The work of Dr. Mahmoud Marsafy

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REMEMBER: you must do your own experiment risk assessment, ideas are illustrated but health and safety details are NOT included

22. Measuring pH with Universal Indicator

Comments on the Chart for measuring pH of solutions with UI.

 Universal Indicator solutions are commercially available. Their pH colour scales may vary from one source to the other and their formulae are not disclosed. A suitable indicator can be readily prepared as follows:

In 50 cm3 of Ethanol dissolve the following amounts of the dyes:  12 mg   Thymol Blue,  60 mg  Methyl Red,  150 mg  Bromothymol Blue and 250 mg  Phenolphthalein.

Neutralize the solution (to green) with 0.05M NaOH and dilute to 100 ml with water. The colour scale is similar to the usual products.                                           

• The recommended micro-scale procedure for pH determination for various solutions utilizes a plastic multi-cavity sheet. The sheet utilized was that used for packaging pharmaceutical tablets.
• 10 drops of water (0.5 cm3) are added in any one cavity followed by one drop of the indicator. The colour should be green, proving that all appliances used are clean. Then one drop of the solution for which the pH is to be measured is carefully added and stirred. The colour obtained is compared with the pH scale of colours for the utilized Indicator. All the results in the above chart have been obtained applying this procedure. For powder samples a very small amount may be added by a micro plastic spatula and also stirred.
• The results obtained are similar to those achieved by the conventional techniques carried out in glass beakers or in test tubes. This technique allows a great economy in expenses and in the laboratory space required and in the efforts for preparation and for cleaning after end of experimentation. 

It is recommended to handle the UI indicator in small plastic bottles fitted with an integral dropper closure rather than the conventional, larger size bottles fitted with detachable droppers.  Samples of the solutions investigated were also manipulated in similar dropper plastic bottles.

• Since only drops of solutions were tested, it was possible to dilute the UI indicator with alcohol and add one drop of the diluted indicator and still get similar colour change. Using one drop of colored solutions, such as copper sulphate (CuSO₄) or Cola drinks and diluting with 10 drops of water allows the acidic colour change of the UI indicator to be evident, and not masked with the original solution color.
• However utilizing various types of commercial indicators may give different colours for the same sample of solution. This is illustrated in the above chart for Vinegar and sodium carbonate solutions tested by two different indicators A&B. Vinegar has a pH of 2.8 and was distinctly red with the Indicator B, (i.e.  same colour as that of strong acids), but orange coloured with indicator A (i.e. distinctly differentiated from the red colour indicating strong acids). While sodium carbonate (Na₂CO₃) solutions were coloured blue with indicator A and Violet with Indicator B. Since the pH of the Na₂CO₃ solution is 11, then it should give a violet colour with the UI indicator. Therefore one must check the reliability of the actual colour scale of the UI indicator solution by comparing the colours obtained by reference samples of various solutions with known pH values.

The following solutions are recommended as reference samples:

This suggested verification  calibration procedure for UI indicator utilized is somewhat similar to the calibration of the pH sensor electrode when pH meters are utilized.

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