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Topic: How to find concentration  (Read 21891 times)

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Offline Borek

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Re: How to find concentration
« Reply #31 on: March 20, 2015, 03:17:07 PM »
Something i missing here?

Pycnometer is designed to hold each time exactly the same volume of the liquid. Its volume is not given, 50 mL is just an approximation. Exact volume of each pycnometer is something you have to measure before using it. However, once you measure its volume in a given temperature, you can be sure volume stored is reproducible to a very high degree.

Please note density you got is already quite informative - you can be sure this is more or less pure acid (around 100%, perhaps just a typical 98% stock solution). Density of the 60% solution would be around 1.5 g/mL.
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Offline Trizocy

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Re: How to find concentration
« Reply #32 on: March 20, 2015, 04:24:24 PM »
Quote
Quote
Something i missing here?
Pycnometer is designed to hold each time exactly the same volume of the liquid. Its volume is not given, 50 mL is just an approximation. Exact volume of each pycnometer is something you have to measure before using it. However, once you measure its volume in a given temperature, you can be sure volume stored is reproducible to a very high degree.

Please note density you got is already quite informative - you can be sure this is more or less pure acid (around 100%, perhaps just a typical 98% stock solution). Density of the 60% solution would be around 1.5 g/mL.

Thanks!

Okay, did go to the store and bought some "battery water" for cars. It stands: demineralised water.

So what was my result?

Well, this time i had 3 - 0,01g weight.

the pycnometer weight was between 34,11g to 34,13g - 0,02g deviation between
pycnometer with destilled water was between 84,38g to 84,44g - 0,06g deviation between

the temprature was 20 degree celsius

So then the calculation will be:
84,41g - 34,12g = 50,29g water

50,29g water / 50ml = 1,0058 g/mL

So what conclusion will it be?


I also tryed this, because i know this is distilled water, and have a pycnometer, maybe something hidden or invisible dirt inside or out side the pycnometer, also i havent calibrated the scales (but had 3 of them)
But anyway, i found this site : http://antoine.frostburg.edu/chem/senese/javascript/water-density.html - and if i put 20 degree celsius on it, it cames up 0,9982.

So if i take my result:
1,0058 - 0,9982 = 0,0076 deviation.


If i take this deviation on the H2SO4 my result will be like this:
1,8552 g/mL - 0,0076 deviation = 1,8476 g/mL

Well 1,84 g/mL is 100% H2SO4, but im getting closer :)

Offline Arkcon

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Re: How to find concentration
« Reply #33 on: March 20, 2015, 07:07:02 PM »
Hello Arkcon

Im using these density tables:

http://www.inyoprocess.com/images/chem_appl/sulfuric%20acid%20properties.pdf
http://www.norfalco.com/EN/ProductsServices/Documents/NorFalco_H2SO4Property02.pdf
http://www.sschemical.com/wp-content/uploads/2013/05/Conversion_Table.pdf

Tabl1 1 specifies 60° F, that is 16°C, you can't use a pycnometer of sulfuric acid at 10°C with that table.  Likewise table 3.  Table 2 can be used.  Check your pycnometer with water first.  If it is accurate with a few readings, fine.  Otherwise keep using it properly until you can use it properly.

Since the way a pycnometer is used is to basically overfill it, and let the liquid run out an overflow as you bring it to temperature, I hope you're being very careful with conc sulfuric acid.
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Offline Borek

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Re: How to find concentration
« Reply #34 on: March 20, 2015, 07:30:10 PM »
50,29g water / 50ml = 1,0058 g/mL

No, that's not how you do it. You should calculate exact volume from the measured mass and density of pure water taken from the density tables.

Alternatively, you can measure the water weight, the sulfuric acid weight then divide the latter by the former. What you will get this way is called "specific gravity" and is - for most practical purposes - numerically identical with density (although it slightly differs, typically in the 0.01% range).
« Last Edit: March 20, 2015, 07:56:27 PM by Borek »
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Offline Trizocy

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Re: How to find concentration
« Reply #35 on: March 21, 2015, 03:43:46 AM »
Quote
Tabl1 1 specifies 60° F, that is 16°C, you can't use a pycnometer of sulfuric acid at 10°C with that table.  Likewise table 3.  Table 2 can be used.  Check your pycnometer with water first.  If it is accurate with a few readings, fine.  Otherwise keep using it properly until you can use it properly.

Since the way a pycnometer is used is to basically overfill it, and let the liquid run out an overflow as you bring it to temperature, I hope you're being very careful with conc sulfuric acid.

Have tryed to fill it with water and got 1,0058 g/mL on average
It must be atleast 0,35g-0,40g missing weight ,but how is that possible? the pycnometer is filled and the fluid is running out.


