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Course: MCAT > Unit 3

Lesson 2: Foundation 5: Chemical structures, reactions, and interactions

Ions in solution: Qualitative analysis of water supply


A variety of chronic disorders may develop from the accumulation of non-essential trace elements. Humans have been exposed to such elements only recently due to extensive mining. Because humans did not encounter these elements during the course of evolution, they have not developed homeostatic mechanisms to accommodate or eliminate these elements from their bodies. For example, cadmium has been linked to the development to itai-itai disease which has symptoms such as renal failure, weak bones, and joint and spine pain. Studies indicate that elevated cadmium levels, due to inefficient removal from the body by the kidneys ,leads to mitochondrial damage and eventually renal failure.
Patients all originating from one manufacturing town and its environs have been reporting at various hospitals with wide-ranging symptoms, including headache, vomiting, muscle pain, memory loss, constipation, and high blood pressure. To determine what may be responsible, an investigator can use qualitative analysis to identify the ions present in a sample by systematically conducting different chemical tests. The most common test is selective precipitation, which separates cations into groups based on their solubility under specific conditions. The reagents are added sequentially to the same solution throughout testing. After separation, the individual ions can be identified by confirmatory tests, which can include observing their color, odor, appearance, and solubility.
A systematic qualitative analysis is conducted on a water sample at a local lab, and the results are in the table below:
Table 1. Qualitative Analysis of Water Sample by Precipitating Cation Groups with Reagents
Cation CategoryAnion PrecipitatedReagent Added (in sequential order)Precipitate Color
1Cldilute HClWhite
2S2 (acid insoluble)H2S in concentrated HNO3No precipitate
3OHNH4Cl/NH3 buffer (solid NH4Cl and 16M NH3)Brown
4S2 (base insoluble)H2S in NH4Cl/NH3 bufferWhite
5PO43(NH4)3PO4 and concentrated NH3White
6NoneNoneNo precipitate
Many of the tests for identifying cations have been superseded by techniques like mass spectrometry and absorption spectrophotometry due to the range of chemicals the instruments can detect. However, traditional qualitative analysis can be still useful in field work or other situations where access is not expedient. A neighboring facility conducted a spectroscopic analysis of a sample with another suspected pollutant, and the results are below:
Figure 1. Absorption Spectrogram of Water Sample from Adjacent Town
The addition of nitric acid to the water sample produces a colorless and odorless gas. What is the likely reaction when nitric acid is added to the sample?
Choose 1 answer: