Soap Analysis – Free Alkali Determination | EverExams

Laundry Soap Analysis – Free Alkali Determination

Quantitative analysis of free alkali in soap using acid-base titration with 0.1 M HCl

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SAFETY: Wear protective gloves and goggles when handling concentrated HCl and preparing soap solutions

Activity 2: Determine Free Alkali in Laundry Soap

Objective

To determine the percentage of free alkali in a given sample of laundry soap using 0.1 M HCl through acid-base titration.

Background Theory

Commercial laundry soaps often contain free alkali (usually NaOH) that wasn’t completely reacted during saponification. Excess free alkali can be harmful to skin and fabrics. Titration with standardized acid allows quantitative determination of this free alkali content.

Chemical Equation: HCl + NaOH → NaCl + H₂O

Key Information

  • Sample: Laundry soap (15 g)
  • Titrant: 0.1 M HCl solution
  • Indicator: Phenolphthalein
  • Volume of soap solution used: 10 cm³
  • Total soap solution prepared: 1 dm³ (1000 cm³)

Expected Result

Based on the procedure and sample data:

6.67%

The percentage of free alkali in the given sample of laundry soap

Why Test Soap for Free Alkali?

Quality Control: Free alkali indicates incomplete saponification. High free alkali content can:

  • Cause skin irritation and dryness
  • Damage fabrics and colors
  • Reduce soap shelf life
  • Indicate poor manufacturing process

Industry Standard: Good quality soap should have free alkali content below 0.1%

Theoretical Background

Acid-Base Titration Theory

Titration is based on the concept of neutralization, which occurs when an acid reacts with a base to form water and a salt. In a titration, a known concentration of one solution (titrant) is slowly added to a known volume of another solution (analyte) until the reaction reaches completion.

Key Concepts:

  • Equivalence Point: When stoichiometric amounts have reacted
  • End Point: Observable colour change (should match equivalence point)
  • Indicator: Substance that changes colour at/near equivalence point

Soap Chemistry

Soap is produced by saponification – the reaction between fats/oils and alkali (NaOH or KOH):

Saponification: Fat + NaOH → Soap + Glycerol

In commercial soap production, excess alkali is often used to ensure complete reaction. The remaining unreacted alkali is called “free alkali.”

Types of Alkali in Soap:

  • Free Alkali: Unreacted NaOH/KOH (determined in this experiment)
  • Combined Alkali: Alkali that has reacted with fats to form soap
  • Total Alkali: Free alkali + combined alkali

Titration Reaction

The titration involves neutralization of free NaOH in soap with standardized HCl:

Balanced Equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Mole Ratio: 1 mole HCl : 1 mole NaOH

Indicator Reaction: Phenolphthalein changes from pink (basic) to colourless (acidic) at pH ≈ 8.2

The volume of HCl required to neutralize the free alkali is used to calculate the percentage of free alkali in the soap sample.

Experimental Procedure

Apparatus Required

Volumetric Flask

1 dm³ capacity for preparing soap solution

Burette

50 cm³ capacity for HCl delivery

Pipette

10 cm³ for transferring soap solution

Conical Flask

250 cm³ for titration

Analytical Balance

For weighing soap sample (±0.001 g)

Indicator Bottle

Phenolphthalein solution

Step-by-Step Procedure

Step 1: Preparation of Soap Solution

Weigh exactly 15 g of laundry soap using analytical balance. Transfer to 1 dm³ volumetric flask. Add about 500 cm³ distilled water and shake well to dissolve. Add more water up to the mark (1000 cm³). Shake gently to avoid frothing.

Step 2: Fill Burette with HCl

Rinse burette with small amount of 0.1 M HCl, then fill it with the same solution. Ensure no air bubbles in the tip. Record initial reading.

Step 3: Transfer Soap Solution

Using pipette, transfer exactly 10 cm³ of soap solution to clean conical flask.

Step 4: Add Indicator

Add 2-3 drops of phenolphthalein indicator. The solution will turn pink due to basic nature of free alkali.

