A manual of practical chemistry for public health students : especially arranged for those studying for the D.P.H / by A.W. Stewart.

  • Stewart, Alfred W. (Alfred Walter), 1880-1947
Date:
1913
Catalogue details

Licence: In copyright

Credit: A manual of practical chemistry for public health students : especially arranged for those studying for the D.P.H / by A.W. Stewart. Source: Wellcome Collection.

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    All pipettes and burettes before being used should be rinsed out with a little of the solution that is going to be placed in them. A little vaseline put on the burette taps will enable them to turn easily. Two titrations should be made and the mean taken. Supposing it was 24-6 c.c. : this means that to every 20 c.c. of the acid liquid 4-6 c.c. of water must be added to make it decinormal. Measure in a cylinder the volume of the remaining acid and calculate how much water to add. Example: Volume of acid goo c.c. Amount of H„0 to be added will therefore be 46 x 900 207 c.c. Measure out this quantity, pour in about 180 c.c., keeping the remainder in reserve. If all the water is added at once the new solution on titration will probably be found too weak. The diluted acid liquid is now titrated as described above in the first operation. This time 20 c.c. will probably require 20-3 c.c. TN5 Na2C03. The reserve, water is then added by 5 or 10 c.c. portions until 20 c.c. exactly require 20 c.c. of Na2C03. Such a solution would then be decinormal. Preparation of Decinormal Caustic Soda, Tv NaOH. From the formula NaOH we see that there is only one hydroxyl group (OH), consequently 23+16+1 40 of caustic soda dissolved in 1,000 c.c. water would constitute a normal solution, and 4 grm. would be As caustic soda usually contains traces of impurities, such as carbonate and cyanide, and as it cannot be accurately weighed out on account of its being hydro- scopic, we have to make the solution slightly stronger than decinormal, and by titration determine the amount of. water to be added in order to make it exact. For this purpose approximately 4-5 grm. of NaOH are weighed out, dissolved in water, and finally made up to 1,000 c.c. Pipette out 20 c.c., add a little water and a few drops of phenolphthalein. Run in from a burette H.-.SO until the pink colour is just discharged. The process is then continued precisely in the same manner as described under the preparation of ^ H^SO^.