Políticas

Because even the smallest detail can influence the result of a jar test and all samples in a series of tests should be handled as nearly alike as possible, proper maintenance of equipment is necessary to assure good operation.

A. Stirring Machine

The stirring machine has three to six paddles, each capable of operating at variable speeds from 0 to 100 rpm. Maintenance of the stirring equipment should be performed on a regular basis as recommended by the manufacturer. The stirring machine should be kept clean and lubricated (according to manufacturer's instructions) so that smooth revolution of the paddles at the specified speed may be assured. Also, the rotation speeds should be checked periodically to ensure that all paddles are rotating at uniform speeds.

B. Floc Illuminator

Located at the base of the stirrer, the floc illuminator enables observation of small floc particles. Care should be taken to keep the glass clean. The lamp should be checked routinely and replaced when needed. A replacement lamp should be kept on hand for this purpose.

The six 1,500ml beakers should be washed and rinsed after each test. Prior to starting a test, the beakers should be rinsed with tap water and turned upside-down for a few minutes to drain.

4.4 pH Meter

The pH meter consists of a sensitive voltmeter, a glass electrode, a reference electrode (made of either calomel or a silver/silver-chloride (Ag/AgCl) compound), and a temperature compensating device. In some units, the two electrodes are mounted in a single unit called a combined electrode. When the electrodes are immersed in the test solution, a circuit is completed through the voltmeter. The meter then converts the voltage change caused by the change in pH to a meter reading expressed in pH units. The temperature compensating device is needed because the pH of a sample is affected by temperature.

A. Maintenance

Some precautions to be taken in the care and maintenance of the pH meter are:

1. Glass electrodes should not be allowed to become dry during periods of inactivity. Follow the manufacturer's instructions for the storage of electrodes, since recommended solutions for short-term storage of electrodes vary with the type of electrode and the manufacturer. Generally, tap water is a better substitute than distilled water, but pH 4 buffer is best for the single glass electrode. Saturated KCI (potassium chloride) solution is preferred for calomel and Ag/AgCl reference electrodes;

2. Keep the pH electrodes free of oil, grease, or precipitates. These materials coat the pH electrode and may interfere with pH readings by causing a sluggish response;

3. Electrode troubles generally can be traced to a clogged junction which can be cleared by applying suction to the tip or by boiling the tip in distilled water until the electrolyte flows freely when suction is applied to the tip or pressure is applied to the fill hole.

4. The electrode may have a crystal buildup and should be flushed with distilled water and refilled with proper solution.

5. Routine maintenance should be performed in accordance with the manufacturer's instructions.

B. Calibration

Proper calibration (standardization) is essential for accurate pH measurements. The purpose of standardization is to adjust the response of the glass electrode to the instrument. When only occasional pH measurements are made, the instrument should be standardized before each measurement. When frequent measurements are made and the instrument is stable, standardize less frequently. If sample pH values vary widely, standardize for each sample with a buffer having a pH within 1 to 2 pH units of the

sample. Calibrate the electrode system against standard buffer solutions of known pH. Because buffer solutions may deteriorate as a result of mold growth or contamination, prepare fresh solutions as needed for accurate work.

When calibrating the pH meter, the following method should be used:

1. Before use, remove electrodes from the storage solution, rinse with distilled water, and blot dry;

2. Place the electrodes in the initial buffer solution with a known pH, set the meter scale or needle to the pH of the buffer, and adjust for temperature;

3. Select a second buffer within 2 pH units of the sample pH and bring sample and buffer to the same temperature;

4. Remove the electrodes from the first buffer, rinse thoroughly with distilled water, blot dry, and immerse in the second buffer;

5. Record the temperature of measurement and adjust the temperature dial on the meter so that the meter indicates the pH value of the buffer at the test temperature;

6. Remove the electrodes from the second buffer, rinse thoroughly with distilled water, blot dry and immerse in a third buffer below pH 10, approximately 3 pH units different from the second;

7. The reading should be within 0.1 pH unit for the pH of the third buffer. If it is not, then look for trouble with the electrodes or potentiometer. Failure to obtain a correct value for the pH of the third reference buffer solution could indicate a cracked glass electrode, failure to maintain the KCI in the calomel electrode, or oily substances or precipitate coating the surface.