Step 5: Perform Titration

Slowly add HCl from burette while swirling conical flask continuously. Continue until pink colour just disappears (colourless or very light pink). This is the end point.

Step 6: Record Volume

Record final burette reading. Calculate volume of HCl used (V₁ = Final – Initial).

Step 7: Repeat Titration

Repeat steps 3-6 two more times until you obtain three concordant readings (within ±0.1 cm³).

Procedure Tips

Avoid Frothing: Soap solutions tend to foam. Shake gently and allow bubbles to settle before pipetting.

End Point Detection: Stop titration at first permanent disappearance of pink colour. Over-titration gives high results.

Concordant Readings: Take at least 3 readings that agree within 0.1 cm³. Discard the first rough titration.

Observations and Calculations

Observation Table

Sr. No. Initial Reading (cm³) Final Reading (cm³) Volume of HCl used (cm³)
1 0.0 2.5 2.5
2 2.5 4.9 2.4
3 4.9 7.4 2.5
Mean Volume of HCl (V₁): 2.5 cm³

Given Data:

  • Molarity of HCl solution (M₁) = 0.1 M
  • Volume of soap solution taken (V₂) = 10 cm³
  • Mass of soap dissolved = 15 g
  • Total volume of soap solution = 1000 cm³ (1 dm³)
  • Molar mass of NaOH = 40 g/mol

Step-by-Step Calculations

Step 1: Calculate Molarity of NaOH in soap solution (M₂)

Using formula: M₁V₁/n₁ = M₂V₂/n₂ (1:1 reaction)

(0.1 × 2.5)/1 = (M₂ × 10)/1

0.25 = M₂ × 10

M₂ = 0.25/10 = 0.025 M

Step 2: Calculate Strength of NaOH in soap solution

Strength = Molarity × Molar mass

Strength = 0.025 M × 40 g/mol

Strength = 1 g/dm³

This means 1 dm³ of soap solution contains 1 g of free NaOH

Step 3: Calculate free NaOH in original soap sample

15 g of soap was dissolved in 1 dm³ solution

So, 15 g soap contains 1 g free NaOH

Step 4: Calculate percentage of free alkali

Percentage = (Mass of free NaOH / Mass of soap) × 100

Percentage = (1 g / 15 g) × 100

Percentage = 6.67%

Final Result

6.67%

The percentage of free alkali in the given sample of laundry soap

Interpretation: This soap contains 6.67% unreacted NaOH, which is very high for commercial laundry soap.

Calculation Memory Tip

Remember the Formula Chain:

  1. M₁V₁ = M₂V₂ (for molarity calculation)
  2. Strength = M × Molar mass (for g/dm³)
  3. % = (mass NaOH / mass soap) × 100 (for percentage)

Always check units: cm³ for volumes, M for molarity, g for masses.

Short Questions with Answers

1. Why is phenolphthalein used as an indicator in this experiment instead of methyl orange?

Answer: Phenolphthalein is used because it changes colour in the pH range of 8.2-10.0 (colourless in acid, pink in base). In this titration, we’re titrating a strong base (NaOH) with a strong acid (HCl). The equivalence point occurs at pH 7, but phenolphthalein’s colour change occurs close enough to this point for accurate results. Methyl orange changes colour in the acidic range (pH 3.1-4.4), which would occur after the equivalence point in this titration, leading to overshooting and inaccurate results.

2. What does “free alkali” mean in the context of soap analysis, and why is it important to determine its percentage?

Answer: Free alkali refers to unreacted sodium hydroxide (NaOH) or potassium hydroxide (KOH) remaining in the soap after the saponification process. It’s important to determine its percentage because:

  1. Skin Safety: High free alkali content can cause skin irritation, dryness, and dermatitis
  2. Fabric Protection: Excess alkali can damage fabrics, especially natural fibers like cotton and silk
  3. Quality Control: Indicates incomplete saponification or poor manufacturing process
  4. Standard Compliance: Commercial soaps must meet regulatory limits for free alkali content
  5. Shelf Life: High free alkali can reduce soap stability and shelf life

Good quality soap should have free alkali content below 0.1%.

