5.13
沉淀滴定曲线：分析
沉淀滴定曲线显示了一种反应物浓度随滴定剂体积所发生的变化。在用硝酸银来滴定氯离子的过程中，沉淀滴定曲线可以分为三个区域：等当点之前、当点之间和当点之后。在等当点之前，由于微溶氯化银沉淀的再溶解度较低，从而导致银离子的浓度较低。然而，在代表当量点的第二区域中，随着氯离子被完全消耗，银离子的浓度便会急剧…
在用硝酸银滴定氯离子的过程中，由于微溶的氯化银沉淀再溶解有限，初始的银离子浓度较低。
在等当点时，氯离子被完全消耗，银离子浓度急剧上升。
等当点之后，较高的银离子浓度主要来自过量的滴定剂。
滴定曲线的形状受沉淀物溶度积的影响。
碘化银较低的溶度积表明银离子浓度较低，并且在等当点处产生更大的突跃。
较大的氯化银溶度积表明银离子浓度较高，且在等当点处的突跃较小。
当碘化物和氯化物的混合物用硝酸银滴定时，由于碘化银的溶度积较小，会首先析出沉淀。
接近第一个等当点时，随着碘离子几乎被完全消耗，银离子浓度逐渐升高，直至达到氯化银的溶度积。当所有氯离子完全沉淀后，出现第二个等当点，银离子浓度再次上升。
View the full transcript and gain access to JoVE Core videos
Q1: Why is the silver ion concentration low before the equivalence point in a chloride titration?
Before the equivalence point, silver ion concentration remains low because silver chloride is a sparingly soluble precipitate. Limited redissolution of this precipitate keeps silver ions in solution at minimal levels. As titrant is added, most silver ions are consumed forming the solid precipitate rather than remaining dissolved.
Q2: What causes the sharp increase in silver ion concentration at the equivalence point?
At the equivalence point, chloride ions are completely consumed, so silver ions are no longer being removed from solution by precipitation. This sudden shift causes silver ion concentration to rise sharply. Beyond this point, excess silver nitrate titrant directly increases the silver ion concentration in solution.
Q3: How does the solubility product affect the shape of a precipitation titration curve?
The solubility product determines the magnitude of the concentration break at the equivalence point. Silver iodide, with a lower solubility product than silver chloride, produces a larger break because less silver remains dissolved. Conversely, silver chloride's higher solubility product results in a smaller break and higher silver concentration before the equivalence point.
Q4: Why does silver iodide precipitate before silver chloride in a mixed halide titration?
Silver iodide precipitates first because it has a much lower solubility product than silver chloride. This means iodide ions are removed from solution at lower silver ion concentrations. Once iodide is nearly consumed, silver concentration rises until it reaches the solubility product threshold for silver chloride precipitation.
Q5: What are the two equivalence points observed when titrating a mixture of iodide and chloride?
The first equivalence point occurs when all iodide ions are consumed. After this point, silver concentration increases until silver chloride begins to precipitate. The second equivalence point marks complete consumption of chloride ions, after which silver concentration rises again due to excess titrant.
Q6: How does the titration curve change in the region after the equivalence point?
After the equivalence point, the silver ion concentration is high and increases linearly with added titrant volume. This region is dominated by excess silver nitrate, which directly contributes silver ions to the solution. The curve shows a steep positive slope as no precipitation reaction consumes the added silver ions.
Q7: What role does reactant concentration play in determining the precipitation titration curve shape?
Reactant concentration influences the overall magnitude and steepness of the titration curve. Higher concentrations of chloride or other halides affect how quickly the equivalence point is reached and the degree of concentration change observed. Combined with the solubility product, reactant concentration determines the curve's characteristic shape and break magnitude.