METHOD OF PROCEDURE IN THE ANALYSIS OF WATER FOR CHLORINE "Pour 25 cubic centimeters of the water to be tested into a white porcelain dish. Add about one-half a cubic centimeter of chromate solution and run in standard silver-nitrate solution from a burette until the first faint reddish tint appears. This is more easily noted if for comparison a dish containing the same amount of water and chromate is kept beside the dish in which the test is made.
"If one or more cubic centimeters of silver-nitrate are necessary to reach an end point, the test may be made without evaporation, but if less is required then evaporate 250 cubic centimeters to 25 cubic centi meters volume before making the test. It may at times be necessary to evaporate more than this if the chlorine present is very close to zero in amount.
"It is best always to titrate with 25 cubic centimeters of the water. In this case 0.1 cubic centimeter is subtracted from the results as an indicator error. If more than this amount is used in titration, subtract 0.1 cubic centimeter for each 25 cubic centimeters of the volume of water titrated.
"If 250 cubic centimeters of water are taken, the number of cubic centimeters of silver-nitrate solution used to obtain an end point minus 0.1 cubic centimeter multipled by 2, gives the chlorine in parts per million.
"Example: 250 cubic centimeters are evaporated to a volume of 25 cubic centimeters and chromate solution added. In the titration 3.5 cubic centimeters of silver-nitrate are used. Then (3.5 — 0.1) X 2 = 6.8. The water, then, contains 6.8 parts per million of chlorine." The temperature of waters encountered in drilling oil wells varies so widely that differences are sometimes observed and recorded without an effort to measure the exact temperature. In many cases, it will be found advisable to determine accurately the temperature of water, as well as of oil, for the purpose of identifying its source.
Where several water bearing strata contribute to the supply of water in a well there may be difficulty in determining the tem perature from each stratum. The most favorable conditions for
such determination will probably exist while the well is drilling, and where it may be possible to exclude, at least temporarily, certain flows.
Oil Encountered during Drilling.—It might seem superfluous to say that the occurrence of oil should always be recorded in the log of a well, in view of the fact that its discovery is the prime reason for drilling. Many wells, however, have failed to yield profitable production on account of carelessness in observing and recording oil which was actually encountered. Many experi enced oil operators claim they can distinguish a difference be tween samples of sand which carry oil or water. Such a claim is untenable. There is no inherent difference between sands which carry oil and those which carry water.
The only certain method of determining that rock is oil-bearing is actually to find oil in it. Sometimes the oil can be seen readily or its presence may be detected by the odor. Washing an oil sand in water will frequently liberate the oil, which can then be seen floating on the water.
Some kinds of oil will discolor chloroform or ether, giving the liquid a brown color. The presence of such oils in rock can be detected by dropping finely crushed fragments of the impreg nated rock into a glass test tube containing chloroform or ether. Water in the rock may interfere with this test, but that obstacle can be avoided by first soaking the rock in alcohol which will absorb the water. Sometimes a sample may be carefully dried so that the presence of oil may be detected by the chloroform or ether test. If too much heat is applied in drying the sample, however, there is risk of driving off all the oil.
Tests of rock, by ether or chloroform, which disclose the pres ence of oil do not prove that it is present in commercial quanti ties. However, this test is widely used, and the prudent opera tor will always make use of it where there may be danger of over-looking productive formations.