- Chemical structure
- Nomenclature of acid salts
- Acidic hydric salts
- Ternary acid salts
- Another example
- Training
- Phosphates
- Citrates
- Examples
- Acidic salts of transition metals
- Acid character
- Applications
- References
The acid salts or oxy salts are those derived from the partial neutralization of the hydrohalic and oxoacids. Therefore, binary and ternary salts can be found in nature, either inorganic or organic. They are characterized by having acidic protons available (H +).
Due to this, their solutions generally lead to obtaining acidic media (pH <7). However, not all acid salts exhibit this characteristic; some in fact originate alkaline solutions (basic, with pH> 7).
Sodium bicarbonate
The most representative of all acidic salts is what is commonly known as sodium bicarbonate; also known as baking powder (top image), or with their respective names governed by traditional, systematic or compositional nomenclature.
What is the chemical formula for baking soda? NaHCO 3. As can be seen, it only has one proton. And how is this proton bound? To one of the oxygen atoms, forming the hydroxide group (OH).
So the two remaining oxygen atoms are considered as oxides (O 2–). This view of the chemical structure of the anion allows it to be named more selectively.
Chemical structure
Acid salts have in common the presence of one or more acidic protons, as well as that of a metal and a nonmetal. The difference between those that come from hydracids (HA) and oxoacids (HAO) is, logically, the oxygen atom.
However, the key factor that determines how acidic the salt in question is (the pH it produces once dissolved in a solvent), rests on the strength of the bond between the proton and the anion; it also depends on the nature of the cation, as in the case of the ammonium ion (NH 4 +).
The force HX, being X the anion, varies according to the solvent that dissolves the salt; which is generally water or alcohol. Hence, after certain equilibrium considerations in solution, the acidity level of the mentioned salts can be deduced.
The more protons the acid has, the greater the possible number of salts that can emerge from it. For this reason in nature there are many acid salts, most of which lie dissolved in the great oceans and seas, as well as nutritional components of soils in addition to oxides.
Nomenclature of acid salts
How are acid salts named? Popular culture has taken it upon themselves to assign deeply rooted names to the most common salts; however, for the rest of them, not so well known, chemists have devised a series of steps to give them universal names.
For this purpose, the IUPAC has recommended a series of nomenclatures, which, although they apply the same for hydracids and oxacids, present slight differences when used with their salts.
It is necessary to master the nomenclature of acids before moving on to the nomenclature of salts.
Acidic hydric salts
Hydracids are essentially the bond between hydrogen and a non-metallic atom (of groups 17 and 16, with the exception of oxygen). However, only those that have two protons (H 2 X) are capable of forming acid salts.
Thus, in the case of hydrogen sulfide (H 2 S), when one of its protons is replaced by a metal, sodium, for example, we have NaHS.
What is the NaHS salt called? There are two ways: traditional nomenclature and composition.
Knowing that it is a sulfide, and that sodium has only a valence of +1 (because it is from group 1), we continue below:
Salt: NaHS
Nomenclatures
Composition: Sodium hydrogen sulfide.
Traditional: Sodium acid sulfide.
Another example can also be Ca (HS) 2:
Salt: Ca (HS) 2
Nomenclatures
Composition: Calcium bis (hydrogen sulfide).
Traditional: Acid calcium sulfide.
As can be seen, the prefixes bis-, tris, tetrakis, etc. are added, according to the number of anions (HX) n, where n is the valence of the metal atom. So, applying the same reasoning for Fe (HSe) 3:
Salt: Fe (HSe) 3
Nomenclatures
Composition: Tris (hydrogenoselenide) of iron (III).
Traditional: Acid iron (III) sulfide.
Since iron mainly has two valences (+2 and +3), it is indicated in parentheses with Roman numerals.
Ternary acid salts
Also called oxysalts, they have a more complex chemical structure than acidic hydracid salts. In these, the non-metallic atom forms double bonds with oxygen (X = O), classified as oxides, and single bonds (X-OH); the latter being responsible for the acidity of the proton.
The traditional and composition nomenclatures maintain the same norms as for oxoacids and their respective ternary salts, with the only distinction of highlighting the presence of the proton.
On the other hand, the systematic nomenclature considers the types of XO bonds (of addition) or the number of oxygens and protons (that of hydrogen of anions).
