Science, Tech, Math › Science Complex Ions and Precipitation Reactions Qualitative Analysis Reactions Share Flipboard Email Print Cultura RM Exclusive/Matt Lincoln / Getty Images Science Chemistry Molecules Basics Chemical Laws Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 23, 2018 Among the most common reactions in the qualitative analysis are those involving the formation or decomposition of complex ions and precipitation reactions. These reactions may be performed directly by adding the appropriate anion, or a reagent such as H2S or NH3 may dissociate in water to furnish the anion. Strong acid may be used to dissolve precipitates containing a basic anion. Ammonia or sodium hydroxide may be used to bring a solid into solution if the cation in the precipitate forms a stable complex with NH3 or OH-. A cation is usually present as a single principal species, which may be a complex ion, free ion, or precipitate. If the reaction goes to completion the principal species is a complex ion. The precipitate is the principal species if most of the precipitate remains undissolved. If a cation forms a stable complex, the addition of a complexing agent at 1 M or greater generally will convert the free ion to complex ion. The dissociation constant Kd can be used to determine the extent to which a cation is converted to a complex ion. The solubility product constant Ksp can be used to determine the fraction of cation remaining in a solution after precipitation. Kd and Ksp are both required to calculate the equilibrium constant for dissolving a precipitate in a complexing agent. Complexes of Cations with NH3 and OH- Cation NH3 Complex OH- Complex Ag+ Ag(NH3)2+ -- Al3+ -- Al(OH)4- Cd2+ Cd(NH3)42+ -- Cu2+ Cu(NH3)42+ (blue) -- Ni2+ Ni(NH3)62+ (blue) -- Pb2+ -- Pb(OH)3- Sb3+ -- Sb(OH)4- Sn4+ -- Sn(OH)62- Zn2+ Zn(NH3)42+ Zn(OH)42- Cite this Article Format mla apa chicago Your Citation Helmenstine, Anne Marie, Ph.D. "Complex Ions and Precipitation Reactions." ThoughtCo, Aug. 27, 2020, thoughtco.com/complex-ions-and-precipitation-reactions-608170. Helmenstine, Anne Marie, Ph.D. (2020, August 27). Complex Ions and Precipitation Reactions. Retrieved from https://www.thoughtco.com/complex-ions-and-precipitation-reactions-608170 Helmenstine, Anne Marie, Ph.D. "Complex Ions and Precipitation Reactions." ThoughtCo. https://www.thoughtco.com/complex-ions-and-precipitation-reactions-608170 (accessed September 26, 2021). copy citation Double Replacement Reaction Definition Ionic Compound Properties, Explained A to Z Chemistry Dictionary Ammonium Hydroxide Facts and Formula Definition of Precipitation Reaction Chemistry of Hard and Soft Water Rhodium, a Rare Platinum Group Metal, and Its Applications Formulas of Ionic Compounds Normality Definition in Chemistry Chemistry Vocabulary Terms You Should Know Qualitative Analysis in Chemistry Methods for Protein Purification in Biotechnology Bronsted-Lowry Acid Definition Precipitate Definition and Example in Chemistry Minerals of the Earth's Surface What Is the Difference Between Molarity and Normality?