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Measuring the pH of a Solution

Another frequently used measure of water quality along with electrical conductivity and turbidity is the pH. pH is a measure of the hydrogen ion activity in a solution. Solutions with high hydrogen (H+) or hydronium ion (H3O+) activity are called acids and solutions with low hydrogen ion activity are called bases. Another way of looking at it is by looking at the reciprocal quantity of hydroxyl ions (OH-). The more hydroxyl ions, the more basic a solution is and so on. pH is a mathematical way of quantifying these concentrations and placing them on a relative scale. It is defined as:

In a parallel fashion the quantity pOH is defined as:

Conversely, to determine the activity or concentrations given the pH or pOH, we have:

The relationship between pH and pOH at standard temperature conditions (25 degrees Celsius) is:

pH + pOH = 14

From here, we can see that the relative scale used is from a pH of 0 to a pH of 14 with 0 being very acidic, 14 being very basic, and 7 being neutral.

There are 2 basic types of techniques for measuring pH. The first type is called colorimetric measurement. The use of litmus paper is a colorimetric technique. The second type is called electrometric and is done using an electrical instrument. The instrument is calibrated with 3 standard solutions at known pHs of 4, 7, and 10. Then the instrument can be used to measure unknown solutions. The typical range for drinking water is ideally around 6 – 8.5.

Turbidity and Suspended Solids

Another method of analyzing water quality is through the measurement of turbidity and its correlation with total solids.  Turbidity is a relative measure of the clarity of a water sample by measuring the scatter of light as it passes through a sample. A sample with high turbidity will appear unclear while a sample with low turbidity will appear more clear because light passes through it with less scatter and absorption. It is relative in that it is measured against a standardized sample of stabilized formazin or gelex. The preferred units used in turbidity measurements are nephelometric turbidity units or NTUs. It is usually measured with a nephelometric turbidimeter or spectrophotometer. A key point here is that the turbidity can be measured in just a few minutes including calibration.

Total solids can be categorized into 4 categories: suspended solids, dissolved solids, settable solids, and volatile solids. They include all the solid materials that are contained in the water matrix. They can be measured as a concentration of the sample volume using gravimetric measurement techniques. This requires that we use an Imhoff cone to measure settlable solids, a filter to measure suspended solids, and the filtrate to measure dissolved solids. Usually suspended solids is the primary concern. These constituents are what typically affect the clarity of the water. However since it is time consuming and not necessarily practical to collect a sample, put it in an evaporating dish, evaporate the water in an oven over the course of 24 hours or more, and weight the solids remaining, we attempt to find a correlation between suspended solids and turbidity. Turbidity cannot by itself be used to quantify the concentration of suspended solids in a body of water. However by analyzing a few samples we can find and plot the correlation between concentration of suspended solids and turbidity. This gives us a relationship (valid only for a particular location and time) so that we can determine the amount of suspended solids in a body of water by simply measuring the turbidity.

The primary reasons that suspended solids are a concern is first, simple aesthetics and second, the harboring of pathogens or pathogen supporting envirnoments. The EPA has set primary drinking water quality standards at less than 1 NTU. Prior to 2002, less than 5 NTU was acceptable.

Electrical Conductivity and Water Purity

Our first environmental engineering lab was concerning one method of water quality assessment called electrical conductivity. The theory behind this technique is based on the fact that pure water is not a good conductor of electricity. As the amount of inorganic, ionic elements or compounds in the water increases, the conductivity also increases. Conductivity also increases with temperature, therefore most results are standardized at 25 degrees Celsius. With this understanding we can make comparative analysis of conductivity values to assess the purity of the water. Conductivity in this context is usually in units of microsiemens per centimeter. In the lab we utilized a handheld electrical conductivity meter like the one above. Below is a table we were given of the typical value range found in different types of water.

The reason for the high conductivity in seawater is the high salt content and thus a high number of sodium and chloride ions in this aqueous solution. If you are interested in additional information, I found a good EPA website concerning water quality monitoring and electrical conductivity.