Returning with the baking soda, it is named as follows:
Salt: NaHCO 3
Nomenclatures
Traditional: sodium carbonate.
Composition: Sodium hydrogen carbonate.
Systematics and hydrogen addition of the anions: Hidroxidodioxidocarbonato (-1) sodium, hydrogen (trioxidocarbonato) sodium.
Informal: Baking soda, baking soda.
Where do the terms 'hydroxy' and 'dioxide' come from? 'Hydroxy' refers to the -OH group remaining in the anion HCO 3 - (O 2 C-OH), and 'dioxide' to the other two oxygen on which the C = O double bond “resonates” (resonance).
For this reason the systematic nomenclature, although more exact, is a bit complicated for those initiated into the world of chemistry. The number (-1) is equal to the negative charge of the anion.
Another example
Salt: Mg (H 2 PO 4) 2
Nomenclatures
Traditional: Magnesium diacid phosphate.
Composition: magnesium dihydrogen phosphate (note the two protons).
Systematics and hydrogen addition of the anions: dihidroxidodioxidofosfato (-1) magnesium, bis magnesium.
Reinterpreting the systematic nomenclature, we have that the anion H 2 PO 4 - has two OH groups, so the two remaining oxygen atoms form oxides (P = O).
Training
How are acid salts formed? They are the product of neutralization, that is, of the reaction of an acid with a base. Since these salts have acidic protons, neutralization cannot be complete, but partial; otherwise the neutral salt is obtained, as can be seen in the chemical equations:
H 2 A + 2NaOH => Na 2 A + 2H 2 O (Complete)
H 2 A + NaOH => NaHA + H 2 O (Partial)
Also, only polyprotic acids can have partial neutralizations, since acids HNO 3, HF, HCl, etc., only have a single proton. Here, the acidic salt is NaHA (which is fictitious).
If instead of having neutralized the diprotic acid H 2 A (more precisely, a hydracid), with Ca (OH) 2, then the corresponding calcium salt Ca (HA) 2 would have been generated. If Mg (OH) 2 were used, Mg (HA) 2 would be obtained; if LiOH was used, LiHA; CsOH, CsHA, and so on.
From this it is concluded with regard to the formation, that the salt is made up of the anion A that comes from the acid, and from the metal of the base used for neutralization.
Phosphates
Phosphoric acid (H 3 PO 4) is a polyprotic oxo acid, which is why a large amount of salts are derived from it. Using KOH to neutralize it and thus obtain its salts, we have:
H 3 PO 4 + KOH => KH 2 PO 4 + H 2 O
KH 2 PO 4 + KOH => K 2 HPO 4 + H 2 O
K 2 HPO 4 + KOH => K 3 PO 4 + H 2 O
KOH neutralizes one of the acidic protons of H 3 PO 4, being replaced by the K + cation in the potassium diacid phosphate salt (according to traditional nomenclature). This reaction continues to take place until the same KOH equivalents are added to neutralize all protons.
It can then be seen that up to three different potassium salts are formed, each with its respective properties and possible uses. The same result could be obtained using LiOH, giving lithium phosphates; or Sr (OH) 2, to form strontium phosphates, and so on with other bases.
Citrates
Citric acid is a tricarboxylic acid present in many fruits. Therefore, it has three –COOH groups, which is equal to three acidic protons. Again, like phosphoric acid, it is capable of generating three types of citrates depending on the degree of neutralization.
In this way, using NaOH, mono-, di- and trisodium citrates are obtained:
OHC 3 H 4 (COOH) 3 + NaOH => OHC 3 H 4 (COONa) (COOH) 2 + H 2 O
OHC 3 H 4 (COONa) (COOH) 2 + NaOH => OHC 3 H 4 (COONa) 2 (COOH) + H 2 O
OHC 3 H 4 (COONa) 2 (COOH) + NaOH => OHC 3 H 4 (COONa) 3 + H 2 O
Chemical equations look complicated given the structure of citric acid, but if represented, the reactions would be as simple as those for phosphoric acid.
The last salt is neutral sodium citrate, whose chemical formula is Na 3 C 6 H 5 O 7. And the other sodium citrates are: Na 2 C 6 H 6 O 7, sodium acid citrate (or disodium citrate); and NaC 6 H 7 O 7, sodium diacid citrate (or monosodium citrate).
These are a clear example of acidic organic salts.
