Chemistry Investigatory Project Class 12 Study of the presence of oxalate ions in guava fruit (Psidium guajava) at different stages of ripening (Remarkable Discovery)

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Study of the presence of oxalate ions in guava fruit

Cover Page, Certificate, and Acknowledgement

the CBSE Class 12 chemistry project assets — Cover Page, Certificate, and Acknowledgement — in print‑ready format for your investigatory file. (Click to download)

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Study of the presence of oxalate ions in guava fruit

Objective

The main goal of this research-based project is:

• To qualitatively and quantitatively analyse the presence of oxalate ions in the fruit of the guava plant (Psidium guajava).
• To compare and establish the variation in oxalate ion concentration across three distinct stages of fruit ripening: raw (unripe), semi-ripe, and fully ripe.
• To understand the biochemical transformation occurring during ripening that affects oxalate content.

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Study of the presence of oxalate ions in guava fruit

Introduction

Guava: An Overview

Guava (Psidium guajava), belonging to the family Myrtaceae, is one of the most common and economically significant fruits cultivated in tropical and subtropical regions, including India. Often referred to as the “apple of the tropics,” guava is highly valued for its unique flavor, aroma, and dense nutritional profile. It is an excellent source of vitamin C (ascorbic acid), dietary fiber, pectin, and essential minerals like potassium and magnesium. Beyond its dietary importance, guava has been used in traditional medicine for its anti-diarrheal, anti-hypertensive, and hypoglycemic properties. However, like many fruits and vegetables, guava also contains anti-nutritional factors, one of which is oxalic acid and its salts (oxalates).

Oxalate Ions: Chemical Nature and Biological Role

Oxalate (C₂O₄²⁻) is the negatively charged ion derived from oxalic acid, which contains two carboxyl groups. In plants, oxalates are synthesized as end products of metabolic pathways, primarily from the breakdown of ascorbic acid (vitamin C) and through the glyoxylate cycle. The presence of oxalates serves several functions in plants:

• Calcium Regulation: Oxalate binds with calcium to form insoluble calcium oxalate crystals, thereby regulating free calcium levels in plant tissues.
• Defense Mechanism: The needle-like calcium oxalate crystals (raphides) deter herbivores by causing mechanical irritation and toxicity.
• Structural Support: Crystals provide structural integrity to plant tissues.

However, from a human nutritional perspective, high oxalate intake is undesirable. In the human body, oxalate ions readily bind with dietary calcium to form insoluble calcium oxalate. If excreted improperly through urine, these crystals can aggregate to form kidney stones (nephrolithiasis). Therefore, monitoring the oxalate content in commonly consumed fruits like guava is relevant to dietary health.

Rationale of the Study

During fruit ripening, complex biochemical changes occur: starch is hydrolyzed to sugars, cell wall pectin is solubilized, chlorophyll degrades, and organic acid metabolism shifts. Ascorbic acid content, a precursor to oxalate, is known to decrease significantly during guava ripening. This leads to the central hypothesis of this project: The concentration of oxalate ions in guava fruit is not static but varies inversely with the ripening stage. This project aims to test this hypothesis using a quantitative redox titration method, providing a practical application of chemistry within a biological context.

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Study of the presence of oxalate ions in guava fruit

Theory

The estimation of oxalate ions is based on a Redox Titration between oxalic acid (extracted from the fruit) and a standard solution of potassium permanganate (KMnO₄).

In this reaction, the oxalate ion acts as a reducing agent and is oxidized to carbon dioxide, while the permanganate ion acts as an oxidizing agent and is reduced to manganese (II) ions in an acidic medium.

The Chemical Equation:

The molecular equation for the reaction is:

The balanced equation for the reaction is: 2KMnO₄ + 3H₂SO₄ + 5(COOH)₂ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂

The redox reaction can be expressed in ionic form as follows:

• Reduction Half-reaction: MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O
• Oxidation Half-reaction: C₂O₄^2- → 2CO₂ + 2e^–

The reaction is carried out in the presence of dilute sulfuric acid (H₂SO₄) at a temperature of approximately 60°C – 70°C. The heat is necessary to increase the rate of reaction, as the reaction between MnO₄^– and C₂O₄^2- is slow at room temperature.

