Group V Cations: Insoluble Carbonates

Introduction

After several less-soluble chloride, sulfide and hydroxide groups have been removed, a classical scheme can separate calcium, strontium and barium as carbonate solids. These group 2 cations have the same +2 charge, so their carbonate formulas are MCO₃. Their precipitation depends on free CO₃²⁻ at the chosen pH, not merely on total added carbonate material.

Core explanation

For CaCO₃(s) ⇌ Ca²⁺ + CO₃²⁻, Ksp = [Ca²⁺][CO₃²⁻] in the dilute model. SrCO₃ and BaCO₃ have parallel expressions. When free carbonate becomes high enough that Qsp exceeds each Ksp, the solid can form. Because the three carbonates have different solubilities and cation concentrations may differ, their onset values are not identical. Group separation means they are collected as a fraction under a selected condition; later tests distinguish individual members.

Carbonate is a base and participates in equilibria CO₂ ⇌ HCO₃⁻ ⇌ CO₃²⁻. In acidic solution, much of the carbon pool is protonated and free carbonate is low. A less acidic medium makes a carbonate precipitate more likely. If acid is added to a carbonate solid, protonation consumes CO₃²⁻ and can promote dissolution with carbon dioxide formation. This is why carbonate group chemistry is especially pH-dependent.

The group ions are not identified by a shared white precipitate. Calcium carbonate, strontium carbonate and barium carbonate are all pale or white. Flame colours and differences in sulfate or other salt solubilities can provide additional evidence. Calcium tends to give an orange-red flame, strontium a red flame, and barium a green flame under standard visual charts, but overlapping red hues and contamination require confirmation.

Magnesium is also a group 2 element, yet in this chosen classical cation scheme it is usually handled later with the soluble remainder rather than grouped with Ca, Sr and Ba carbonates. That is an operational classification under specified pH and reagent conditions, not a claim that MgCO₃ can never form. Other schemes may place magnesium differently or use additional separations. Use the scheme stated in the problem.

After precipitation, the filtrate can retain sodium, potassium, ammonium and possibly magnesium. This phase accounting matters because a flame-colour signal from the original mixture may be dominated by sodium even if barium is present; the carbonate fraction isolates candidates for a clearer follow-up. A residual dissolved calcium level remains because Ksp is finite, so “all calcium removed” is an approximation.

In a quantitative question, free carbonate must be determined from pH, acid-base constants and carbon mass balance. Substituting total inorganic carbon CT for [CO₃²⁻] can grossly overpredict precipitation at a pH where HCO₃⁻ dominates. If CO₂ gas can escape, CT itself can change, so state whether the system is open or sealed.

The general strategy mirrors earlier groups: choose conditions where the intended carbonates cross their Ksp thresholds, separate the solid, and confirm individual ions using independent behaviour. It is not a single-shot full identification.

Step-by-step reasoning

1. Identify the filtrate after earlier group solids are removed. 2. Determine pH-dependent free CO₃²⁻ rather than using total carbon directly. 3. Compare M²⁺CO₃²⁻ ion products with each Ksp. 4. Treat the white carbonate solids as a group fraction. 5. Distinguish Ca, Sr and Ba with later independent evidence.

Visual explanation

Draw Ca²⁺, Sr²⁺ and Ba²⁺ arrows into separate MCO₃ solids. Beside them show CO₂ /HCO₃⁻/CO₃²⁻ proportions shifting with pH to control the common precipitating ion.

Real-world analogy

Three boats dock at the same harbor when the water level reaches their thresholds. The harbor identifies a group of boats, while their markings are needed to distinguish each one.

Real-world example

A white carbonate fraction from a later cation-analysis stage may contain calcium and barium together. An orange-red and a green flame clue, interpreted with other tests, can support a mixed assignment.

Why?

Why does acid commonly dissolve MCO₃ solids? It protonates CO₃²⁻ to HCO₃⁻ and CO₂-related forms, lowering the free-carbonate ion product and allowing more solid to dissolve.

Common misconception

“Magnesium is Group V because it is a group 2 element.” Analytical groups are defined by separation conditions, not periodic columns; this chosen scheme puts Mg²⁺ in a later fraction.

Worked example

Let [Ca²⁺] = 2.0×10⁻³ M and a hypothetical Ksp(CaCO₃) = 1.0×10⁻⁸ for an exercise. Free carbonate onset is Ksp/[Ca²⁺] = 5.0×10⁻⁶ M. If total carbonate-family concentration is 1.0×10⁻³ M but only 0.1% is free CO₃²⁻, then free carbonate is 1.0×10⁻⁶ M and Qsp is below the supplied Ksp. Total carbon alone would have led to the wrong prediction.

Quick check

1. What carbonate quantity belongs in Ksp for BaCO₃: free CO₃²⁻ or total CO₂ + HCO₃⁻ + CO₃²⁻? Answer: Free CO₃²⁻ activity, not total dissolved inorganic carbon.

Exam focus

Use MCO₃ formulas, note pH-dependent carbonate availability and keep this analytical group distinct from the periodic-table group.

Advanced insight

Carbonate precipitation can be coupled to atmospheric CO₂ uptake or loss. An open system needs a gas-exchange boundary condition as well as acid-base and Ksp relations.

Summary

Ca²⁺, Sr²⁺ and Ba²⁺ can form sparingly soluble carbonate solids in a later group fraction. Their shared white precipitate requires follow-up identification. Free carbonate, controlled by pH and CO₂ exchange, determines precipitation through each Ksp.

Practice questions

1. What formula does Sr²⁺ form with CO₃²⁻? Answer: SrCO₃. 2. Does a white carbonate precipitate uniquely identify Ba²⁺? Answer: No. Ca²⁺ and Sr²⁺ can also form white carbonates. 3. Why is CT not substituted directly into carbonate Ksp? Answer: CT includes protonated carbon species; Ksp uses free CO₃²⁻. 4. Why may a problem place Mg²⁺ in a different analytical group from Ca²⁺? Answer: Group assignment follows operational separation conditions, not periodic-table membership alone.