Examples
Many acid salts are found in flowers and many other biological substrates, as well as in minerals. However, the ammonium salts have been omitted, which, unlike the others, are not derived from an acid but from a base: ammonia.
How is it possible? It is due to the neutralization reaction of ammonia (NH 3), a base that deprotonates and produces the ammonium cation (NH 4 +). NH 4 +, as well as the other metal cations, can perfectly substitute any of the acidic protons of the hydracid or oxacid species.
In the case of ammonium phosphates and citrates, it is enough to substitute NH 4 for K and Na, and six new salts will be obtained. The same is true with carbonic acid: NH 4 HCO 3 (acidic ammonium carbonate) and (NH 4) 2 CO 3 (ammonium carbonate).
Acidic salts of transition metals
Transition metals can also be part of various salts. However, they are less well known and the syntheses behind them present a higher degree of complexity due to the different oxidation numbers. Examples of these salts include the following:
Salt: AgHSO 4
Nomenclatures
Traditional: Acid silver sulfate.
Composition: Silver hydrogen sulfate.
Systematics: Silver hydrogen (tetraoxidosulfate).
Salt: Fe (H 2 BO 3) 3
Nomenclatures
Traditional: iron (III) diacid borate.
Composition: Iron (III) dihydrogenoborate.
Systematics: Iron Tris (III).
Salt: Cu (HS) 2
Nomenclatures
Traditional: Acidic copper (II) sulfide.
Composition: Copper (II) hydrogen sulfide.
Systematic: Bis (hydrogen sulfide) of copper (II).
Salt: Au (HCO 3) 3
Nomenclatures
Traditional: Acid gold (III) carbonate.
Composition: Gold hydrogen carbonate (III).
Systematics: Golden Tris (III).
And so with other metals. The great structural richness of acid salts lies more in the nature of the metal than that of the anion; since there are not many hydracids or oxacids that exist.
Acid character
Acidic salts generally when dissolved in water give rise to an aqueous solution with a pH lower than 7. However, this is not strictly true for all salts.
Why not? Because the forces that bind the acidic proton to the anion are not always the same. The stronger they are, the less will be the tendency to give it to the middle; Likewise, there is an opposite reaction that makes this fact regress: the hydrolysis reaction.
This explains why NH 4 HCO 3, despite being an acidic salt, generates alkaline solutions:
NH 4 + + H 2 O <=> NH 3 + H 3 O +
HCO 3 - + H 2 O <=> H 2 CO 3 + OH -
HCO 3 - + H 2 O <=> CO 3 2– + H 3 O +
NH 3 + H 2 O <=> NH 4 + + OH -
Given the previous equilibrium equations, the basic pH indicates that the reactions that produce OH - occur preferentially to those that produce H 3 O +, an indicator species of an acid solution.
However, not all anions can be hydrolyzed (F -, Cl -, NO 3 -, etc.); These are those that come from strong acids and bases.
Applications
Each acid salt has its own uses for different fields. However, they can summarize a number of common uses for most of them:
-In the food industry they are used as yeasts or preservatives, as well as in confectionery, in oral hygiene products and in the manufacture of medicines.
-Those that are hygroscopic are intended to absorb moisture and CO 2 in spaces or conditions that require it.
-Potassium and calcium salts generally find uses as fertilizers, nutritional components or laboratory reagents.
-As additives for glass, ceramics and cements.
-In the preparation of buffer solutions, essential for all those reactions sensitive to sudden changes in pH. For example, phosphate or acetate buffers.
-And finally, many of these salts provide solid and easily manageable forms of cations (especially transition metals) with great demand in the world of inorganic or organic synthesis.
References
- Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 138, 361.
- Brian M. Tissue. (2000). Advanced Weak Acid and Weak Base Equilibria. Taken from: tissuegroup.chem.vt.edu
- C. Speakman & Neville Smith. (1945). Acid Salts of Organic Acids as pH-Standards. Nature volume 155, page 698.
- Wikipedia. (2018). Acid Salts. Taken from: en.wikipedia.org
- Identifying Acids, Bases, and Salts. (2013). Taken from: ch302.cm.utexas.edu
- Acidic and Basic Salt Solutions. Taken from: chem.purdue.edu
- Joaquín Navarro Gómez. Acidic hydric salts. Taken from: formulacionquimica.weebly.com
- Encyclopedia of Examples (2017). Acid salts. Recovered from: examples.co