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Topic: 5% acid  (Read 3046 times)

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Amnesiac

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5% acid
« on: April 27, 2014, 11:54:20 PM »
Okay I joined up just to ask this. I've only worked in one lab and we order the regular 68-70% HNO3. When we say to make a 5% HNO3 solution, we'll do 5 mL of the 68% and dilute to 100mL using DI water. Now I understand this is really a solution that is less than 5% HNO3, but we refer to it as such and when I get picky about it, people say ' well its 5% conc HNO3 ' .

Do other labs practice this?

Offline Archer

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Re: 5% acid
« Reply #1 on: April 28, 2014, 02:58:12 AM »
No other lab would get picky about it because this is a bench reagent where the solution is approximately 5% nitric acid and usually used in excess in its application.

You don't know that it's less than 5% because you don't know know the precise starting concentration.

What you are describing is a standardised 5% nitric acid solution which is rarely needed.

If you have to be exact about it then you should purchase the desired solution or dilute the concentrate to about 6%, measure the amount of acid using a primary standard and then dilute accordingly.

It's the same as 1M NaOH as a bench reagent.

You could weigh 3.997 grams into 100 ml but would this give the correct molarity to such high precision?
« Last Edit: April 28, 2014, 03:28:43 AM by Archer »
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Offline Borek

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Re: 5% acid
« Reply #2 on: April 28, 2014, 03:23:56 AM »
Actually - thanks to the different densities of these solutions - your recipe (in ideal case, assuming stock solution of 68%) produces solution that is 4.66 %. That's close enough to 5% to be labeled this way. As Archer wrote, bench reagents are only approximate, as long as they are within 10% or even 20% of the labeled concentration they are in most cases perfectly OK.

Exact theoretical volume of the 68% stock solution required is 5.4 mL.
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Offline billnotgatez

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Re: 5% acid
« Reply #3 on: April 28, 2014, 10:33:45 AM »
Is there an added symbol sometimes used to indicate approximate

for instance in math
~5% to me means
 about 5%

although after looking up math symbols I am not sure I am using ~ wrong
Maybe I should use ≈

Offline Borek

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Re: 5% acid
« Reply #4 on: April 28, 2014, 11:36:14 AM »
Is there an added symbol sometimes used to indicate approximate

None that I am aware of - or perhaps I should say, none that I am aware of being generally accepted.

Quote
~5% to me means
 about 5%

That would be my understanding as well.
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Offline billnotgatez

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Re: 5% acid
« Reply #5 on: April 29, 2014, 01:46:05 AM »
I see it would be difficult to label it 5% HNO3 ± 10%
 ;)

As an after thought
I take it that it is not common to label it 5% HNO3 ± 1%
 :)

Offline Borek

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Re: 5% acid
« Reply #6 on: April 29, 2014, 03:16:06 AM »
I see it would be difficult to label it 5% HNO3 ± 10%
 ;)

As an after thought
I take it that it is not common to label it 5% HNO3 ± 1%
 :)

Yes, that was 10-20% relative to the value on the label.
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