Estimation of lead chromate in turmeric powder.

 

Aim

To detect and estimate the presence of lead chromate adulteration in turmeric powder using spectrophotometric/colorimetric method.

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Principle

Lead chromate (PbCrO₄) is sometimes added to turmeric powder to enhance its yellow color. In acidic medium, chromate ions are converted into dichromate ions, which produce an intense color that can be measured using a spectrophotometer or colorimeter.

The absorbance of the colored solution is proportional to the concentration of chromium present according to Beer–Lambert law.

$$A=εclA = \varepsilon c l

Where

• A = Absorbance

• ε = Molar absorptivity

• c = Concentration of analyte

• l = Path length of the cuvette

Thus, the amount of lead chromate present in turmeric can be determined from the absorbance value.

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Apparatus

• Spectrophotometer / Colorimeter

• Volumetric flasks

• Beakers

• Pipette

• Funnel

• Filter paper

• Glass rod

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Chemicals / Reagents

• Turmeric powder sample

• Dilute hydrochloric acid (HCl)

• Distilled water

• Standard potassium chromate solution

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Procedure

1. Preparation of Sample Solution

1. Weigh about 1 g of turmeric powder accurately.

2. Transfer it into a beaker.

3. Add dilute hydrochloric acid and heat gently to dissolve any lead chromate present.

4. Filter the solution to remove insoluble impurities.

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2. Preparation of Test Solution

1. Transfer the filtrate into a volumetric flask.

2. Dilute the solution to a known volume using distilled water.

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3. Measurement

1. Take a portion of the prepared solution in a cuvette.

2. Set the spectrophotometer/colorimeter at appropriate wavelength (around 440–450 nm).

3. Use distilled water as blank.

4. Measure the absorbance of the sample solution.

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4. Calculation

1. Compare the absorbance of the sample with the standard chromate solution.

2. Determine the concentration of lead chromate present in turmeric powder using Beer–Lambert law.

   Observation Table

   1. Preparation of Standard Chromate Solution

   Sr. No.	Concentration of Standard (mg/L)	Absorbance
   1	0
   2	2
   3	4
   4	6
   5	8
   6	10

   (A calibration curve of absorbance vs concentration is plotted.)

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   2. Sample Measurement

   Sr. No.	Volume of Sample Taken (ml)	Wavelength (nm)	Absorbance
   1	10	440–450
   2	10	440–450
   3	10	440–450

   Average Absorbance = ______

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   Calculations

   Let

   • $A_s$ = Absorbance of sample

   • $C_s$ = Concentration of sample (mg/L)

   • $A_{std}$= Absorbance of standard

   • $C_{std}$ = Concentration of standard (mg/L)

   According to Beer–Lambert law:

   $$\frac{A_s}{A_{std}} = \frac{C_s}{C_{std}}$$

   Therefore,

   $$C_s = \frac{A_s \times C_{std}}{A_{std}}$$

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   Amount of Lead Chromate in Turmeric

   $$\text{Lead chromate (mg/g)} = \frac{C_s \times V}{W}$$

   Where

   • $C_s$ = Concentration of chromium obtained (mg/L)

   • $V$= Final volume of solution (ml)

   • $W$= Weight of turmeric sample taken (g)

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   Percentage of Lead Chromate

   $$\text{Percentage of Lead Chromate} = \frac{\text{Amount of lead chromate}}{\text{Weight of turmeric sample}} \times 100$$

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   Result:

   Lead chromate present in turmeric = _____ mg/g

   or

   Percentage of lead chromate = _____ %