1. Why a beer laboratory needed to measure acidity and alkalinity
Enzymes are very sensitive to acidity and alkalinity. Beer is made when starch from malt breaks down into sugar, which yeast then ferments. The enzyme called amylase, which breaks down starch, works best when the pH of the brewing mixture is around 5.4 to 5.8. It is said that if it is too acidic or too alkaline, the enzyme's activity drops, and it may even stop working [5]. When the wort is boiled, its pH drops by 0.2 to 0.3, and a pH around 5 is considered suitable for starting yeast fermentation [5]. These numbers are general guidelines for brewing and vary by brand.
Sørensen (1868–1939) was the director of the Carlsberg Laboratory in Copenhagen from 1901 to 1938, where he studied enzymes and proteins [2]. Because the concentration of hydrogen ions was particularly important for enzyme activity, he introduced the pH scale in 1909 as a simple way to represent it [2]. So, what rules determine these numbers?
2. What the "p" in pH means: the rules of the hydrogen ion index
The concentration of hydrogen ions (how many moles are in 1 liter) is represented as C_H, or as 10 to the power of negative X. This X is called the hydrogen ion index [1]. Sørensen called the p in this form the "hydrogen ion index," which is why we write pH [3]. If the concentration is 0.1 moles per liter, the pH is 1; if it is 0.01 moles per liter, the pH is 2 [1]. The larger the number, the fewer hydrogen ions there are.
Pure water at 25°C has a pH of 7 and is neutral. If the value is less than 7, it is acidic; if it is greater than 7, it is alkaline (basic) [3]. Note that neutral being pH 7 applies specifically at 25°C; the neutral value changes if the temperature changes.
The origin of the letter 'p' is not clear. Sørensen proposed the term "hydrogen ion index" but did not explain why he chose the letter 'p' [3]. There are theories that it comes from the French word *puissance* or the German word *Potenz* (both meaning "power" or "exponent"), but this is not a confirmed fact [3]. By the way, if the number differs by 1, how much does the actual concentration change?
3. Comparing rain and stomach acid: how much their hydrogen ion concentrations differ
According to the Japan Meteorological Agency, when carbon dioxide in the air is fully dissolved in pure water, the pH is 5.6, which is the standard for ordinary rain. There is no absolute numerical definition of acid rain: the term refers to rain made more acidic by acidic substances from human activities, and pH 5.6 or lower is sometimes used as a guideline [6]. The pH of ordinary rain is not 7.
Stomach acid is strongly acidic, with a pH of 1.0 to 1.5. Its hydrochloric acid activates pepsinogen, a digestive enzyme precursor, to help digest food, and also acts to kill bacteria [8]. Since values vary by source, these are treated as approximate figures.
The difference between rain (pH 5.6) and stomach acid (pH 1.0–1.5) is 4.1 to 4.6. This corresponds to a concentration ratio of approximately 13,000 to 40,000 times (calculated values for the upper and lower limits).
According to the Ministry of the Environment's acid rain survey, the annual national average pH of precipitation in Japan for fiscal years 1993 to 1995 was 4.8 to 4.9 [7]. Since 10 to the power of 0.8 is about 6.3, rain with a pH of 4.8 contains about 6.3 times more hydrogen ions than rain with a pH of 5.6. These are values from the 1990s survey, not current values.
4. Two ways to measure pH: electrodes and color comparison
According to the English Wikipedia, Sørensen's 1909 paper dealt with the measurement and meaning of hydrogen ion concentration in enzyme reactions and presented two methods: an electrical method using platinum electrodes, and a method comparing the color of indicators with standard colors [2]. Thus, color comparison was included in the same paper as the electrode method.
In school, students learn that pH equals negative log of concentration. However, the current IUPAC definition uses the "activity" of hydrogen ions rather than concentration. Because the activity of a single ion requires an activity coefficient, pH cannot be measured on its own and remains a "conceptual definition" [4]. Therefore, actual measurements are performed by comparing with standard solutions.
This scale is not just a tool for knowing acidity and alkalinity. It also plays an important role inside the body.
5. Why blood pH is kept within a narrow range of just 0.1
Normal blood is slightly alkaline, with a pH of approximately 7.35 to 7.45. Even a slight deviation from the normal range can have major effects on many organs, so the body strictly regulates blood pH [9].
A range of 0.1 in pH corresponds to a hydrogen ion concentration difference of about 1.26 times (10 to the power of 0.1). This is a much narrower range compared to the 10,000-fold difference between rain and stomach acid.
Blood pH is protected from sudden changes toward acidity or alkalinity by chemical buffer systems, such as carbonic acid and bicarbonate ions [9]. The scale created in 1909 is still used today as a tool to express that range in numbers.
6. How to investigate pH further at home
Look at the containers of detergents, soaps, and cosmetics at home to find items labeled "weakly acidic" or with "pH" values. If you find them, simply copy the numbers into a notebook. Do not mix them or put them in your mouth.
If you want to read more, try opening the Japan Meteorological Agency's page on acid rain [6] or the Ministry of the Environment's announcement on the acid rain survey [7].
You might also become curious to see the original 1909 paper by Sørensen to see what diagrams and tables showed the two measurement methods.