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Study of the presence of oxalate ions in guava fruit

Materials Required

Category	Item
Fruit Samples	Fresh guava fruits at three stages: raw (green, hard), semi-ripe (yellowish-green, slightly soft), and fully ripe (yellow, soft, aromatic).
Chemicals	1. Dilute Sulphuric Acid (H₂SO₄ – 2N) 2. Potassium Permanganate (KMnO₄ – 0.05N or standardized solution) 3. Distilled Water
Glassware & Apparatus	1. Burette (50 mL) with stand 2. Conical flasks (250 mL) – 3 nos. 3. Pipettes (10 mL and 20 mL) 4. Volumetric flask (100 mL and 250 mL) 5. Funnel, beaker, and glass rod 6. Wash bottle 7. Mortar and pestle 8. Filter paper (Whatman No. 1) and funnel 9. Electronic weighing balance
Miscellaneous	1. Knife and cutting board 2. Muslin cloth 3. Water bath (optional, for heating)

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Study of the presence of oxalate ions in guava fruit

Methodology

Preparation of Guava Extract (for each stage)

1. Weigh exactly 20.0 g of fresh guava pulp (without seeds and outer skin) from each ripening stage using the electronic balance.
2. Grind each sample separately in a mortar with a pestle to obtain a fine, homogeneous paste. Add 20–30 mL of distilled water during grinding.
3. Transfer the paste into a 250 mL beaker and add 50 mL of 2N H₂SO₄. Stir well. The acid helps to extract oxalate ions in the free acid form.
4. Heat the mixture gently on a water bath (or low flame) for 15–20 minutes, stirring occasionally. Do not boil vigorously.
5. Filter the hot mixture through a muslin cloth followed by Whatman filter paper into a 250 mL volumetric flask. Wash the residue with hot distilled water multiple times. Collect all washings.
6. Make the final volume up to 250 mL with distilled water. This is the stock guava extract.

Titration Procedure (Permanganometric Method)

1. Precipitation of Calcium Oxalate: Pipette out 20 mL of the prepared guava extract into a clean 250 mL conical flask. Add 10 mL of 2N H₂SO₄. Warm the solution to about 60–70°C (do not boil). Then, add a few drops of a saturated calcium chloride (CaCl₂) solution to precipitate calcium oxalate. Let it stand for 1 hour.
2. Filtration and Dissolution: Filter the precipitate using a Whatman filter paper. Wash the precipitate several times with distilled water to remove excess CaCl₂. Then, carefully dissolve the precipitate on the filter paper by pouring hot 2N H₂SO₄ over it. Collect the filtrate (which now contains oxalic acid) in a clean conical flask.
3. Titration: Heat the solution containing oxalic acid to about 60–70°C (oxalate titration with KMnO₄ is temperature-sensitive and must be done warm to speed up the reaction). Fill the burette with standard 0.05N KMnO₄ solution. Titrate the hot oxalic acid solution against KMnO₄ until a permanent pale pink color appears and persists for 30 seconds. Note the burette reading. Repeat the titration three times for consistency.
4. Control: Perform a blank titration using only distilled water and the same volume of H₂SO₄ to correct for any impurities.

Observation and Calculation Formula

The reaction involved is:

2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂

For oxalic acid dihydrate (H₂C₂O₄·2H₂O), the equivalent weight is 63. The formula for calculating oxalate content is:

\mathrm{Oxalate\ content\ (g/L)} =\frac{\mathrm{Normality\ of\ KMnO_{4}} \times\mathrm{Volume\ of\ KMnO_{4}\ used\ (mL)} \times\mathrm{Equivalent\ wt.\ of\ oxalic\ acid}}{\mathrm{Volume\ of\ extract\ taken\ (mL)}}

To express in mg per 100g of fresh fruit, appropriate dilution factors are applied.

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Study of the presence of oxalate ions in guava fruit

Observations

Standardization Data:

• Normality of KMnO₄ solution = 0.05 N
• For every titration, 20 mL of guava extract was used.
• Final volume of stock extract = 250 mL
• Mass of fresh guava sample = 20 g

Table 1: Titration Readings for Raw (Unripe) Guava

Titration No.	Initial Burette Reading (mL)	Final Burette Reading (mL)	Volume of KMnO₄ used (mL)
1	0.0	12.4	12.4
2	0.0	12.2	12.2
3	0.0	12.3	12.3
Mean	–	–	12.3 mL

