Stock HCl solution?

Ian · Post by **Ian** » 23 Oct 2006, 15:17

Hi there everyone,
I know that this is a very basic question, and so anybody with more than a years experience as a Lab tech should be able to answer me. I do not fit into that catagory, hence would appreciate some advice

Our concentrated HCl comes as a 32% (w/w) solution. The manual that I found when I started the job here, tells me that 32% HCl is in fact 11.1 Molar solution. That woiuld suggest 90mL HCl to 910mL H[sub]2[/sub]O for 1L of 1M solution.

I went to a "Beginners Lab Technition's Course" earlier in the year where they told me that 32%HCl was really only 8.77M. That would imply 114mL of HCl to 886 mL H[sub]2[/sub]O for my 1M solution.

I tried to calculate it myself, based on the stated density of 1.17g/cc and noting that the 32% was a w/w measurement, not v/v. I calculated about 10.25M for the 32% stuff which would mean 97.6mL HCl to 902.4mL of Water. (Yeah, OK. I come from a Physics background, not Chem. Be kind o me, OK!)

My teachers would like to simply add 50ml of 1M HCl to 50mL of 1M NaOH and have it turn Universal Indicator green. NONE of these three concentrations seem to want to exactly neutralise 1M NaOH!!

Are we being a little too simplistic here? Is there something that I have missed?

How do you make up a 1M solution of HCl from 32% solution any way?

Regards
Ian

worktofish · Post by **worktofish** » 23 Oct 2006, 15:37

I work on conc HCl being 10 Molar. Makes a simple 1 in 10 dilution for 1M. I dont think your conc HCl is of a particularly standard concentration and you are loosing acid as soon as you open the bottle.

Remembe that you cant make a highly accurate solution of NaOH anyway and it breaks down once mixed.

Based on these two factors you will never get to produce 50 ml of one plus 50 of the other giving a pH of 7. As an excerscise for your teachers give them a dropper bottle of the reagents and ask them to get you a pH 7 solution. They will struggle. Once they have then they will not ask you to prepare solutions to do what they are suggessting.

Post by **Robb** » 23 Oct 2006, 22:33

Hi All,

Just to add to this topic, Depending on the manufacturer the concentration of 32% will no doubt differ. Having said this value is only approximate. This is why some people say it is one concentration where others may say its another.

The only way to determine the concentration of the 32% (10M approx) is to use a process called Standardisation. This involves using Burettes, Volumetric Flasks and Analytical Grade reagents, to titrate a known diluted sample of the HCl, and determine the concentration by working the answer back to the original volume and hence the initial Concentration.

I hope you follow it all up to here.

Just remember for something as touch-and-go as 50mL - 50mL HCl and NaOH then the glassware you require should be of an "A-Grade" Standard.

Accuracy is important. Being out by 0.1mL in a titre reading means possibly +/- 0.2M.

Once you complete the Titration for the acid the base itself needs to be Standardised as well. This depends on the initial make up of solution also. Very tricky but worth it in the end.

I don't know how this will go for someone with your experience but this process will guarantee an accurate answer every time. Also you get to determine the Concentration of the 32% HCl. Keep in mind that each day the acid/base solution/s are left in the bottle, the solutions will have to be Standardised once again to check for integrity.

I hope this has broadened your method. I have completed a number of the standardisation processes to determine the concentrations of various solutions particularly those of Phosphoric Acid and Ethanoic Acid. But certainly is worth the time.

The sodium Hydroxide will keep for a good while, so the solution will remain fine if this is kept in HDPE bottles, otherwise it begins to attack the glass bottles it is kept in. The sodium Hydroxide doesn't break down as such as worktofish has suggested, as long as the base is kept as described above then there should not be a problem to store and use this over a 1-2 month period.

I hope this has offered an answer not too technical, and certainly a positive towards a mystery first expected.

