Talk:Concentration
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My first effort at wikipedia, the last paragraph still needs work I think. Let me know what you think. Aglimme
Sorry, my english is too bad to correct the article, however I signal that molarity and normality are different and distinct concepts. Svante 23:48, 3 Jul 2004 (UTC)
There should be something about equivalants per liter on this page?
Today I expanded this article significantly, adding a table, image, further examples, etc. I still feel it needs some work - in particular, the definition of "normality" is inadequate, and the whole article can be further wikified. Plus I need to double-check all the examples. I'll be revising and tidying later this week, if someone else doesn't. -- FirstPrinciples 14:48, Sep 25, 2004 (UTC)
- Further units to add: molality & formal -- FirstPrinciples 16:02, Sep 25, 2004 (UTC)
- OK I added those too. Still needs tidying up. -- FirstPrinciples 07:29, Sep 26, 2004 (UTC)
[edit] Weight-volume percentage
Removed the following from the article until clarified.
- Weight-volume percentage, (sometimes referred to as mass-volume percentage and often abbreviated as w/v) denotes the mass of a substance in a mixture as a percentage of the volume of the entire mixture. For instance: in the previous example 40 grams of ethanol was mixed with 60 grams of water. However, ethanol is less dense than water (with a specific gravity of 0.789), so the total volume of the bottle is 110.7 millilitres; this means that the bottle contains around 36% ethanol, w/v.
- Note: It is usual practise to label alcoholic beverages with weight-volume percentages, although some less scrupulous manufacturers give the proportion of alcohol as weight-for-weight, thereby making drink appear slightly stronger than it is. In many areas this practice is restricted by fair trading laws.
My reasoning
There is a problem here, volumes of alcohol and water do not add. 60g water/1.0g/mL = 60mL. 40g ethanol/0.789g/mL =50.7mL. But 60mL water plus 50.7mL ethanol does not = 110.7mL, but something less than that for a total volume.
This is a common demo in introductory chemistry. Combine 10mL water with 10mL alcohol; the expected combined volume is 20mL. Students are quite surprised to find the combined volume to be measurably less.
So, if alcohol trade laws are based on this concept they are in error.
I have not heard of this weight-volume concentration term and find it troubling even without the above mentioned glitch. It may be an odd term used in the alcohol industry, if so we need to define and explain it better before putting it back in.-Vsmith 22:11, 30 Sep 2004 (UTC)
OK, I recant:-) I have heard of weight-volume. Weight-volume percentage is often used for solutions made from solid reagents. It is the weight of the solute in grams multiplied by one hundred divided by the volume of solution in milliliters. Need to re-write this without the volume mixing problems and re-insert. More to do. -Vsmith 01:44, 1 Oct 2004 (UTC)
Still digging. For liquid:liquid solutions such as alcohol:
"Volume-volume percentage or % (v/v) describes the ml solute per 100 mL solution. This is most useful when liquids are being mixed. For example, beer is about 5% ethanol by volume. This means every 100 mL beer contains 5 mL ethanol (ethyl alcohol)." (chem lecture notes) Need to re-write and include as volume-volume concentration. Still more to do :) -Vsmith 02:19, 1 Oct 2004 (UTC)
- My apologies, I entered the original information from handwritten notes. Now that I look at it critically, it does seem clearly incorrect. I can't actually figure out what I was trying to say... I believe I was confused about the dissolution of solids (for which 'mass-volume percentage' is aprropriate) but gave a nonsensical example using ethanol. Anyway, I suggest we discard the above 'weight-volume' rubbish; the article should give a simple description mass-mass, mass-volume and volume-volume percentages, and give a concise, accurate example calculation for each. -- FirstPrinciples 13:00, Oct 2, 2004 (UTC)
- The article is great now. Congratulations to Vsmith. -- FirstPrinciples 08:27, Oct 8, 2004 (UTC)
- The comment about alcohol is (partly) correct: it is usual to give percentage alcohol by volume, but sometimes it can be given by weight, usually with the deliberate intent of causing confusion. (This information may not be particularly relevant for the main article, all the same.) -- FirstPrinciples 13:00, Oct 2, 2004 (UTC)
I fixed the equation for calculating w/v. It read mg/mL × 100, which I changed to g/mL × 100. If you wish to verify this, see the following:
Sorry, but the "Generic formula" in the "Table of concentration measures" needs to be fixed and the "Typical units" note dropped. The formula shown and the [% g/L] are dangerously wrong: % w/v is used all the time in making injectable drug formulations and "(grams solute x 100 / litres solution)" is off by a factor of 10 -- which can be fatal. The generic formula should be:
(grams solute) / (100 x millilitres solution)
and the units are % w/v.
