# Calibrating a pH meter

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```					                  Unit 1 - Calibrating an Instrument or Device

Calibrating a pH meter

Introduction –

We need to calibrate instruments and devices to put a scale on them in the
first place. We then calibrate it against a known scale to check that the
readings it gives us are correct.

   Speed cameras – if they are not calibrated correctly, the speed they
give is wrong and photos from that camera will not stand up in court.
   Breath testers for alcohol – If they are not calibrated correctly the
measurements will be inaccurate. Why is this important?
   Thermometers need to be calibrated so they can read the correct
temperature. Why is this important in a hospital?
   Ph testers in a swimming pool – if they are not calibrated correctly the
reading of pH will be incorrect. A swimming pool needs to be kept at
pH 7.8, which is the best pH to restrict the growth of bacteria without
causing eye and skin irritation. If the pH tester is calibrated wrong, this
will result in the pool operator adding too much acid or alkali, causing a
reduction in the pool waters disinfecting ability, and an increase in eye
irritation. Pools usually use indicators to monitor their pH such as
phenol red
   Can you think of two more instruments which should be calibrated and
why?

Aim - To calibrate a pH meter using standard buffer solutions at pH 4, 7 and
10.

Instrument or Device. The method is on the movie. Write it in the past tense.

Safety
Sulphuric acid is corrosive
Bleach =irritant
Sodium Hydroxide = irritant
Cleaning Fluid =Irritant
Biological washing powder = irritant
Unit 1 - Calibrating an Instrument or Device

Results
Dawsonsarah/ links/calibrating - Manufacturing Soluble Salts by
Neutralization Reactions to fill in the colours in the universal indicator section.

pH of solution using following method
Universal Indicator
Solution             Colour             pH               pH probe

Analysis – The accuracy of the buffer solutions was + or – 0.02% according to
their bottles.

Work out the accuracy (percentage error) of the pH meter calibrated at 4, 7
and 9.22. The 1st result is done for you.

Calibrated to pH 4 –actually read – 4.03.

The percentage error =
(change/original) x 100
(4.03-4)/4 x 100
3%
At pH 4, The pH meter has an accuracy of + or – 3%

Calibrated to pH 7 – actually read – …………….
The percentage error =
(change/original) x 100
(………..-7)/7 x 100
………%
At pH 4, The pH meter has an accuracy of + or – ……..%

Calibrated to pH 10 – actually read – …………….
The percentage error =
(change/original) x 100
(………..-10)/10 x 100
Unit 1 - Calibrating an Instrument or Device

………%
At pH 10, The pH meter has an accuracy of + or – ……..%

Evaluation

   describe any problems encountered when carrying out the tests, e.g.
different buffers
pH.
   discuss the reliability, accuracy and sensitivity of the techniques and
equipment used
   highlight where random, human or equipment error might occur when
   review your work and evaluate the procedures used, highlighting strengths
and weaknesses of this technique
   propose how you might make improvements to this technique, especially if
more time was available

Discussion of vocational implications

This should discuss the commercial implications of what you did.
 are similar techniques used in industry?
 where are similar and different levels of accuracy and sensitivity required
in industry?
 is the scale used in industry the same?
 what additional (?) safety features might by be used in industry?

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