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30 Manual of basic techniques for a health laboratory
pH of the water has to be around 7.0 (neutral water) and has to be kept neutral.
This is achieved, if possible, by dissolving buffer salts in the water (buffered water).
Materials and reagents
● Measuring cylinders, 10ml and 1000ml
● Volumetric flask, 1000ml
● Universal indicator paper (for measuring pH from 1 to 10)
● Indicator paper of limited pH range: for the 5.0–7.0 range and for the 6.0–8.0
range
● Distilled (or demineralized) water
● Acetic acid, 5% solution (reagent no. 1), diluted 1:10 with distilled water
● Disodium hydrogen phosphate (Na 2
HPO 4·2H 2
O), hydrated
● Phenol red, 1% solution (reagent no. 42)
●
Potassium dihydrogen phosphate (KH 2
PO 4
), anhydrous
● Sodium carbonate, 0.2% solution (reagent no. 51).
Method
1. Weigh out accurately 3.76g of disodium hydrogen phosphate.
2. Transfer the chemical to a 1000-ml volumetric flask through a funnel (Fig.
2.36).
3. Rinse out the weighing container into the volumetric flask several times with
water. Rinse the funnel into the flask.
4. Weigh out accurately 2.1g of potassium dihydrogen phosphate and proceed as
in steps 2 and 3.
5. Add a little more water and mix the solution until the chemicals are dissolved.
6. Fill the flask to the 1000-ml mark with water.
7. Replace the flask stopper and mix the solution well.
8. Store the solution in a white glass reagent bottle and keep in a refrigerator.
Fig. 2.36 Transferring disodium hydrogen phosphate into a volumetric flask