Sciencemadness Discussion Board - Understanding (1+1) Reagent

Understanding (1+1) Reagent

RU_KLO - 14-11-2023 at 11:04

In some books one can read a (1+1) H2SO4 solution.

From the AOAC:
"in expressions (1+2) (5+4), etc used in connection with name of a reagent, first numeral indicates volume reagent used, and second numeral indicates volume of H2O. For example: HCl (1+2) means reagent prepared by mixing one volume of HCl + 2 Volumes of H2O."

From this:
Where does concentration (molarity) takes part? (I think is not the same (1+1) 36%HCl than (1+1) 20%HCl)

Also it speaks of volume, and not weight. (what I mean is v/v , w/w; w/v?)

for example: if 100% H2SO4 is 18.11M, then solultion 1+1 H2SO4 is 9.05 M?

Is there a basic formula which transforms (x+x) Reagent, to Molarity?

Texium - 14-11-2023 at 11:23

What a confusing and unintuitive convention. There can't be a simple formula for translating to molarity because it would depend entirely on the reagent being diluted. There is a good reason everyone uses molarity these days.

Bedlasky - 14-11-2023 at 12:31

Concentration of acid is usually most commonly available. So 36% HCl, 65% HNO3, 96% H2SO4 etc.

B(a)P - 14-11-2023 at 13:02

Can you provide an example of a book/document that uses this convention?
By AOAC do you mean Association of Official Agricultural Chemists? Which document is the quoted text in your original post from?
It is not possible to come up with a formula until you understand if the '(1+1)' refers to volume, mass or something else.

Sulaiman - 15-11-2023 at 03:00

Quote: Originally posted by RU_KLO

Is there a basic formula which transforms (x+x) Reagent, to Molarity?

eg sulphuric acid (1+2)
Assume 18M acid, 1 Iitre=18 moles
Add to 2 litres of water to make 3 litres total containing 18 moles acid
18 moles in 3 litres = 6 moles/litre = 6M

I think that here may be very slightly less than 3 litres total when 1 litre of acid is added to 2 litres of water, but it is not significant
(unless you are doing precsion titrations etc. but normally you would titrate the diluted acid in such a case)
Just from memory..... not certain.

I believe that when using dilute acid for general reactions,
precise concentration is not critical:
a little more concentrated and the reaction rate will increase
and less concentrated will be slower.
Don't concern yourself too much with precision in such cases.

Also, when you buy concentrated acid, the precise molarity is rarely known.
And if exposed to air, sulphuric acid slowly absorbs moisture, diluting itself,
. 37% HCl_(aq) is more concentrated than the 20.2% azeotrope so it will loose HCl as a (highly corrosive to steel) gas., until the azeotrope is reached.
. if you just look at 69% nitric acid too long it will slowly decompose the HNO₃
etc.
........................
PS if you want a formula;
Let
M1 = starting molarity of concentrated liquid
M2 = final molarity of diluted liquid
Mixture = (a + b)

M2 = M1 x [a/(a + b)]

eg sulphuric acid 18M (1 + 2)
M2 = 18 x 1/(1 + 2) = 18/3 = 6

[Edited on 15-11-2023 by Sulaiman]

woelen - 15-11-2023 at 04:47

This "(a+b)" convention indeed is horrible.
What do they mean with "(1+1)" HCl? You say 1 volume of HCl with one volume of water. But what is one volume of HCl? Most concentrated acid you can buy is 37% by weight. Most common concentration is 30...31%, at least where I live. Still quite concentrated, but not fuming as nastily as a 37% solution. But 10% HCl also is quite common these days.
And also, mixing 1 volume of compound A with 1 volume of compound B may lead to a total volume, different from 2 volumes. The difference can be a few percent.

This notation only is useful if the acid itself is well-defined, according to some written standard, otherwise it remains a matter of guessing and it even depends on where you live.

RU_KLO - 15-11-2023 at 04:54

Quote: Originally posted by B(a)P

Can you provide an example of a book/document that uses this convention?
By AOAC do you mean Association of Official Agricultural Chemists? Which document is the quoted text in your original post from?
It is not possible to come up with a formula until you understand if the '(1+1)' refers to volume, mass or something else.

"Officila Methods of Analysis" 15th ed, 1990. Associaton of Official Analytical Chemist"

As Bedlasky said, probably, as some Acid concentration are listed in a previous paragraph, these figures (1+1) reg, refers to them.

here is the page:

Bedlasky - 18-11-2023 at 14:43

Quote: Originally posted by woelen

This "(a+b)" convention indeed is horrible.
What do they mean with "(1+1)" HCl? You say 1 volume of HCl with one volume of water. But what is one volume of HCl? Most concentrated acid you can buy is 37% by weight. Most common concentration is 30...31%, at least where I live. Still quite concentrated, but not fuming as nastily as a 37% solution. But 10% HCl also is quite common these days.
And also, mixing 1 volume of compound A with 1 volume of compound B may lead to a total volume, different from 2 volumes. The difference can be a few percent.

This notation only is useful if the acid itself is well-defined, according to some written standard, otherwise it remains a matter of guessing and it even depends on where you live.

Sulaiman described it well. This is not something that is used for precise things. These A+B acids are usually used for acidification in titrations or tests. There is no need for precise concentration. As I said, concentrated acids are used in labs mostly in these concentration: 35-37% HCl, 65-68% HNO3, 96% H2SO4, 100% acetic acid, 24-26% NH3 etc. I don't find this convetion confusing at all.