readings. 

8) Calculate the normality by using the formula N1V1 = N2V2. 

Example 1: Preparation of 500 mL of 0.5 M Hydrochloric acid (HCl) from a 2 M solution of HCl.

(H𝐶𝑙) 𝑏𝑒𝑓𝑜𝑟𝑒 𝑑𝑖𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑛2 (𝐻𝐶𝐿) 𝑎𝑓𝑡𝑒𝑟 𝑑𝑖𝑙𝑢𝑡𝑖𝑜𝑛          

                                                                           𝑐1𝑉1=𝑐2𝑉2       

2 𝑥 𝑉1=0.5 𝑥 .5            

𝑉1=0.5 𝑥 .5𝐿 2 

𝑉1=0.125 𝑜𝑟 125𝑚𝐿                    

 This volume of 2 M HCl is measured and placed in a 500 mL volumetric flask containing about 250 mL distilled water. Then, enough distilled water is added to make up to the 500 mL mark. The solution should be mixed well to obtain a homogeneous solution of 0.5 M HCl.

 

Preparation of normal saline

Requirements 

1) 2L conical flask 

2) 2L measuring cylinder 

3) 1L Volumetric flask 

4) Sodium Chloride (AR) 

5) Distilled water 

6) Analytical balance 

7) Butter paper 

8) 1L Reagent bottle 

 9) Magnetic stirrer 

Procedure 

1) Weigh 8.50 g sodium chloride on an analytical balance by using a butter paper. 

2) Transfer it to a 2-liter conical flask. 

3) Add 900 ml of distilled water. 

           4) Mix by using a magnetic stirrer. 

5) Transfer to IL volumetric flask and add distilled water up to the mark.

6) Mix well and store in a clean and dry reagent bottle.

7) Label it appropriately and store it at room temperature (25°C + 5°C). 

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