This error shows up elsewhere on the Internet, including a commercial vendor's site. It needs to be clarified.
For more background, see the standard text "Pharmaceutical Calculations" by H.C. Ansel and M.J. Stoklosa (Lippincott Williams & Wilkins, Philadelphia)
p.s., I don't know how to edit the graphics in the table. Otherwise I would make the changes now myself.
Decisiondoctor 18:31, 5 May 2007 (UTC)
- Fixed, I think - don't know much about pharmacological usage, but this has to be about the dumbest distortion of percent. Anyway, the table isn't an image - just uses some alien looking math language markup. Cheers, Vsmith 01:02, 6 May 2007 (UTC)
[edit] ppt?
Parts per thousand (ppt)
Parts per trillion (ppt)
OK we have a problem here ppt cannot mean both per thosand and per trillion. In my experience ppt means parts per trillion.
- The trouble is, in different contexts it does vary. There are many acronyms with more than one meaning. -- FirstPrinciples 13:00, Oct 2, 2004 (UTC)
Parts per thousand is represented by the per mil symbol: (% symbol with an extra zero), don't know what the code is for it. Looks sorta like this o/oo . -Vsmith 22:57, 30 Sep 2004 (UTC)
OK, found it: parts per thousand or 'per mil' ‰. [1]
-Vsmith 00:58, 1 Oct 2004 (UTC)
- I still suggest that a warning be added about ppt. For instance, in the context of salinty measurement, 'ppt' very frequently means 'parts per thousand'. Also, at least one acronym dictionary lists ppt as meaning both parts per thousand and trillion (http://www.acronymfinder.com/af-query.asp?String=exact&Acronym=ppt&Find=Find). -- FirstPrinciples 13:00, Oct 2, 2004 (UTC)
[edit] Normality
I suggest this section needs to be revised and expanded with examples. I don't have sufficient chemistry knowledge to do a good job, but maybe someone else can? -- FirstPrinciples 08:27, Oct 8, 2004 (UTC)
Please check 195.229.241.180's edit. — Jeandré 2005-06-28t10:54z
I've updated the normality section, its referencing disambiguation page, and the equation at the bottom, to reflect the standard chemical definition of normality, which is concerned with the gram equivalents in solution. I thought the old version of the equation was a bit misleading -- "valence" may or may not refer to the charge of the ion or the number of bonds connected to the atoms, but we're talking about gram equivalents here, which can include equivalents of electrons - a concept outside the realms of "valence". Hope that makes it clear. Unfortunatley the definition of gram equivalent is somewhat ambiguous in the chemical community, but so it goes.--Lineweaver 23:21, 2 June 2006 (UTC)
[edit] Osmolarity, tonicity
Do these concepts also belong under 'concentration'? -- FirstPrinciples 03:51, Oct 12, 2004 (UTC)
Not really familiar with them (actually never heard of tonicity until I just looked it up), so don't know if we want to include them ??
How about activity, fugacity, and partial pressure? -Vsmith 17:11, 12 Oct 2004 (UTC)
Hmm. Ideally, I think the article should be comprehensive as possible. Hence, a mention of most of these terms is probably warranted, even if it's only a sentence or brief paragraph. I get the feeling that osmolarity, activity and fugacity are more relevant than tonicity and partial pressure. -- FirstPrinciples 05:59, Oct 13, 2004 (UTC)
[edit] CaCO3 as an example of Formal
When I envisage adding a mole of chalk to a litre of water, I really don't see complete dissociation occuring. I thought calcium carbonate was essentially insoluble in water. If this is right, perhaps a better example is possible.
