pH of samples using universal indicator सार्वभौमिक सूचक से नमूनों का pH
Aim: To find the approximate pH of common samples by the colour they give with universal indicator, and to classify them as acidic, neutral or basic.
Apparatus उपकरण
- Test tubes and test-tube stand
- Dropper
- Universal indicator solution (or pH paper) and its colour chart
- Samples: dilute HCl, dilute NaOH, dilute ethanoic acid, lemon juice, water, sodium hydrogencarbonate solution and others
Theory सिद्धांत
pH is a scale from 0 to 14 that shows how acidic or basic a solution is. pH 7 is neutral, below 7 is acidic and above 7 is basic; the lower the pH, the higher the concentration of hydrogen ions. Universal indicator is a mixture of indicators that shows a different colour at each pH: red for strongly acidic, orange and yellow for weakly acidic, green for neutral, blue for weakly basic and violet for strongly basic. Matching the colour with the chart gives the approximate pH.
Procedure विधि
- Take about 2 mL of each sample in a clean, labelled test tube.
- Add two or three drops of universal indicator to each test tube and shake gently.
- Hold the test tube against a white background and compare its colour with the colour chart.
- Note the colour and the matching pH. Classify the sample as acidic, neutral or basic.
- Rinse the dropper with distilled water between samples.
Simulated experiment अनुकरण प्रयोग
Pick a sample, add universal indicator, then tap the colour on the chart that matches the test tube best.
Lemon juiceनींबू का रस
Observation table प्रेक्षण तालिका
| S. No. | Sample | Colour | pH | Nature | Check |
|---|---|---|---|---|---|
| Test at least six samples, including an acid, water and a base. | |||||
Record your first sample.
pH values are typical approximate values; real samples vary with brand, freshness and dilution, and indicator colours differ slightly between makers.
Precautions सावधानियाँ
- Handle acids and bases carefully; do not touch or taste them.
- Use clean test tubes and a rinsed dropper so samples do not mix.
- Add the same small amount of indicator to every sample.
- Compare colours in daylight against a white background.
Sources of error त्रुटि के स्रोत
- Colour matching is judged by eye and can be off by about one pH unit.
- Coloured samples (like orange juice) can hide the indicator colour.
- Water absorbs carbon dioxide from air and becomes slightly acidic.
- Dirty test tubes or droppers can change the pH of a sample.
Viva voce questions मौखिक प्रश्न
Q1. What does pH stand for and what is its range?
pH comes from "potenz" (power) of hydrogen. The common scale runs from 0 (very acidic) to 14 (very basic).
Q2. What is the pH of pure water and why?
pH 7. Pure water has equal concentrations of hydrogen and hydroxide ions, so it is neutral.
Q3. Which is the stronger acid, one with pH 2 or one with pH 5?
pH 2. A lower pH means a higher concentration of hydrogen ions.
Q4. Why is universal indicator preferred to litmus?
Litmus only tells whether a solution is acidic or basic. Universal indicator also tells roughly how strong it is, through its pH.
Q5. Why does tooth decay start when the pH in the mouth falls below 5.5?
Acids made by bacteria from sugar dissolve tooth enamel (calcium hydroxyapatite) below about pH 5.5.