Nitrate and Sulfate Ions

H2O, H+


Nitrate ion

Nitrate is the anion of nitric acid. Because nitric acid is a strong acid (pKa = -1), nitrate does not cause a significant  pH change in a solution. All nitrates are water soluble. Most metals form nitrate complexes, which are ion pairs to be specific. An ion pair differs from a complex in its bond: Complexes form a coordinate bond, when ion pairs are bonded only temporarely by the electrostatic force.


Sulfate ion

Hydrogen sulfate and sulfate are the anions of sulfuric acid. Sulfuric acid is a strong acid based on it’s first protolysis reaction (pKa1 = −3). Hydrogen sulfate is also a relatively strong acid (pka2 = 1,9). Sulfate on the other hand is nearly neutral. Most sulfates are water soluble, slightly soluble ones are lead(II), strontium and barium sulfates. Sulfate also forms mostly ion pairs.



 Silver(I) nitrate dissolves readily in water (solubility = 234 g/100g H2O). Silver(I) sulfate (pKs = 4,82) is slightly soluble in water. Silver(I) ions also form a sulfate complex (lgβ1 = 1,3).



Lead(II) nitrate is water soluble (solubility = 59,7 g/100 g H2O). Sulfate precipitates white lead(II) sulfate (pKs = 7,79), which is slightly soluble in acids. Lead(II) sulfate dissolves in concentrated hydroxide, acetate and tartrate solutions. Lead(II) ions also form a sulfate complex (lgβ1 = 2,69).



Mercury(I) nitrate and mercury(II) nitrate are water soluble (solubility g/100 g H2O). Mercury(I) sulfate and mercury(II) sulfate are slightly soluble (pKs = 6,13 and). Mercury(I) and (II) ions also form sulfate complexes (lgβ(I) = 1,3 and 2,4, lgβ(II) = 1,34 and 2,4).



 For dissolution and hydrolysis see H2O, OH. Bismuth(III) ions form weak nitrate (lgβ = 1,7, 2,5, 1,1 and 0,4) and sulfate complexes (lgβ = 1,98, 3,41, 4,08, 4,34 and 4,60).



Copper(II) nitrate and sulfate dissolve readily in water (145 and 22,0 g/100g H2O). Copper(II) ions also form a sulfate complex (lgβ1 = 2,36).



Cadmium nitrate and sulfate are water soluble (solubility = 156 ja 76,7 g/100 g H2O). Cadmium ions also form a sulfate complex (lgβ1 = 2,37).





Cobalt nitrate and sulfate dissolve in water (solubility = 56,2 ja 38,3 g/100 g H2O) but the sulfate dissolves slowly. Cobalt ions also form a sulfate complex (lgβ1 =2,30).



Nickel(II) nitrate and sulfate are water soluble (solubility = 99,2 ja 40,4 g/100 g H2O). Nickel(II) ions also form a sulfate complex (lgβ1 = 2,30).



Manganes(II) nitrate and sulfate are water soluble (solubility = 161 ja 63,7 g/100 g H2O). Manganese(II) ions also form a sulfate complex (lgβ1 =2,25).



 Iron(II) nitrate and sulfate are water soluble (solubility = 87,5 and 29,5 g/100 g H2O). Iron(III) nitrate and sulfate are also water soluble (solubility = 82,5 and 440 g/100 g H2O). Iron(II) and (III) ions also from suflate complexes (lgβ1(II) = 2,39 and lgβ1(III) = 4,05).


Zinc nitrate and sulfate are water soluble (solubility = 120 and 55,7 g/100 g H2O). Zinc also has sulfate complexes (lgβ1 = 2,34).



Aluminium nitrate and sulfate are water soluble (solubility = 68,9 ja 38,5 g/100 g H2O). Sulfate has a tendency to form alums with aluminium ions, which are double sulfates of alkali metals with the general formula of MAl(SO4)2∙12H2O, where M = monovalent cation or ammonium. Aluminium ions also form a suflate complex (lgβ1 = 3,89).



Chrome (III) nitrate and sulfate are water soluble (solubility = xxx, 64 g/100 g H2O).  Chrome(III) ions also form a suflate complex (lgβ1 = 2,60).



 Magnesium nitrate and sulfate are water soluble (solubility = 71,2 and 35,7 g/100 g H2O). Magnesium ions also form a suflate complex (lgβ1 = 2,26).



Calcium nitrate is water soluble (solubility = 144 g/100 g H2O). White calcium sulfate is somewhat slightly soluble (pKs = 4,61(dihydrate or gypsum)) and it is precipitated from concentrated calcium solutions.  Calcium ions also form a suflate complex (lgβ1 = 2,36).



Strontium nitrate is water soluble (solubility = 80,2 g/100 g H2O). White strontium sulfate is slightly soluble (pKs = 6,62). Strontium ions also form a suflate complex (lgβ1 = 2,30).



Barium nitrate is water soluble (solubility = 10,3 g/100 g H2O). White barium sulfate is insoluble in water (pKs = 9,98) and dissolves only in hot concentrated sulfuric acid. Barium ions also form a suflate complex (lgβ1 = 2,13).



