SALINITY TESTS

At times you may be assigned to perform salinity tests with a field water, quality control kit. These tests serve to identify mineral characteristics of the water.

The quality control kit provides the necessary materials and equipment for the tests. Two bottles of reagents, one small and one large, are included. As the solution is used up during the test, the small bottle is refilled from the larger bottler The test bottles have two marks-the lower one at 50-ml capacity and the upper one at 100-ml capacity. The test solutions are measured with pipettes. These pipettes deliver a total of 1 ml from the upper graduation mark and are calibrated in 1/10-ml divisions. Each pipette is to be used only for the test for which it is marked in the pipette case and is to be returned directly to its place when the test is completed.

Four types of salinity tests, which you may perform, are as follows-the alkalinity test, the hardness test, the chloride test, and the sulfate test. Each of these tests is discussed in subsections below.

The Alkalinity Test

Alkalinity of water results from the presence of bicarbonate, carbonate, hydroxides of calcium, magnesium, sodium, and other minerals. The term alkalinity has little or no relation to the pH of the water but refers to the acid-neutralizing capacity of the water. In other words, alkalinity of water refers to the amount of various alkalies in the water that are capable of neutralizing acids. One method of determining the alkalinity of a water sample is by titration with standard sulfuric acid first to the phenolphthalein (PT) end point, and then to the methyl purple or methyl orange end point. Although methyl orange is the "standard indicator," methyl purple is much easier for the average operator to use because its color change is easier to see, and the results obtained with it are good enough for almost all uses.

REAGENTS - The reagents used in testing the alkalinity of water are as follows:

Phenolphthalein (PT) Indicator Solution

Methyl Purple Indicator Solution

Methyl Orange Indicator Solution

Standard Sulfuric Acid (N/50)

PROCEDURE WITH METHYL PURPLE - In determining the alkalinity of water with methyl purple, use the following procedure:

1. Measure 100 ml of the clear sample (filtered if necessary) into an evaporating dish or Erlenmeyer flask.

2. Add 4 drops of phenolphthalein indicator solution. If a pink or red color develops, phenol- phthalein alkalinity (alkalinity fraction contributed by hydroxide and half of carbonate) is present.

3. Fill the burette with acid and add to the sample slowly just until the pink color disappears.

4. Record the ml of acid used.

5. Now, add 2 to 4 drops of methyl purple indicator.

6. Continue titration, adding the acid in 0.5-ml portions until a greenish tint appears where the acid hits the sample. Then continue the addition more slowly, about 3 drops at a time. The color will change from green to gray and then to purple. The appearance of the purple tint marks the end point.

7. Record the total ml of acid required to reach this end point. This includes the ml of acid used in the phenolphthalein alkalinity titration and that used in the methyl purple titration.

PROCEDURE WITH METHYL ORANGE - In determining the alkalinity of water with methyl orange, follow the procedure given below.

1. Measure 100 ml of the clear sample (filtered if necessary) into an evaporating dish or Erlenmeyer flask.

NOTE: If an evaporating dish is used to get a white background for better color observation, the sample must be stirred with a stirring rod during addition of the standard acid.

2. Add 4 drops of phenolphthalein indicator solution. If a pink or red color develops, phenolphthalein alkalinity (alkalinity fraction contributed by hydroxide and half of carbonate) is present.

3. Fill the burette with the acid and add to the sample slowly just until the pink color disappears.

4. Record the ml of acid used.

5. Now, add 2 to 4 drops of methyl orange indicator.

6. Continue titration, adding the acid in 0.5-ml portions until the reddish color that appears where the acid hits the sample begins to persist. Then continue the addition more slowly, about 3 drops at a time, until the first pinkish tinge is seen throughout the sample. This is the end point.