Assertion (A) : The calorimetric method of pH measurement is based on the assumption that if an indicator has the same colour in two solutions, then the pH of both the solutions are same. Reason (R) : The colour developed depends not only on the pH but also on other factors.
Both (A) and (R) are correct and (R) is the correct explanation of (A)
Both statements are true, and the reason explains precisely why the assertion has to be phrased as an assumption rather than as a fact — option 1.
What the colorimetric method does. An indicator is a weak acid whose acid and base forms differ in colour, and the ratio of the two forms is governed by the pH:
\(pH=pK_{a}+\log_{10}\dfrac{\left[\text{In}^{-}\right]}{\left[\text{HIn}\right]}\)
Matching the observed colour against a chart of standards therefore gives an estimate of pH — and the whole procedure rests on taking equal colour to mean equal pH.
Why that is only an assumption. Colour is not determined by pH alone, and several other influences act on it:
| Factor | Effect on the reading |
|---|---|
| Temperature | Shifts the indicator's pKa |
| Ionic strength / salt error | Alters activities, so the same pH gives a different ratio |
| Coloured or turbid samples | Mask or distort the indicator's own colour |
| Oxidising or reducing agents | Bleach or chemically alter the dye |
| Protein error | The dye binds to protein and changes colour |
| Indicator concentration | The dye itself perturbs a weakly buffered sample |
The logical relationship is what makes the answer option 1. Notice that the reason does not contradict the assertion — it accounts for its status. Precisely because colour depends on more than pH, the identity of colour cannot be derived; it has to be assumed, and it holds only when those other factors are matched between sample and standard. The reason therefore tells us why the assertion is an assumption at all, which is exactly what "correct explanation" means here.
What follows in practice. The method is quick and cheap and needs no power, which is why indicator papers remain useful for a rough check. But its accuracy is only about ±0.5 pH, and for anything better the potentiometric method is used instead — a glass electrode developing
\(E=E_{0}-\dfrac{2.303RT}{F}\,pH\)
about 59 mV per pH unit at 25 °C. That method depends on a potential rather than a perception, so it is unaffected by colour and turbidity, and its temperature dependence is explicit in the equation and can be compensated.
Hence, both (A) and (R) are correct and (R) is the correct explanation of (A).
Similar Questions
Read the following statements regarding errors in measurement :
(a) Gross errors are mostly due to lack of knowledge and care on the part of the experimenter.
(b) Systematic errors are mostly caused by inherent short-comings of the instruments and components used in measurement.
(c) Random errors are due to sources and causes that cannot be specified for their effects with.
(d) Gross and systematic errors are non-correctable.
Which of the above are correct ?
Blood pressure is measured by
Assertion (A) : In a well-type manometer, ratio of cross-sectional areas of well and the tube is not the factor that determines the scale of the manometer.
Reason (R) : Accuracy of a manometer is not affected by the shape or size of the tubes.
Electrocardiogram is a graphical representation of electrical activity of heart muscle myocardium, in the form of
For the quantitative measurement of acidity or alkalinity, the instrument used is
The amplitude of an electrocardiograph signal is usually of the order of
In the bio-electric phenomenon, ERG is used to detect
An ammeter of 1 mA possesses a resistance of 100 ohms. This ammeter is to be converted into 1A ammeter. The value of shunt resistance required will be :
Followings is the sequence of blocks in spectrum Analyser :
A. Detector & video Amplifier
B. Fixed Frequency Active filter
C. Attenuator (Adjustable)
D. Mixer & Tunable Detector
Choose the correct answer from the options given below :
Read the following statements regarding biomedical instrumentations :
(a) Sphygmos is used for blood pressure measurement.
(b) EMG represents a record of the electrical activity of the brain.
(c) ERG represents the record of potential from the retina.
(d) ECG represents the record of electrical activity of brain.
Which of the above statements are correct ?
Important Questions from Miscellaneous
The ration of the number of girls and boys in a school is 8 : 7. If the percentage increase in the number of girls and boys is 10% and 20% respectively, what will be the new ratio?
The cheapest means of transport is:
The property of Catenation is most readily predominant in:
Which newspaper edited by Bal Gangadhar Tilak, was one of the strongest critics of the British rule?
Which of the following is NOT a cause of food and water contamination?
