1. It is not always feasible to determine pH on board ship, owing to bad light and other conditions, although it may be possible to collect water samples.

2. By preserving the water samples on collection with mercuric chloride solution (four drops per 100 c.c. of water) and by taking care to leave only a small air-space in the bottles it is possible to keep the water practically unchanged for many days.

3. Test tubes containing such water (10 c.c.) plus 0·02% cresol red (0·5 c.c.) have been-kept unchanged for three weeks at 12–14° C. and for two days at 33° C, whereas unpreserved water showed a drop in pH of 0·05 unit in one day at 12° C.

4. McClendon buffer solutions may also be preserved by mercuric chloride, but in this case a small correction must be applied since complex formation leads to a removal of effective buffer salts from the solution and a small decrease in pH.