Boric acid

Key takeaways

Boric acid belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Boric acid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Boric acid from memory before moving on to harder problems.

Reference excerpt

Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula B(OH)3. It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves in water, and occurs in nature as the mineral sassolite. It is a weak acid that yields various borate anions and salts, and can react with alcohols to form borate esters. Boric acid is often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other boron compounds. The term "boric acid" is also used generically for any oxyacid of boron, such as metaboric acid HBO2 and tetraboric acid H2B4O7.

History Orthoboric acid was first prepared by Wilhelm Homberg (1652–1715) from borax, by the action of mineral acids, and was given the name sal sedativum Hombergi ("sedative salt of Homberg"). However, boric acid and borates have been used since the ancient history for cleaning, preserving food, and other uses.

Molecular and crystal structure The three oxygen atoms form a trigonal planar geometry around the boron. The B-O bond length is 136 pm, and the O-H is 97 pm. The molecular point group is C3h. Two crystalline forms of orthoboric acid are known: triclinic with space group P1, and trigonal with space group P32. The former is the most common; the second, which is a bit more stable thermodynamically, can be obtained with a special preparation method. The triclinic form of boric acid consists of layers of B(OH)3 molecules held together by hydrogen bonds with an O...O separation of 272 pm. The distance between two adjacent layers is 318 pm. While the layers of the triclinic phase are nearly trigonal with γ = 119.76°, a = 701.87 pm, and b = 703.5 pm (compared to a = 704.53(4) pm for the trigonal form), the stacking of the layers is somewhat offset in the triclinic phase, with α = 92.49° and β = 101.46°. The triclinic phase has c = 634.72 pm and the trigonal one has a = 956.08(7) pm.

Preparation Boric acid may be prepared by reacting borax (sodium tetraborate decahydrate) with a mineral acid, such as hydrochloric acid:

Na2B4O7·10H2O + 2 HCl → 4 B(OH)3 + 2 NaCl + 5 H2O It is also formed as a byproduct of hydrolysis of boron trihalides and diborane:

B2H6 + 6 H2O → 2 B(OH)3 + 6 H2 BX3 + 3 H2O → B(OH)3 + 3 HX (X = Cl, Br, I)

Reactions

Pyrolysis When heated, orthoboric acid undergoes a three-step dehydration. The reported transition temperatures vary substantially from source to source. When heated above 140 °C, orthoboric acid yields metaboric acid (HBO2) with loss of one water molecule:

B(OH)3 → HBO2 + H2O Heating metaboric acid above about 180 °C eliminates another water molecule forming tetraboric acid, also called pyroboric acid (H2B4O7):

4 HBO2 → H2B4O7 + H2O Further heating (to about 530 °C) leads to boron trioxide:

H2B4O7 → 2 B2O3 + H2O

Aqueous solution When orthoboric acid is dissolved in water, it partially dissociates to give metaboric acid:

B(OH)3 ⇌ HBO2 + H2O The solution is mildly acidic due to the ionization of the acids:

B(OH)3 + H2O ⇌ [BO(OH)2]− + H3O+ HBO2 + H2O ⇌ [BO2]− + H3O+ However, Raman spectroscopy of strongly alkaline solutions has shown the presence of [B(OH)4]− ions, leading some to conclude that the acidity is exclusively due to the abstraction of OH− from water:

B(OH)3 + HO− ⇌ B(OH)−4 Equivalently,

B(OH)3 + H2O ⇌ B(OH)−4 + H+ (Ka = 7.3×10−10; pKa = 9.14) Or, more properly,

B(OH)3 + 2 H2O ⇌ B(OH)−4 + H3O+ This reaction occurs in two steps, with the neutral complex aquatrihydroxyboron B(OH)3(OH2) as an intermediate:

B(OH)3 + H2O → B(OH)3(OH2) B(OH)3(OH2) + H2O → [B(OH)4]− + H3O+ This reaction may be characterized as Lewis acidity of boron toward HO−, rather than as Brønsted acidity. However, some of its behaviour towards some chemical reactions suggest it to be a tribasic acid in the Brønsted-Lowry sense as well. Boric acid, mixed with borax Na2B4O7·10H2O (more properly Na2B4O5(OH)4·8H2O) in the weight ratio of 4:5, is highly soluble in water, though they are not so soluble separately.

Sulfuric acid solution Boric acid also dissolves in anhydrous sulfuric acid according to the equation:

B(OH)3 + 6 H2SO4 → [B(SO4H)4]− + 2 [HSO4]− + 3 H3O+ The product is an extremely strong acid, even stronger than the original sulfuric acid.

Esterification Boric acid reacts with alcohols to form borate esters, B(OR)3 where R is alkyl or aryl. The reaction is typically driven by a dehydrating agent, such as concentrated sulfuric acid:

B(OH)3 + 3 ROH → B(OR)3 + 3 H2O

With vicinal diols The acidity of boric acid solutions is considerably increased in the presence of cis-vicinal diols (organic compounds containing similarly oriented hydroxyl groups in adjacent carbon atoms, (R1,R2)=C(OH)−C(OH)=(R3,R4)) such as glycerol and mannitol. The tetrahydroxyborate anion formed in the dissolution spontaneously reacts with these diols to form relatively stable anion esters containing one or two five-member −B−O−C−C−O− rings. For example, the reaction with mannitol H(HCOH)6H, whose two middle hydroxyls are in cis orientation, can be written as:

B(OH)3 + H2O ⇌ [B(OH)4]− + H+ [B(OH)4]− + H(HCOH)6H ⇌ [B(OH)2(H(HCOH)2(HCO−)2(HCOH)2H)]− + 2 H2O [B(OH)2(H(HCOH)2(HCO−)2(HCOH)2H)]− + H(HCOH)6H ⇌ [B(H(HCOH)2(HCO−)2(HCOH)2H)2]− + 2 H2O Giving the overall reaction:

B(OH)3 + 2 H(HCOH)6H ⇌ [B(H(HCOH)2(HCO−)2(HCOH)2H)2]− + 3 H2O + H+ The stability of these mannitoborate ester anions shifts the equilibrium to the right, thereby increasing the solution's acidity by five orders of magnitude compared to that of pure boric oxide. This lowers the pKa from 9 to below 4 for a sufficient concentration of mannitol. The resulting solution is referred to as mannitoboric acid. The addition of mannitol to an initially neutral solution containing boric acid or simple borates lowers its pH enough for it to be titrated by a strong base such as NaOH, including with an automated potentiometric titrator. This property is used in analytical chemistry to determine the borate content of aqueous solutions, for example to monitor the depletion of boric acid by neutrons in the water of the primary circuit of light-water reactor when the compound is added as a neutron poison during refueling operations.

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Illustrations

Worked examples

Example 1 — a first encounter with Boric acid

Start with the simplest possible case. Write down what Boric acid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Boric acid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Boric acid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Boric acid

In research: Boric acid appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Boric acid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Boric acid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Antifungals for dermatologic use, Antiseptics, Ants, so understanding it makes those chapters shorter.
In everyday life: Look for Boric acid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Boric acid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Boric acid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Boric acid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Boric acid in simple terms?

Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula B(OH)3. It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid.

Why does Boric acid matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Boric acid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Boric acid.