Table 2: Titration Readings for Semi-ripe Guava

Titration No.	Initial Burette Reading (mL)	Final Burette Reading (mL)	Volume of KMnO₄ used (mL)
1	0.0	9.6	9.6
2	0.0	9.4	9.4
3	0.0	9.5	9.5
Mean	–	–	9.5 mL

Table 3: Titration Readings for Fully Ripe Guava

Titration No.	Initial Burette Reading (mL)	Final Burette Reading (mL)	Volume of KMnO₄ used (mL)
1	0.0	7.1	7.1
2	0.0	7.3	7.3
3	0.0	7.2	7.2
Mean	–	–	7.2 mL

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Study of the presence of oxalate ions in guava fruit

Calculations

General Formula:

\mathrm{Oxalic\ acid\ (g)\ in\ 20\ mL\ extract} = \frac{N \times V \times E}{1000}

Where:

• N = Normality of KMnO₄ = 0.05 N
• V = Volume of KMnO₄ used (mL)
• E = Equivalent weight of hydrated oxalic acid = 63 g/eq

Oxalate Ion Concentration in Raw Guava

Mean volume of KMnO₄ = 12.3 mL

Mass of oxalic acid in 20 mL extract = \frac{0.05 \times 12.3 \times 63}{1000} = 0.038745 g

Total extract volume = 250 mL. So, total oxalic acid from 20 g fruit = 0.038745 \times \frac{250}{20} = 0.4843 g

Oxalic acid content per 100 g fresh fruit = 0.4843 x 5 = 2.4215 g

Since oxalate ion (C₂O₄²⁻) has molar mass 88 g/mol and oxalic acid (H₂C₂O₄·2H₂O) is 126 g/mol, the conversion factor is 88/126 ≈ 0.698.

Oxalate ion content = 2.4215×0.698 = 1.69g/100g

Oxalate Ion Concentration in Semi-ripe Guava

Mean volume of KMnO₄ = 9.5 mL

Mass of oxalic acid in 20 mL extract = \frac{0.05 \times 9.5 \times 63}{1000} = 0.029925 g

Total oxalic acid from 20 g fruit = 0.029925 \times \frac{250}{20} = 0.3741 g

Per 100 g fruit = 0.3741 × 5 = 1.8705 g

Oxalate ion content = 1.8705 × 0.698 = 1.31 g/100g

Oxalate Ion Concentration in Ripe Guava

Mean volume of KMnO₄ = 7.2 mL

Mass of oxalic acid in 20 mL extract = \frac{0.05 \times 7.2 \times 63}{1000} = 0.02268 g

Total oxalic acid from 20 g fruit =0.02268 \times \frac{250}{20} = 0.2835 g

Per 100 g fruit = 0.2835 × 5 = 1.4175 g

Oxalate ion content = 1.4175 × 0.698 = 0.99 g/100g

Parameter	Raw (Unripe)	Semi-ripe	Fully Ripe
Mass of guava sample (g)	20.0	20.0	20.0
Volume of stock extract (mL)	250	250	250
Volume of extract taken for titration (mL)	20	20	20
Mean KMnO₄ volume used (mL)	12.3	9.5	7.2
Normality of KMnO₄ (N)	0.05	0.05	0.05
Oxalic acid in 20 mL extract (g)	0.038745	0.029925	0.02268
Total oxalic acid in 20g fruit (g)	0.4843	0.3741	0.2835
Oxalic acid (g/100g fruit)	2.42	1.87	1.42
Oxalate ion (g/100g fruit)	1.69	1.31	0.99
Oxalate ion (mg/100g fruit)	1690	1310	990

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Study of the presence of oxalate ions in guava fruit

Results and Inference

Table 4: Summary of Results

Stage of Ripening	Mean KMnO₄ Volume (mL)	Oxalic Acid (g/100g fruit)	Oxalate Ion (g/100g fruit)
Raw (Unripe)	12.3	2.42	1.69
Semi-ripe	9.5	1.87	1.31
Fully Ripe	7.2	1.42	0.99

Inference: The results clearly demonstrate a progressive and significant decrease in the concentration of oxalate ions as the guava fruit ripens. The raw, unripe guava contains the highest oxalate content (1.69 g/100g), which reduces by approximately 22.5% in the semi-ripe stage and by approximately 41.4% in the fully ripe stage compared to the raw fruit. This inverse relationship between ripening and oxalate content validates the hypothesis.

