# The Naked Scientists Forum

### Author Topic: Why did they start at seven?  (Read 4853 times)

#### Geezer

• Neilep Level Member
• Posts: 8328
• "Vive la résistance!"
##### Why did they start at seven?
« on: 22/10/2010 20:52:26 »
Slightly related to another thread here, why does a neutral substance have a pH of 7?  I always wondered why neutral is not zero, with bases and acids having positive and negative values.

Is there any truth to the story that chemists found the concept of negative numbers too challenging?

#### Vereava

• Jr. Member
• Posts: 23
##### Why did they start at seven?
« Reply #1 on: 22/10/2010 22:49:31 »
Perhaps it's because they thought one couldn't have less than zero of anything? But yeah, seven is a very odd number, isn't it?

Edit: after doing some research, I found that the reason it's seven is because that is the value of hydrogen ion activity in distilled water at 25°C, thus creating an average for them to work with. It has to do with math that I don't understand and can't explain... :( (at least I'm honest!)
« Last Edit: 22/10/2010 22:54:44 by Vereava »

#### maffsolo

• Sr. Member
• Posts: 280
##### Why did they start at seven?
« Reply #2 on: 23/10/2010 01:22:27 »
I think Vereava is on track!

"In 1909, the Danish biochemist Sören Sörensen invented the pH scale for measuring acidity.  The pH scale is described by the formula:

pH = -log [H+] Note: concentration is commonly abbreviated by using square brackets,
thus [H+] = hydrogen ion concentration.
When measuring pH, [H+] is in units of moles of H+ per liter of solution.

For example, a solution with [H+] = 1 x 10-7 moles/liter has a pH equal to 7 (a simpler way to think about pH is that it equals the exponent on the H+ concentration, ignoring the minus sign). The pH scale ranges from 0 to 14. Substances with a pH between 0 and less than 7 are acids (pH and [H+] are inversely related - lower pH means higher [H+]). Substances with a pH greater than 7 and up to 14 are bases (higher pH means lower [H+]). Right in the middle, at pH = 7, are neutral substances, for example, pure water. The relationship between [H+] and pH is shown in the table below alongside some common examples of acids and bases in everyday life."

http://www.visionlearning.com/library/module_viewer.php?mid=58

Then I found:

"Historical background for pH:

In the late 1880′s, Svante Arrhenius proposed that acids were substances that delivered hydrogen ion to the solution. He has also pointed out that the law of mass action could be applied to ionic reactions, such as an acid dissociating into hydrogen ion and a negatively charged anion. This idea was followed up by Wilhelm Ostwald, who calculated the dissociation constants (the modern symbol is Ka) of many weak acids.

Ostwald also showed that the value of the constant is measure of an acid’s strength. By 1894, the dissociation constant of water (today called Kw) was measured to the modern value of 1×10-14. In 1904, H. Friedenthal recommended that the hydrogen ion concentration be used to characterize solutions. He also pointed out that alkaline (modern word = basic) solutions could also be characterized this way since the hydroxyl concentration was always (1×10-14 / the hydrogen ion concentration). Many consider this to be the real introduction of the pH scale."

http://profiles.incredible-people.com/soren-peter-lauritz-sorensen/

« Last Edit: 23/10/2010 02:04:54 by maffsolo »

#### Geezer

• Neilep Level Member
• Posts: 8328
• "Vive la résistance!"
##### Why did they start at seven?
« Reply #3 on: 23/10/2010 01:33:48 »
Ah! That's very clever. Thanks!

#### maffsolo

• Sr. Member
• Posts: 280
##### Why did they start at seven?
« Reply #4 on: 23/10/2010 04:06:41 »
Ah! That's very clever. Thanks!
Yes it is clever
I never ever gave it any thought.
I Thank you for bringing it to attention, and Vereava for the clue to put me in the correct direction, now I know!
« Last Edit: 23/10/2010 04:08:38 by maffsolo »

#### The Naked Scientists Forum

##### Why did they start at seven?
« Reply #4 on: 23/10/2010 04:06:41 »