Acids and Bases

Acids and bases

Acid: An "acid" is defined, according to (Lowry and Bronsted), as a substance  which, in solution, tends to liberate hydrogen ions (protons).

These hydrogen ions may

1.   remain as such

2.   may combine with the solvent forming, e.g., hydroxonium ions, H30⁺

3.   may combine with negatively charged ions.

 Base: A “base " is defined as a substance which tends to accept hydrogen ions.

Thus

H⁺ donor ↔ H⁺ + H⁺ acceptor

                                  (Acid)     (Base)

·        Acids, may not always be  undissociated molecules.

·        Bases, may be molecules, though the class includes all anions.

Thus the cation NH₄⁺ liberated from ammonium salts is an acid because it is capable of producing hydrogen ions according to the

equation:

NH₄⁺ ↔ H⁺ + NH₃

                                                (Acid)            (Base)

Similarly the anion HPO₄^-2 is a base because it can accept H⁺ to form H₂PO₄^- and the acid molecule H₃P0₄.

The anions H₂PO₄^- and HPO₄^-2 can also act as acids because they can lose H⁺ to form the base PO₄^-3 (" amphoteric").

Strength of acids

The " strength " of an acid is measured by its tendency to donate hydrogen ions ( by its degree of dissociation)

·        The freely dissociated mineral acids (HCI, etc.) are strong acids; slightly dissociated acids such as carbonic acid are weak.

·        A strong base has a great affinity for hydrogen ions whereas a weak base is a poor acceptor.

·        The anion of a strong acid is a weak base, e.g., Cl-, and the anion of a weak acid is a strong base, e.g., HCO3 -, OH-.

As I was searching literature for a short  and crisp note on acids and bases for the students of medicine, I came across a brilliant paper by 

Frazer and Stewart (J. clin. Path. (1959), 12, 195). I would advise the readers to go through the paper once if time permits. On my part I would definitely strive to elaborate upon all the subtopics explained in the paper and of course all the related material as well. Subsequent posts will lay stress on the physiological buffers and acidosis/ alkalosis.

I wish to have suggestions from the readers in order to improve my blog. Please feel free to ask any doubts related to these topics. As far as the explanation is concerned I think these topics can act as ready-made notes to be covered before each professional examination.

Poorvi

Serum anion gap

Anion Gap

Anion gap (plasma or serum) is an entity which can be calculated from the electrolytes measured in laboratory.

Definition:

“Anion gap is defined as the sum of serum chloride and bicarbonate concentrations subtracted from the serum sodium concentration.”

Normal value:  can vary widely, reflecting both differences in the methods that are used to measure its constituents and substantial interindividual variability. The average serum anion gap in healthy individuals measured varies from 11 ± 2.5   to 15 ±2.5 m eq/l .

Indications: This entity is used mainly for the detection and analysis of acid-base disorders, assessment of quality control in the chemical laboratory, and detection of such disorders as multiple myeloma, bromide intoxication, and lithium intoxication.

Gamblegram:

serum cations=serum anions

The sum of both entities must always be equal. This equivalency is the foundation of the derivation of the serum anion gap.

A graphic display of the ionic environment of the serum (Gamble J, 1950)

Total cations in serum = total anions in serum =155 meq/l

As a routine procedure, only sodium, potassium, chloride, bicarbonate ions are measured. Therefore other remaining cations and anions are called unmeasured cations (UC) and unmeasured anions (UA) respectively.

UC include magnesium (3 meq/l) and calcium (5 meq/l) ions mainly.

UA include proteins (16 meq/l); phosphate (2 meq/l), sulphate (1 meq/l), other miscellaneous anions( 6 meq/l).

Therefore,

Na⁺ + K⁺ + UC = Cl^- + HCO₃^-+ UA……….(i)

By rearranging:

Na⁺ + K⁺  - (Cl^- + HCO₃^-) = UA-UC..........(ii)

Under normal conditions, in a healthy individual, UA> UC (UA=25 meq/l; UC=8 meq/l); therefore, there is anion gap.

The concentration of potassium in the blood usually is relatively small compared with that of sodium, chloride, and bicarbonate;

therefore, many a times when calculating the anion gap potassium ion is omitted as

UA-UC = Na⁺ - (Cl^- + HCO₃^-) …………(iii)

Measured cations (MC)= 147 meq/l

Measured anions (MA) = 130 meq/l

MC-MA = UA-UC

147-130= 25-8

17=17

taken from an indepth review by Kraut and Madias (Clin J Am Soc Nephrol 2: 162–174, 2007).

Deviations in the normal values of serum anion gap would be discussed in some other post.