Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doís and Doníts of Using a pH Meter

pH control is very important for many plating and water liquids used for preparation for plating. Knowing how to use and care for the pH measuring system is crucial to achieving accurate readings.

A pH meter is an instrument that measures the logarithm of the hydrogen ion activity of a water solution. What that means is that the meters measures how much acid, if the number is lower than 7. The lower the number the stronger is the acid. It also measures the alkalinity (hydroxyl ions) when the number is higher than 7. The higher number the stronger is the alkalinity. The meter is a standard accurate volt meter that uses two electrodes. One is called the reference electrode and consists of a sturdy glass tube with a tiny opening at the bottom containing a saturated concentration of potassium chloride. The main chamber also contains a saturated solution of potassium chloride for conductivity. The second electrode has a very thin special type of glass bulb that allows measurement of hydrogen ion activity in the solution. The accuracy is +/- 0.01

Use and care of pH electrodes

Measurement errors can be caused by high sodium content of the solution being measured. There is a carry over when high sodium compounds have been tested by the by the glass electrode. It should be cleaned, and immersed into a low pH (acid) for a short time.
 
Here are some guidelines:

1. Cool the solution to below 100 Degrees F before testing. The accuracy is improved and the electrodes will last longer. It is best to test the same solution using the same temperature each time for best accuracy.
2. Errors can be caused by having the bridge of the reference electrode clogged.
3. Maintain the potassium chloride solution in the reference electrode.
4. Glass electrodes should be cleaned using nitric acid (50% by vol.) or replaced at regular intervals.
5. Millivolt measurements in standard buffers should be compared to gage constant operation. This is done by removing the calomel reference electrode and replacing it with a new glass electrode. Now you have two glass electrodes in the buffer solution. Turn the switch to "volts" and read the number. You should have a zero reading. If there is a significant voltage, clean the old electrode in 50% nitric acid for a few minutes, rinse and test again, or it is time to replace the old glass electrode.
6. Frequent exchange of buffer solutions is recommended.
7. Electrodes stored in DI water can become coated with a film of algae that results in inaccurate measurements.
8. Donít forget to "slope" the meter between two buffer solutions that cover the range you wish to measure.

New glass electrodes should be conditioned by soaking in a 50/50% mixture of pH 4 buffer solution and saturated solution of potassium chloride.

Jack Horner says "pH papers can be very helpful for measuring pH of plating solutions. For best results when testing hot solutions, time the dip at about 3seconds, remove and count off 10 seconds before reading the results. Then do not take a second reading with the same paper because the color is likely to continue to change. You can correlate any differences using a timed pH paper for a given batch with a room temperature pH meter for a given batch of pH paper, and you will be pretty close."1.

Doís

Uncover the reference electrode fill hole. It must be uncovered when making a pH reading.

Use fresh buffers for calibration

Calibrate frequently

Stir frequently; Both buffers and samples

Keep electrodes clean

For short time storage, store electrodes in a buffer solution when not in use.

Clean and calibrate electrodes after storage in DI water to remove algae.

When finished pH testing a sample, rinse the electrodes with distilled water and leave the electrodes in distilled water until the next test.

Doníts

Allow the electrodes to dry

Scratch the glass electrode. The glass is very thin. Scratched glass electrodes will give incorrect readings. The glass bulb is thin and delicate.

Forget to "slope" the meter with two different buffers that cover the pH range you expect when starting to use the meter.

Donít use a reference electrode containing silver for testing electroless nickel solutions. Reducing agents react with silver to cog the frit resulting in incorrect pH readings.

  1. Jack Horner Research Manager Allied Kelite, retired.

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