Quote
Quote
50,29g water / 50ml = 1,0058 g/mL

No, that's not how you do it. You should calculate exact volume from the measured mass and density of pure water taken from the density tables.

Alternatively, you can measure the water weight, the sulfuric acid weight then divide the latter by the former. What you will get this way is called "specific gravity" and is - for most practical purposes - numerically identical with density (although it slightly differs, typically in the 0.01% range).

hmm? "You should calculate exact volume from the measured mass and density of pure water taken from the density tables."?

1,855g/ml H2SO4 / 1,0058g/ml water = 1,8443 g/mL still over 1,84 :(

Offline Borek

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Re: How to find concentration
« Reply #36 on: March 21, 2015, 04:55:45 AM »
hmm? "You should calculate exact volume from the measured mass and density of pure water taken from the density tables."?

Which part you don't get? Just get the definition of the density, solve for volume, and plug your knowns.

Quote
1,855g/ml H2SO4 / 1,0058g/ml water = 1,8443 g/mL still over 1,84

What is the balance you are using, is it leveled? Are your weights calibrated? What is the balance accuracy?

Pycnometer - while not a very sophisticated device - requires a good technique to make the results reliable. For example, after filling it has to be carefully dried out on the outside. I do remember there were some small tricks we were taught, but it was 30 years ago, so I don't remember them.
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Offline Arkcon

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Re: How to find concentration
« Reply #37 on: March 21, 2015, 09:26:24 AM »
Until recently, I've been using a pycnometer almost constantly all day.  And yes they are simple, if tedious, to work with.  However, if you're soppy, you won't get good results.  There is simply no shortcut to the tedium.

A pycnometer must be scrupulously clean.  My standard method was to soak it in hot soapy water, and rinse with distilled water thoroughly, before drying the cap, and pycnometer in a 105 C oven.  The thermometer can't be treated that way (I absent-mindedly tried once, sent all the mercury to the end and it never left,) but still must be cleaned and dried.  The assembly is never touched, not even with gloved hands, I carry it in a folded up kimwipe as a holder. For quarterly calibration, the empty dry pycnometer is weighed in triplicate.  This is, in some ways not needed, the mass is never different, not even at the 4th decimal place.  But at least it establishes that the balance is reliable.

Then there's triplicate runs of distilled water.  This is where simple, but tedious comes in.  You fill with water, you drop the thermometer in, some overflows, but you check the temperature.  You warm in your hand until almost 25 C, using a kimwipe to soak up droplets as it overflows from warming.  The last mounded droplet, on the pycnometer side-arm, at exactly 25 C is removed with the corner of a kimwipe.  If you suck the water out to below the side arm's level by capillary action -- screw you, start over.  If its OK, put the cap on, and read the mass.  In triplicate.

Here's something I observed, that you don't hear about anywhere else.  The first reading is often lower than subsequent readings.  I don't know why that is, perhaps the ground glass joints hold water within them?  Or maybe the glass itself has microscratches that make it porous.  Anyway, I always fill, set it up and then dump the water out a few times before the first mass.  And of course, you don't take any sample readings without a through rinse, especially when the sample is nothing like water, for example, a solution of an emulsified fat that's meant to add viscosity to a formulation.

Trizocy:, you seem to want really good results, but you don't mention how much work you're going through to insure your accuracy and precision.  You keep telling us one number, on average, but how close are your numbers to each other?  How close are they to the theoretical density of water at room temperature?

P.S.  of course, I neglect to mention, I also have to calibrate my pycnometer fitting thermometer to a NIST standardized thermometer.  That's a little too much to expect of you Trizocy:.  However, bear in mind that in my experience they always deviate -- at least by 0.5 C, sometimes as much as ±1 or 1.5 C.  You can certainly check at two points -- 0 C for ice and water, and 100  C for boiling water.  If the thermometer can handle boiling temps (c.f. above.)

*EDIT -- I spell better later in the day*
« Last Edit: February 14, 2016, 08:31:58 AM by Arkcon »
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Offline Borek

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Re: How to find concentration
« Reply #38 on: March 21, 2015, 11:44:42 AM »
You can certainly check at two points -- 0 C for ice and water, and 100  C for oiling water.

I wouldn't trust boiling without checking the pressure first.
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Offline Arkcon

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Re: How to find concentration
« Reply #39 on: March 21, 2015, 02:40:16 PM »
You can certainly check at two points -- 0 C for ice and water, and 100  C for oiling water.

I wouldn't trust boiling without checking the pressure first.

Of course, now that you mention it, we can't rely on the boiling point being a standard temperature without taking local atmospheric pressure into account.  I guess I oversimplified.  ;D  Sorry Trizocy: see what happens when we look for shortcuts.
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