How to determine pH of any solution

pH

Solution pH is measure of the concentration of active acid or base in solution. Exactly defined, pH is the negative logarithm of the hydrogen ion concentration, but a simpler explanation is that pH is a number between 0 and 14, denoting various degrees of acidity or alkalinity. Neutral water has  pH of 7. Values below 7 and approaching 0 are increasingly acid while values from 7 to 14 are increasingly alkaline.

Pure water ionizes to produce hydrogen or acid ions (H+) and hydroxyl or alkaline ions (OH-) as illustrated.

H2O = H+ + OH-

When pure water ionizes in this manner, 0.0000001 gram of hydrogen ion is liberated per liter. This number, which can also be written as 1 x 10-7 is inconvenient to handle, but on the pH scale is written simply as 7.

H+    H2O    OH-

On the pH scale of value of 7 is exactly neutral. Proceeding to the left on the diagram and approaching “zero” indicates an increasing number of hydrogen ions present and increasing acidity. Proceeding to the right on the scale and more closely approaching a pH value of 14 indicates an increasing number of hydroxyl ions and increasing alkalinity.

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The further the pH is located from the neutral point of 7, the greater is the concentration of hydrogen of hydroxyl ions, but not in direct arithmetical proportion to the distance between each number. A pH of 7 means 0.0000001-gram of ionizable hydrogen is liberated per liter.

A pH of 6 means 0.000001gram of ionizable hydrogen per liter is liberated or 10 times the amount of a pH of 7. In a similar manner, a pH of 5 indicates 10 times the number of hydrogen ions liberated as compared to those available at a pH of 6 and 100 times the number as compared to a pH of 7. In short, each number on the pH scale is 10 times the concentration of hydrogen or hydroxyl ions when compared with its adjoining number.

The pH of most natural waters will fall within the range of 6.0 to 8.0 although more acid conditions, and therefore a lower pH will result where the water contains high concentration of free carbon dioxide or where the water originates from acid mine drainage. A pH above 8.0 is seldom encountered except where pollution by alkaline trade wastes exists or where the water has been chemically treated such as by the lime-soda process.

Since pH is a measure of relative acidity or alkalinity of a water, it is a most important factor influencing either scale forming or corrosion tendencies. Water of low pH will give rise to corrosion of equipment with which that water comes in contact. Conversely, high pH values may cause precipitation of calcium carbonate from solution which results in turn as scale on the surfaces of pilelines, heat exchange equipment, condensers, etc. The langelier index is a measure of the calcium carbonate scaling characteristics of a particular water. In the calculation of this indnex, determination of pH is required. Likewise in clarification processes, pH is probably the most important determination. The control of coagulation is primarily a matter of pH control since every coagulant possesses an optimum pH range in which it gives the most efficient operation.

For zeolite softening processes, pH must be determined on the influent water the range of acceptable pH depends on the particular type of zeolite material used.

Control of pH is an important factor in such processes as iron removal, recarbonation and acid treatment. The pH value of a boiler water is usually adjusted to a minimum of 10.5 to prevent acidic corrosion and to provide a sufficiently high pH for the precipitation of the scale-forming salts.In summary, pH is the single most important factor in industrial water treatment, and any analysis of water for scale forming or corrosive tendencies is incomplete without a determination pH.

pH, Glass Electrode Method Theory of Test

The pH or hydrogen ion concentration of a solution may be measured by determining, with a potentiometer, the voltage developed between two electrodes which are in contact with the solution. The voltage of one electrode (known as the reference electrode) is  fixed, while the voltage of the other electrode varies with the pH of the sample. Several types of electrodes may be used, but in general a glass electrode is the most applicable for industrial use. The potentials developed at the interface of the glass surface and the sample solution are directly proportional to the pH of the solution. Due to the high resistance of the circuit, it is necessary to use some type of electronic device to amplify the current.

This technology has been incorporated by several manufactures into commercial pH meters and electrode assemblies which are relatively inexpensive, accurate, and reasonably rugged and easy to use for control purposes. Many incorporate the reference and glass electrodes into a single assembly, the pH electrode.

Apparatus Required

pH meter    -  1

Combination pH glass electrode    - 1

Chemicals Required

Buffer solutions of appropriate pH for instrument standardization.

Procedure for Test

The exact mechanical procedure for determining pH will vary slightly with the instruments furnished by different manufacturers.

In general, the instrument is first standardized by using buffer solutions of known pH. Usually two buffer solutions are employed for the standardization. One buffer (generally pH= 7) is chosen for the middle of the pH range and either an acid (generally pH=4) or alkaline (generally pH=10) buffer is used to calibrated the meter in the suspected pH range of the sample. The temperature of the sample to be tested is observed and a temperature correction dial on the instrument is adjusted. The combination electrode is then inserted in the water sample and the pH is read directly from the meter on the instrument.

Limitations of Test

Glass electrode potentials are not affected by oxidizing or reducing agents, gases, dissolved organic compounds, colloids or suspended matter. The pH of viscous solutions or highly colored solutions may be obtained by this method. Accuracy is superior to colorimetric methods. In high concentrations of sodium ions, an error is introduced which can be corrected by suitable curves, or by standardizing the instrument for that range.

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