**1. Introduction**

The journey of exploring acid and base starts long before, but in the last century the advancement was remarkable. In 1890, Wilhelm Ostwald electronically measured hydrogen [1]. Svante Arrhenius won the Noble prize in 1903 for the theory of ionization [2]. In 1908, Henderson and Black showed that bicarbonate and phosphate equilibrated with CO2 at normal body temperature in different solution [3]. In 1923, Bronsted first put forward the idea of acid as a substance that ionizes in solution and donate hydrogen and the base accepts the hydrogen from the solution [4]. Bronsted, Henderson and Van Slyke described acid-base balance in the early part of nineteenth century [5]. Handerson invented bicarbonate as the most important buffer system of the body, and Hasselbalch first measured the real blood pH in the early part of nineteenth century [6–8]. In 1909, S. P. S. Sorensen developed the pH scale [8]. Later Hasselbalch-Henderson developed an equation that helped in relating pH to the blood bicarbonate and PCO2 [7, 9, 10]. In the early 1980s, scientists introduced electrodes specific for each ion. Thereafter, serum electrolyte and the anion gap measurement become routine tools for assessing acidosis.

Acidosis has fatal consequences like CNS damage. Even death is not uncommon. Acidosis is characterized by a decrease in pH, and this change is rapidly

corrected by the body buffer systems. Many clinical conditions develop acidosis, as well as ionic derangements and the only correction of the underlying cause can resolve it. There are equal numbers of cations and anions in the blood and among them there are some unmeasured anions. These unmeasured anions can contribute in the clinically important anion gap. In a healthy individual, there is an acceptable range of normal anion gap. But some conditions can increase or decrease this gap. Increased anion gap usually represents metabolic acidosis. Albumin and many other confounding factors influence the anion gap derangements. Accuracy in measuring anion gap is critically important for the evaluation of acidosis.
