How to Test Water for Chlorine in Swimming Pools

For trouble shooting or more accurate control of swimming pool water, it is essential that reliable water test methods and reagents be used.

Routine Water Tests Required

To obtain the maximum benefits from a modern well designed swimming pool, it is very essential that regular tests be carried out for;

Free chlorine
Total chlorine
pH
Reserve Alkalinity

Correct water test procedure and knowledgeable interpretation of the results will result in good plant control and the most efficient and economical usage of chemicals, water and power.

Samples of water for testing should be taken from about one foot to 18 inches below the surface. The position of sampling should be next to the exit from the pool (not at the inlet or in between, as this may give higher free chlorine readings). Also for comparison purposes the same exit position should be used for all regular sampling and this should preferably be somewhere near the shallow end or area of greatest loading.

The common apparatus used for the measurement of free chlorine residuals, total chlorine and pH, is the Lovibond Comparator.

The basis of each water test with the comparator is that a colour developed by the addition of an indicator solution to a specially prepared sample of water is matched against a scientifically calibrated colour disc. The procedure adopted in preparing the sample for colour comparison varies for each type of water test. Colour discs are provided to cover the different ranges of residual chlorine. Various indicators with appropriate discs are also available to cover different ranges of pH.

It is good practice to replenish test reagents at the beginning of each season. During the season the water test solution may be affected by sunlight, or contaminated during use.

Difficulties in maintaining good conditions in the pool have often been traced to faulty reagents that give false readings resulting in incorrect dosage of chemicals with subsequent discomfort to bathers.

Other contributing causes to false colour development are the test procedure incorrectly performed; dirty glassware and comparator discs; temperature error; colour that is developed by interfering impurities such as iron, manganese and nitrates in the water.

Tests should always be carried out in a clean area where solutions or stoppers from bottles will not pick up chlorine chemicals, alkali or alum dust from bench tops. It is bad practice, therefore, to carry out tests in the same room where solutions of these are prepared or stored.

Plastic beakers should be used for collecting the samples of water. (When using glass containers there is always a hazard associated with breakage).

The sample should be taken immediately to the testing room and kept out of sunlight during this period. Tests should be made without delay otherwise true chlorine readings will not be obtained.

All test tubes or cells should be clean and rinsed with sample water before use. The operator's hands should be washed before tests in case he may have been handling other chemicals prior to sampling.

Test for Free and Total Chlorine Residual.

Reagents required:

Standard acid ortho-tolidine solution
Standard sodium arsenite solution
Colour disc range - 0.1 to 1.0 ppm chlorine
0.15 to 2.0 ppm chlorine

Procedure

Fill one cell with sample of water, up to 10mL mark, and place in left hand side of comparator.
To another cell add 0.1mL acid ortho-tolidine solution from a graduated dropper. Fill to 10mL mark with sample, stir with glass rod, and leave for 5 minutes before placing in right hand side of comparator. This will show the total chlorine (reading 1).
Take two cells, to one (a) add 0.1mL of sodium arsenite solution, to the other (b) add 0.1mL of acid ortho-tolidine solution.
Add 10ml. of sample water to (b) and within 5 seconds pour this into cell (a) containing the sodium arsenite. Place in right hand side of comparator, this will show free chlorine (reading 2).
Reading 1, minus Reading 2, will give the combined chlorine residual (or total amount of chloramines present).
Blank Test

Sometimes impurities in the water may give a false colour reading due to the presence of manganese, iron or nitrites. This can be checked by the following procedure.
Take two cells, to one (a) add 0.1mL of ortho-tolidine, to the other (b) add 0.1mL of sodium arsenite solution. Add 10mL. sample of water to (b). This is the reverse procedure for free chlorine. The sodium arsenite will destroy any chlorine residual present. Pour (b) into (a) and let stand for 5 minutes and take (reading 3).

Should a reading be recorded this reading (3) should be subtracted from reading (1) and (2) to get a true free and total chlorine reading.

It should be stressed that these false readings from interfering substances can occur more frequently than one would imagine.

In one particular case, a pool was registering a free chlorine level of over 1.0 ppm with very little addition of chlorine. It was found by doing a blank test that interfering substances were showing colour equivalent to reading 1.0 ppm of free chlorine. It was subsequently found out that a recent school carnival, brown discolouration had been noticed at one end of the pool. As the blank reading was due to manganese it was surmised that a schoolboy had added potassium permanganate (Condy's crystals) to the pool for a prank.