[edit] ppq
There are a few analytical techniques that can measure ppq levels. I changed the text to reflect this as the old version suggested that there are no analytical techniques that can measure ppq concentrations.
[edit] How about gaseous concentrations?
The article is almost totally focused on concentrations in liquid systems. It needs to also cover concentrations in gaseous systems ... for example, pollutant concentrations in the ambient atmospheric air, which are commonly expressed as mg/m³, ppm by volume, etc.
mbeychok 20:58, 3 March 2006 (UTC)
[edit] Question about volumetric iodine solution
Could anyone explain to me why a 0.1M iodine solution sold for volumetric analysis is sometimes quoted as 0.1M (0.1N) and sometimes as 0.1M (0.05N)? What is actually in this solution? Thanks! OAP boba 08:20, 27 April 2006 (UTC)
[edit] Parts per million etc.
- It is the ratio of the amount of the substance of interest to the amount of that substance plus the amount of the substance it is in. e.g. 10 parts per million (ppm) sugar in water means that there are 10 mg of sugar in 999,990 mg of water.
If I'm not mistaken, mg are a unit of mass, not amount, so ppm is a ratio of the mass of the solute to the mass of the solvent. Might be a bit pedantic but I'd rather the article was correct than not. Hairy Dude 17:49, 31 May 2006 (UTC)
[edit] NIST
Regarding:
The National Institute of Standards and Technology, the United States authority on measurement, considers the term molarity and the unit symbol M to be obsolete, and suggests instead the amount-of-substance concentration (c) with units mol/m3 or other units used alongside the SI such as mol/L [1].
- It is true that NIST has recommended that and various quality assurance organizations that do ISO accreditation have blindly copied it, so that it carries force of law in many industrial labs. However, the idea that molarity is obsolete is by no means generally accepted in academia and currently ACS and IUPAC are considering the matter. I think that under the circumstances wikipedia would be well advised not to proclaim molarity quite dead yet.
- I have added some clarifications on the mass versus volume question and simply added that academia has not accepted NIST's recommendations. (Hope they never will..)
[edit] Amount-of-substance concentration
I would suggest that more needs to be said about amount-of-substance concentration, c, in mol. dm-3 or mol. m-3 as this is the standard measure of concentration in the UK as well as being the alternative to molarity offered by the NIST.
[edit] molarity
molarity- moles of solute per liter of solvent
- No, it's moles of solute per liter of solution (solute and solvent together). Shalom (Hello • Peace) 01:30, 4 November 2007 (UTC)
[edit] volumes
need to add volumes e.g. used in measuring hydrogen peroxide 129.31.72.52 15:22, 11 June 2007 (UTC)
[edit] Explain this!
The last sentence of the lead section says:
- "Analytical concentration includes all the forms of that substance in the solution."
I know what that means, but it's not immediately obvious, nor is it referenced at that point. I'm not sure if it's discussed later. I think a clearer explanation of this point would help readers. Shalom (Hello • Peace) 01:30, 4 November 2007 (UTC)
[edit] Written awfully
Part of the reason I never became a chemist is they explain things so darn poorly. What the heck does the following mean?
Normality highlights the chemical nature of salts: in solution, salts dissociate into distinct reactive species (ions such as H+, Fe3+, or Cl-). Normality accounts for any discrepancy between the concentrations of the various ionic species in a solution. For example, in a salt such as MgCl2, there are two moles of Cl- for every mole of Mg2+, so the concentration of Cl- as well as of Mg2+ is said to be 2 N (read: "two normal"). Further examples are given below.--Filll (talk) 01:30, 5 March 2008 (UTC)
[edit] why are there two molarity sections?
they should be combined. —Preceding unsigned comment added by 72.65.167.164 (talk) 23:32, 12 May 2008 (UTC)
