[Grady]

I know that concentrated hydrochloric acid is an approximately 38% solution of hydrogen chloride in water.  And I also know that "dilute hydrochloric acid" can be anything ranging from a somewhat less than 38% solution to almost pure water.  Nevertheless, we often see in the literature, and especially in older text books, reference made to "dilute hydrochloric acid".  And I vividly recall from my youth, in the 50s, reagent bottles etched with such labels that contained no other information.  Travelling back to those far off days, before molarity was taught in my part of the world, what is it reasonable to infer was the concentration of hydrochloric acid in such bottles and what concentration were writers, who were no more specific, referring to?  While we're here, let's jump over to the other side and ask the same question about "dilute sodium hydroxide".

[IJL]

Unfortunately, it can mean almost anything. Though, in practice, I usually
consider 10% hydrochloric acid to be dilute. But 1 M is even more so. It's
probably important to know what you are hoping to do with the dilute
HCl; that might help us determine a reasonable concentration.

I ran into a funny one not too long ago as I saw a procedure calling for 50%
HCl solution. Since conc. HCl is about 38% that had me scratching my head
for a bit until I realized it meant a 50% dilution of conc. HCl.

So, in other words, 50% HCl = 19% HCl. Yikes! Give me molarity any day!!!

[Borek]

I have forwarded the qustion to CHEMED-L - I see IJL answered here and there (thanks ), but there was also another answer:

The usage of "dilute HCl" was/is relevant to qualitative analysis. The poster should consult any older edition of Vogel's Qualitative Analysis. This book has every minute detail which many modern books lack. Dilute HCl refers to 2 M in Vogel and so is dilute NaOH. Dilute H2SO4 is 1 M. Although I agree it is completely arbitrary and unacceptable for all other purposes.

[Grady]

Many thanks to those who were kind enough to respond to this posting.  There was not, to clear up a point, a specific purpose for which "dilute hydrochloric acid" was required.  I was, rather, enquiring into the way in which the term was used by writers.  But it is as I feared.  There are general agreements amongst groups of people, by convention and usage, about the term.  But those agreements are not neccessarily shared between groups. As with so much in life!

[Borek]

Two more answers from CHEMED-L, somehow I forgot to post the here earlier:

Long ago I was taught that "dilute" simply was half the concentration of the "concentrated" acid/base.  Thus dilute HCl was 6 M and dilute H2SO4 was 9 M.  Of course this meant you had to know the concentration of the "concentrated" reagent.  More recently, we regularly use 6 M as the concentration for the "diluted" species.   Clearly, this is arbitrary.  However, it is safer for students to work with 6 M H2SO4 than the concentrated reagent.

When I was an undergrad (and worked as stockroom assistant), our dilute acids were all 6 M.  In grad school one summer, I was teaching an overload lab section and was asked to refill the supply carboy for dilute HCl.  I dutifully did so with 6M  HCl.  The conversation in the office later was how a lab which had work so well for years suddenly went south.  I was asked if I was sure I had calculated the dilution correctly.  Of course I was.  One guy said that there was  no way 3M HCl would produce the kinds of results the students got.  I said, "What 3 M?".  Mystery solved.  This university defined dilute as 3 M.

I learned a good lesson (even if the students in that lab did not) I and never fail to tell my students that dilute is a relative term - always specify the molarity.