Danish chemist Søren Peter Lauritz Sørensen invented the pH scale in 1909. He developed the concept as a way of measuring acidity while working at the Carlsberg research lab in Copenhagen. Even today, the pH scale is an essential diagnostic tool that’s used to determine the degrees of acidity and alkalinity of a wide range of substances.
Continue reading to learn more about the birth of the pH scale, how it has evolved, and its use in modern chemistry.
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Introduction to the pH scale
The pH scale is a quantitative measurement of the acidity or alkalinity of a substance. Ordinarily, it ranges from 1 to 14, although theoretically the scale could extend infinitely below zero and above fourteen.
As shown in the image below, a pH of 7 is considered neutral (this is the pH of pure water). A substance with a pH above 7 is basic or alkaline, while anything with a pH below 7 is acidic.
Technically, pH is defined as the measure of the activity of hydrogen ions in a solvent. Put simply, the more [H+] ions a solution contains, the more acidic it is. Likewise, the more [OH-] ions a solution contains, the more basic it is.
The concentration of hydrogen/hydronium ions is calculated in terms of the negative log of base 10. Hence, the formula can be written as pH = −log ([H+]). pH is also related and can be calculated from the pKa values of solutions.
The pKa of a solution is based on the titration curve at the equivalence point. Solutions with lower pKa values are less likely to hold on to proton ions, making them acidic.
The birth of the pH scale
Before chemists could determine precise acidity and alkalinity levels, people already had the intuitive – albeit less scientific – means of classifying a substance as acidic or basic. This is particularly true for edible items such as fruits and vegetables.
For example, sour-tasting substances like lemon juice and vinegar can be easily identified as acidic, while bitter-tasting products like beer and dark chocolate are classified as basic.
Understanding which foods and beverages are acidic or basic is very important in the food industry and culinary in general. So it’s perhaps no surprise that the pH scale was invented by a scientist who was working for a beer company! That scientist was Søren Peter Lauritz Sørensen, who in 1909 developed the pH scale while helping Carlsberg with their mission to brew high-quality beer.
Søren Peter Lauritz Sørensen, inventor of the pH scale
In his role as head of chemistry at the Carlsberg Laboratory in Copenhagen, Søren Peter Lauritz Sørensen was tasked with the job of identifying the best method for brewing beer.
As part of his work, he studied the formation of amino acids and how enzymes were made from proteins. After discovering that hydrogen ion concentrations were important to the performance of these enzymes, in 1909 he developed the pH scale as a way to monitor their conditions in a solution.
The original concept of pH scale
Before the invention of the modern pH meter, chemists used indicators to determine the transitions between ranges of acidity and basicity of substances.
For example, the indicator Bromothymol Blue transitions from blue to yellow within a pH range of 6.0 and 7.6.
Measuring the acidity and basicity of substances later involved electrical methods. Pure water is used as the neutral baseline because it doesn’t conduct an electrical current. A highly sensitive and precise galvanometer was then used to measure the migration of ions to oppositely-charged electrodes.
Evolution of the pH scale
When he invented the pH scale in 1909, Sørensen originally used a lowercase p and a subscript uppercase H with a dot – like this: pH• The H clearly represented hydrogen ions, but Sørensen didn’t explain the meaning of the lowercase p.
Some say it must mean “potential” since the method developed by Sørensen involved measuring the electrical potential between oppositely-charged electrodes. However, the exact meaning is still disputed to this day. In modern chemistry, we now use a lowercase p and an uppercase H when referring to the pH scale.
The pH scale in modern science
Modern understanding of the pH scale is based on the dissociation constants of ions in acidic and basic solutions. The lowercase p represents the negative decimal logarithm of hydrogen ions.
Using the dissociation constants gives us a more accurate way of determining the strength of an acidic or a basic solution. The pOH, or the negative decimal natural logarithmic value of hydroxide ion concentrations, can also be derived from this.
Conclusion
Søren Peter Lauritz Sørensen’s invention of the pH scale was a landmark moment because it allowed for more precise quantitative measurements of a solution’s acidity or basicity. Although Sørensen originally devised the concept to improve the beer-making process, his idea soon gained traction in other fields. To this day, the pH scale remains an essential diagnostic tool. It’s used in a variety of sectors, including chemical manufacturing and food processing.