Sodium nitrate and sulfate are water soluble (solubility = 91,2 ja 28,1 g/100 g H2O).



Potassium nitrate and sulfate are water soluble (solubility = 38,3 ja 12,0 g/100 g H2O).



Ammonium nitrate and sulfate are water soluble (solubility = 213 ja 76,4 g/100 g H2O).




Nitrate ion

Both nitric acid as well as nitrate are powerful oxidizing agents, when their nitrogen at oxidation state +5 reduces to lower states. When metals are dissolved in acids the oxidizing agent is usually the hydrogen ion, which itself reduces to free hydrogen. However, when nitric acid dissolves metals hydrogen is not created, because nitrate reduces instead of the hydrogen ions, it being the stronger oxidizer. Depending on the concentration of the nitric acid, the reaction will create nitrogen dioxide, nitrogen monoxide, dinitrogen monoxide or ammonium.


Concentrated nitric acid:


NO3 + 2 H+ + e NO2↑ +2 H2O

E° = +0,775 V

Dilute nitric acid:


NO3 + 4 H+ + 3 e NO + 2 H2O

E° = +0,957 V

Extremely dilute nitric acid:


2 NO3 + 10 H+ + 8 e N2O + 5 H2O

E° = +1,116 V


NO3 + 10 H+ + 8 e NH4+ + 3 H2

NO3 + 6 H2O + 8 e NH3 + 9 OH

E° = +0,875 V


E = -0,12 V

Due to its ability to oxidize all metals except gold and platinum dissolve in nitric acid. However nitric acid will passivate for example chromium as it creates a protective layer of oxides on the surface.


Reduction agents (fine Zn, Al or Devarda’s alloy: 45 % Al, 5 % Zn ja 50 % Cu) reduce in an alkali solution nitrate into ammonia, when the solution is heated in a large test tube using a water bath. The ammonia being released is identified by holding moist pH paper above the test tube. If too much Devarda’s alloy has been added, the reaction will initially create hydrogen.


NO3 + 6 H2O + 8 e NH3 + 9 OH

Zn↓ + 2 OH + 2 H2O [Zn(OH)4]2– + H2

 E = -0,12 V

Sodium hydroxide that is travelling with the hydrogen will also colour the pH paper. Ammonia is identified only after the develepoment of hydrogen has ceased. Ammonium needs to be removed from the solution before nitrate can be indicated, because ammonium also turns to ammonia in a sodium hydroxide solution.


In a concentrated sulfuric acid solution iron(II) ions reduce nitrate into nitrogen monoxide, which reacts with iron(II) ions, forming dark brown nitrosyl iron(II) ions. The experiment is preformed by adding saturated iron(II) sulfate solution to the examined nitrate solution, then carefully and without shaking trickling in concentrated sulfuric acid along the side of the test tube. The complexes that are then created, form a dark brown ring at the boundary surface of the liquids.


4 Fe2+ + 4 H+ + NO3   [Fe(NO)]2+ + 3 Fe3+ + 2 H2O


The reaction is disrupted by bromide and iodide ions, which concentrated sulfuric acid will oxidize into free halogens. These will colour the surface similarily.


Sulfate ion

Hot concentrated sulfuric acid is a strong oxidizing agent. It oxidizes i.a. bromide and iodide ions into the corresponding halogens, reducing itself into sulfur dioxide. Many metals are also oxidized, for example:


Zn↓ + 4 H+ + SO42– Zn2+ + SO2↑ + 2 H2O


Cold dilute sulfuric acid is in turn a very weak oxidizing agent (E° = +0,158 V). The strength differs for two reasons. Firstly, in a hot and concentrated solution the equilibrium between sulfite and sulfur dioxide is strongly on the side of sulfur dioxide.


H2SO3 H2O + SO2


The increase of escaping sulfur dioxide also increases the removal of sulfite from the solution, following Le Chatelier’s principle, and ultimately driving foward the reduction reaction of sulfate. Secondly, in greater temperatures and concentrations the thermodynamic properties of the species of the sulfate-sulfite system change significantly. For example the concentration of sulfate ions becomes insignificant and the concentration of hydrogen ions grows rapidly. Together these changes increase the ablility to oxidize.


Other reactions


Nitrate ion

Diphenylamine turns blue in concentrated sulfuric acid from the effects of nitrate and other oxidizing agents. The reaction is very sensitive.


Solid nitrates decompose when heated:


NH4NO3 N2O↑ + 2 H2O



2 KNO3 KNO2 + O2



2 AgNO3 2 Ag↓ + 2 NO2↑ + O2



             2 Cu(NO3)2 2 CuO↓ + 4 NO2↑ + O2


Sulfate ion

Barium ions precipitate sulfate as fine white bariumsulfate, which is insoluble in dilute hydrochloric acid and nitric acid, but when isolated is slightly soluble in concentrated sulfuric acid. If the concentration of sulfate is low, the white percipitate can be hard to detect. In that case a few drops of 0,02 M potassium permanganate solution can be added. Permanganate ions are absorbed to the surface of barium sulfate turning the precipitate pink. The remaining free permanganate in the solutin can be reduced with 3% hydrogen peroxide. Hydrogen peroxide does not reduce the absorbed permanganate.