3. Calculate the percentage of free alkali if 20 g of soap dissolved in 1 dm³ solution requires 4.0 cm³ of 0.1 M HCl to neutralize 10 cm³ of the soap solution.

Answer:

Step 1: Calculate molarity of NaOH (M₂)

M₁V₁ = M₂V₂ → 0.1 × 4.0 = M₂ × 10 → M₂ = 0.04 M

Step 2: Calculate strength of NaOH

Strength = M × Molar mass = 0.04 × 40 = 1.6 g/dm³

Step 3: Calculate free NaOH in soap

20 g soap contains 1.6 g free NaOH

Step 4: Calculate percentage

Percentage = (1.6 / 20) × 100 = 8.0%

Result: The percentage of free alkali is 8.0%.

4. Why is it necessary to prepare 1 dm³ of soap solution and then use only 10 cm³ for titration, rather than using the soap directly?

Answer: Preparing 1 dm³ of soap solution and using only 10 cm³ for titration serves several important purposes:

  1. Homogeneity: Ensures the soap sample is uniformly distributed in solution
  2. Accuracy: Larger initial mass (15 g) allows more accurate weighing than trying to weigh a small sample directly
  3. Practical Titration Volume: 10 cm³ is an ideal volume for titration – not too small to cause large percentage errors, not too large to require excessive titrant
  4. Multiple Trials: From 1 dm³ solution, multiple 10 cm³ aliquots can be taken for replicate titrations
  5. Dilution: Dilutes the soap to a concentration that gives a reasonable titration volume (2-3 cm³ of 0.1 M HCl in this case)
  6. Solubility: Ensures complete dissolution of soap, which might not dissolve completely in small volumes

This approach follows the principle of working with manageable volumes while maintaining accuracy through proper dilution.

5. What are the possible sources of error in this experiment and how can they be minimized?

Answer: Possible sources of error and their minimization:

1. Weighing Errors:

  • Source: Inaccurate balance, improper weighing technique
  • Minimization: Use analytical balance, calibrate regularly, weigh by difference

2. Volume Measurement Errors:

  • Source: Parallax error, incorrect meniscus reading, air bubbles in burette
  • Minimization: Read at eye level, ensure no air bubbles, use calibrated glassware

3. End Point Detection Errors:

  • Source: Over-titration, indicator error, colour blindness
  • Minimization: Titrate slowly near end point, use white background, consider potentiometric titration

4. Solution Preparation Errors:

  • Source: Incomplete dissolution, incorrect dilution, contamination
  • Minimization: Ensure complete dissolution, use clean glassware, follow standard procedures

5. Calculation Errors:

  • Source: Unit conversion mistakes, formula errors, rounding errors
  • Minimization: Double-check calculations, maintain consistent units, use appropriate significant figures

General Minimization: Perform multiple trials, use concordant readings, follow standardized procedures, and maintain proper laboratory conditions.

Real-World Applications

Soap Manufacturing

Quality control during production, ensuring complete saponification, meeting industry standards for free alkali content.

Quality Assurance

Testing commercial soap products for compliance with safety standards and labeling requirements.

Medical Soaps

Ensuring medicated soaps have appropriate pH levels for sensitive skin conditions.

Waste Treatment

Analyzing alkaline content in industrial wastewater from soap factories for environmental compliance.

Research & Development

Developing new soap formulations with optimal properties and minimal free alkali.

Forensic Analysis

Identifying soap composition in criminal investigations or product tampering cases.

Industry Standards for Free Alkali

Acceptable Limits in Commercial Soaps:

  • Toilet Soaps: Maximum 0.1% free alkali
  • Laundry Soaps: Maximum 0.5% free alkali (some standards allow up to 1%)
  • Medicated Soaps: Maximum 0.05% free alkali
  • Baby Soaps: Maximum 0.025% free alkali

Note: The 6.67% result in our experiment indicates very poor quality soap that would not meet commercial standards.

Soap Analysis Quiz

Test your understanding of soap analysis and titration with these 20 multiple choice questions. Select your answer and click submit to check your score.

Your Soap Analysis Quiz Results

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