Problem 112 - Chemistry The Science in Context

Question

Between 1993 and \(1995,\) sodium phosphate was added to Seathwaite Tarn in the English Lake District to increase its pH. Explain why addition of this compound increased pH.

Step-by-Step Solution

Answer

Question: Explain why the addition of sodium phosphate to Seathwaite Tarn in the English Lake District increased its pH between 1993 and 1995. Answer: The addition of sodium phosphate to Seathwaite Tarn increased its pH due to the dissolution of the compound in water, which released phosphate ions. These phosphate ions, being weak bases, accepted hydrogen ions from the water and formed hydrogen phosphate ions and water. This resulted in a decrease in hydrogen ion concentration in the water, thus reducing its acidity and increasing its pH.

Step 1: Understanding pH

pH is a measure of the acidity or alkalinity of a solution. It is defined as the negative base 10 logarithm of the hydrogen ion concentration in the solution. The pH scale ranges from 0 to 14, where 7 is considered neutral. A pH below 7 indicates an acidic solution, and a pH above 7 represents an alkaline solution. Therefore, in order to increase the pH of Seathwaite Tarn, we need to reduce its acidity or increase its alkalinity.

Step 2: Identifying sodium phosphate

Sodium phosphate is an inorganic compound composed of sodium (Na) and phosphate (PO4) ions. It has the chemical formula Na3PO4. Sodium phosphate is a salt, which means that it can dissolve in water and dissociate into its constituent ions.

Step 3: Understanding the dissociation of sodium phosphate

When sodium phosphate dissolves in water, it dissociates into its ions: Na3PO4 (aq) → 3Na+ (aq) + PO4^3- (aq) The dissociation of sodium phosphate releases three sodium ions and one phosphate ion into the solution for each molecule of sodium phosphate.

Step 4: Explaining how the addition of sodium phosphate increases pH

By adding sodium phosphate to Seathwaite Tarn, we are introducing more phosphate ions (PO4^3-) into the water. Phosphate ions are weak bases, which means they can accept hydrogen ions (H+) from the water. When phosphate ions accept hydrogen ions, they form hydrogen phosphate ions (HPO4^2-) and water: PO4^3- (aq) + H+ (aq) → HPO4^2- (aq) This reaction results in a decrease in the concentration of hydrogen ions in the water, which leads to an increase in the pH of the solution. Therefore, by adding sodium phosphate to Seathwaite Tarn, the acidity of the water is reduced and the pH is increased.

Key Concepts

Understanding the pH Scale

The pH scale is an instrumental tool in chemistry, essential for describing the acidity or alkalinity of a solution. It spans from 0 to 14, where a pH of 7 is neutral, a value below 7 indicates increasing acidity, and a value above 7 signals increasing alkalinity.

Each unit change represents a tenfold difference in hydrogen ion (\( H^+ \) or \( H_3O^+ \) ) concentration, which is why pH is measured on a logarithmic scale. For instance, a solution with a pH of 6 has ten times more hydrogen ions than a solution with a pH of 7. In practice, measuring and adjusting pH is vital for numerous scientific procedures and industries, from controlling the conditions for enzyme activity in biological systems to ensuring the safety of drinking water.

Role of Sodium Phosphate in pH Adjustment

Sodium phosphate, a salt with the formula \( Na_3PO_4 \), serves as an excellent example of an agent used for pH control. When added to a solution like water, it dissolves, breaking down into sodium (\( Na^+ \) ) ions and phosphate (\( PO_4^{3-} \) ) ions. These ions play a central role in balancing pH levels.

Sodium phosphate is favored in applications requiring a shift towards basicity because it doesn't add to the acidity of the solution. Instead, as a source of the basic phosphate ion, it effectively raises the pH of solutions in which it is dissolved.

Decoding Acid-Base Chemistry

Acid-base chemistry is a fundamental concept within the chemical sciences, dealing with the reactions between acids and bases. Acids are substances that can donate hydrogen ions (\( H^+ \) ) to a solution, while bases are those that can accept them. The reaction between an acid and a base typically results in the formation of water and a salt.

This interaction is intrinsic to understanding chemical reactivity and is described by various theories like the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis theory of acids and bases. For students and chemists alike, grasping these principles is key to manipulating and predicting the outcomes of chemical reactions, particularly those involving pH changes.

Chemical Dissociation Explained

Chemical dissociation is the process by which molecules or ionic compounds separate into smaller particles, usually ions, when dissolved in a solvent like water. This is a reversible physical process distinctly different from chemical reactions that alter the structure of molecules.

For sodium phosphate, the dissociation can be presented as:
\( Na_3PO_4 (aq) \rightarrow 3Na^+ (aq) + PO_4^{3-} (aq) \)

In a practical setting, understanding dissociation is crucial for predicting how substances behave in solution, especially when considering how they will impact the pH. It serves a vital role in formulating products ranging from medications to cleaning agents and managing biochemical pathways.