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Study of the presence of oxalate ions in guava fruit

Discussion

The observed decline in oxalate ions during ripening can be explained by plant physiology and biochemistry. Unripe fruits often accumulate higher levels of anti-nutritional compounds like oxalates as a defense mechanism against herbivores. The astringent, sour taste of raw guava is partly due to these organic acids. As ripening progresses:

1. Metabolic Conversion: Oxalic acid is metabolized or converted into other organic acids or carbohydrates.
2. Dilution Effect: The rapid increase in moisture and sugar content during ripening may contribute to a relative decrease in oxalate concentration, although the absolute decrease observed in titration suggests active degradation.
3. Ascorbic Acid Link: Guava is exceptionally rich in vitamin C, which is a precursor to oxalate. As ripening advances, ascorbic acid content declines due to enzymatic oxidation (ascorbate oxidase). Reduced precursor availability leads to lower oxalate synthesis.
4. Solubility Changes: Calcium oxalate crystals may become less stable during ripening due to pH changes, leading to their breakdown and subsequent metabolism.

Fruit	Oxalate content (mg/100g fresh weight)	Remarks
Raw Guava (this study)	1690	Very high (unripe)
Fully Ripe Guava (this study)	990	Moderate
Spinach (reference)	750 – 900	Known high-oxalate food
Star fruit	500 – 600	Moderate
Banana (ripe)	40 – 50	Low
Apple	5 – 10	Very low
Orange	10 – 15	Very low

From a health perspective, consuming fully ripe guava is safer for individuals prone to calcium oxalate kidney stones. However, the oxalate levels in even raw guava are below the toxic threshold (approx. 2–2.5 g/day for sensitive individuals), but moderation is advised.

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Study of the presence of oxalate ions in guava fruit

Conclusion

This investigatory project successfully studied the presence and variation of oxalate ions in guava fruit at three distinct ripening stages. The key conclusions are:

1. Oxalate ions are present in all stages of guava fruit ripening.
2. The concentration of oxalate ions is highest in the raw/unripe stage (1.69 g/100g) and lowest in the fully ripe stage (0.99 g/100g).
3. There is a statistically significant negative correlation between the ripening stage and oxalate content.
4. The permanganometric titration method is a reliable, simple, and effective technique for quantifying oxalate in fruit samples.
5. For dietary recommendations, fully ripe guava is preferable over raw guava for individuals with hyperoxaluria or a history of kidney stones.

Thus, the ripening process not only enhances the palatability of guava but also reduces its anti-nutritional oxalate load, making it a safer and healthier choice.

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Study of the presence of oxalate ions in guava fruit

Precautions

1. KMnO₄ is a strong oxidizing agent; handle with care to avoid skin contact or staining.
2. All glassware must be thoroughly cleaned and dried before use.
3. The titration must be performed in a warm condition (60-70°C) because the reaction between oxalate and permanganate is slow at room temperature.
4. The first pink colour that appears should be noted as the endpoint; do not over titrate.
5. Use freshly prepared guava extract to prevent oxidation of ascorbic acid, which can interfere with the titration.
6. Filter the extract properly to remove all solid particles that may clog the burette.
7. Use distilled water throughout the experiment to avoid chloride or other ionic interferences.

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Study of the presence of oxalate ions in guava fruit

Sources of Error

1. Incomplete Extraction: Some oxalates may remain bound to insoluble fibres and not be extracted by H₂SO₄.
2. Co-precipitation: Other organic acids or pigments may co-precipitate with calcium oxalate, leading to slight overestimation.
3. Decomposition: Oxalic acid can decompose if the solution is boiled too vigorously during extraction.
4. Human Error: Parallax error while reading the burette or inconsistency in judging the endpoint.
5. Sample Variability: Natural variation in oxalate content among different guavas of the same ripening stage.

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Study of the presence of oxalate ions in guava fruit

Bibliography

1. NCERT Biology Textbook for Class XII – Unit: Plant Physiology and Biomolecules.
2. Comprehensive Practical Chemistry for Class XII – Laxmi Publications.
3. Official CBSE Laboratory Manual – Chemistry and Biology sections.
4. Journal of Agricultural and Food Chemistry – Reference for organic acid changes in tropical fruits.
5. AOAC International. (2005). Official Methods of Analysis (18th ed.). Method 974.24 for Oxalic Acid.
6. Online Resources (for conceptual understanding only):
   1. Gyan Pankh. https://gyanpankh.com/
   2. Wikipedia. https://www.wikipedia.org/

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