The key difference between phenolphthalein alkalinity and total alkalinity is that phenolphthalein alkalinity measures hydroxides and half the carbonates at a pH of 8.3, whereas total alkalinity measures all carbonates, bicarbonate, and hydroxide alkalinity at a pH of 4.5.
Alkalinity measures the amount of acid required to neutralize all the basic components in a given sample. There are different types of alkalinity, and phenolphthalein and total alkalinity are two of them.
CONTENTS
1. Overview and Key Difference
2. What is Phenolphthalein Alkalinity
3. What is Total Alkalinity
4. Phenolphthalein Alkalinity vs Total Alkalinity in Tabular Form
5. Summary – Phenolphthalein Alkalinity vs Total Alkalinity
What is Phenolphthalein Alkalinity?
Phenolphthalein alkalinity can be described as the alkalinity in a water sample measured by the amount of standard acid required to lower the pH to a level of 8.3. This pH value is indicated by the color change of phenolphthalein from pink to colorless.
Usually, carbonate, hydrogen carbonate, and hydroxide ions in water can cause alkalinity in water. Phenolphthalein alkalinity usually measures hydroxides and half of the carbonates. This corresponds to the endpoint of the phenolphthalein indicator we can use in the determination of the alkalinity. We can express phenolphthalein alkalinity in terms of milligrams per liter of equivalent to calcium carbonate.
Simply put, phenolphthalein is a specific type of alkalinity. We can determine it by titrating it to pH 8.3. For example, if we are titrating OH- ions with a strong acid, this gives us a titration curve showing the equivalence point. In the curve, we can find the point at which all the OH- ions are neutralized at the phenolphthalein endpoint. But, if we are titrating carbonate ions with a strong acid, it gives the phenolphthalein endpoint at which only half of the carbonate ions are neutralized because carbonate is a dibasic chemical species. Therefore, phenolphthalein alkalinity can give us the total hydroxide level and half of the carbonate level.
What is Total Alkalinity?
Total alkalinity is the water’s ability to resist changes in pH. In other words, it is the measurement of the total concentration of all the alkaline species dissolved in water. The principle alkaline species include hydroxide ions, carbonate, and bicarbonate ions. These ions can buffer the pH of water by neutralizing acids; that’s why we can say that total alkalinity is the ability of water to resist changes in pH.
Moreover, aquatic chemists use the unit milligrams per litre of calcium carbonate (mg/L CaCO3) to measure this parameter. Or else, we can simply use the unit ppm (parts per million). The ideal range of this parameter for good quality water is 80-120 ppm.
What is the Difference Between Phenolphthalein Alkalinity and Total Alkalinity?
Phenolphthalein alkalinity and total alkalinity are specific forms of alkalinity. These are very important in determining the alkaline levels of water. The key difference between phenolphthalein alkalinity and total alkalinity is that phenolphthalein alkalinity measures hydroxides and half the carbonates at a pH of 8.3, whereas total alkalinity measures all the carbonates, bicarbonate, and hydroxide alkalinity at a pH of 4.5.
The following table summarizes the difference between phenolphthalein alkalinity and total alkalinity.
Summary – Phenolphthalein Alkalinity vs Total Alkalinity
Phenolphthalein alkalinity is the alkalinity in a water sample measured by the amount of standard acid required to lower the pH to a level of 8.3. Total alkalinity is the water’s ability to resist changes in pH. The key difference between phenolphthalein alkalinity and total alkalinity is that phenolphthalein alkalinity measures hydroxides and half the carbonates at a pH of 8.3, whereas total alkalinity measures all the carbonates, bicarbonate, and hydroxide alkalinity at a pH of 4.5.
Reference:
1. “Water and Wastewater Terms Beginning P.” Phenolphthalein Alkalinity. Office of Water.
Image Courtesy:
1. “Potassium & water 39” By Ozone aurora / Philip Evans – Own work (CC BY-SA 3.0) via Commons Wikimedia