In precipitating magnesium ammonium phosphate, why can't NH4Cl be used as a substitute for NH3?

Isn't pH critical? The solubility is vastly different at pH 9 versus pH 4.

In addition, how do you obtain NH3 from NH4Cl? The NH3 is for pH adjustment as Bob Pursley writes.

NH4Cl cannot be used as a substitute for NH3 in precipitating magnesium ammonium phosphate for several reasons:

1. pH control: In the precipitation process, maintaining a specific pH is critical for the formation of magnesium ammonium phosphate. NH3 is frequently used to adjust the pH to an alkaline range (around pH 9) because it acts as a weak base and helps in the precipitation process. On the other hand, NH4Cl is acidic in nature and would result in a lower pH (around pH 4) if used, which is not suitable for the precipitation of magnesium ammonium phosphate.

2. Solubility: The solubility of magnesium ammonium phosphate is significantly different at different pH levels. At pH 9 (alkaline), magnesium ammonium phosphate has a relatively low solubility, facilitating the precipitation process. In contrast, at pH 4 (acidic), the solubility of magnesium ammonium phosphate increases, making it difficult to obtain the desired precipitate.

To obtain NH3 from NH4Cl, the following reaction can be used:

2 NH4Cl + Ca(OH)2 → 2 NH3 + 2 H2O + CaCl2

In this reaction, NH3 gas is liberated along with water and calcium chloride. By using calcium hydroxide (Ca(OH)2), which is an alkaline compound, NH3 gas can be released from NH4Cl. The liberated NH3 gas can then be used for pH adjustment in the precipitation process.

In the process of precipitating magnesium ammonium phosphate, NH4Cl cannot be used as a substitute for NH3 due to several reasons. Let me explain why:

1. pH Control: In this precipitation reaction, pH plays a critical role. The solubility of magnesium ammonium phosphate (MAP) is significantly different at different pH levels. At higher pH (around 9), MAP is less soluble and can precipitate out more readily. On the other hand, at lower pH (around 4), MAP is more soluble and does not readily precipitate.

2. Function of NH3: NH3 is used in this process primarily for pH adjustment. It acts as a base, raising the pH of the solution to a desired level. By raising the pH to around 9 using NH3, the solubility of MAP decreases, facilitating its precipitation. NH3 also forms complex ions with magnesium and ammonium ions, further aiding the precipitation process.

Now, let's address your second question regarding obtaining NH3 from NH4Cl:

NH3 can be obtained from NH4Cl by a process called "ammonia liberation." To achieve this, NH4Cl is heated strongly, causing it to decompose into NH3 gas and HCl gas. The chemical reaction can be represented as follows:

NH4Cl (s) → NH3 (g) + HCl (g)

The generated NH3 gas can then be collected and used for pH adjustment in the precipitation process.