Cheers,

Robb.....

juliem · Post by **juliem** » 24 Oct 2006, 11:40

Hi Ian
as a one off or a prac demo
fudge
prepare 1litre of the 1M NaOH + HCl[even more preferable and typical of titration Yr12 pracs is o.1M NaOH + HCl]
then do a few trial and adjust with the 1Litre solutions
regards Juliem

Ian · Post by **Ian** » 24 Oct 2006, 12:39

Thanks Rob and Worktofish for the information so far. A couple of questions, though.

To standardise anything, I guess we need an accurate starting point. I can see that the NaOH will not be any good as it absorbs moisture when exposed to the air. Obviously the stock HCl is no good as it is of unknown concentration. What is a good starting point? I did a titration earlier in the year when we had a prac teacher for a couple of weeks who stepped me through it. I might be able to do it again! (probably nearly 30 years since my last titration before that!)

Once you have made up a standardised standard, how long will it keep for before it is no good?

What difference does the quality of the glasswear make? or are you refering to the accuracy of the measuring cylinder rather than the quality of the glass? (no, I am not being facetious. It is a genuine question)

What are HDPE Bottles?

Again, thanks for your help.

Regards,
Ian

juliem · Post by **juliem** » 24 Oct 2006, 13:47

Hi Ian
standardised solutions should be checked prior to there use and then every fortnight
standardise NaOH solutions against potasium hydrogen pthalate dried @ 110OC for 2/3 hrs
a known weight of 0.5 - 0.7g --> should give a titration of approx 25ml for a 0.1M NaOH phenolphthalein indicator
this comes from Yr 12 Chemistry prac
alternate primary standard is anhydrous dried sodium carbonate against HCl
and a search on the internet will give more

it's fine if this is necessary for quantitative analytical pracs
regards Juliem

Post by **Robb** » 24 Oct 2006, 16:25

Hi All,

Thanks juliem for the answer you gave, my time here in science is always rushed and to answer a question may be sometime into the day. After all that's what were all herre for!!

Just to offer one thing though, screened methyl Orange indicator will give a more accurate determination for the standardisation technique because it requires a pH change when titrating in the 3.8pH unit range, Right on the dot when working with KHP or Potassium hydrogen Phthalate.

If only the Phenolphthalein Indicator is available than this will be fine. But the option is yours. Not everyone has Screened Methyl Orange either.

Just to reiterate you question Re: HDPE bottles. High Density Poly Ethylene Bottles These bottles or containers are an Opaque white colour. These have a good resistance to solution particularly containing Sodium Hydroxide.

Hope this has added to your repertoire...

Robb...........

Mother · Post by **Mother** » 25 Oct 2006, 09:54

Hi Robb
Where would I be able to order these HDPE solution bottles from? Most of my solutions are stored in 1 or 2 litre empty plastic water bottles from the Supermarket!!!!! I'm wondering if other Lab Techs.do the same and is it safe enough to do this?
Cheers
Mother

Ocean Breeze · Post by **Ocean Breeze** » 25 Oct 2006, 10:11

HDPE bottles are the ones with the little recycling 2 stamped on the bottom. An example are thebottles that we buy the de ionised water from the supermarket in. Just reuse

Post by **Robb** » 25 Oct 2006, 10:26

Hi Lee,

There are a couple of suppliers; Q-stores and Lvingstone International.

I use the bottles that I have left over from the 500mL bottles of Ethanol.

These are the HDPE and hold 500mL a good volume to keep.

Q-Stores 2006 Catalogue No. 1476114 1000mL Page 610 (Although these are 1L).

Livingstone International
Phone: 1300 855 988

School Supplies Catalogue 14th edition: A11094 500mL or A11095 1000mL both on page 72

Hope this Helps,

Robb...

Mother · Post by **Mother** » 25 Oct 2006, 11:05

HI all
Thanks for the feedback!!! I had a look underneath my solution bottles and to my delight there was the HDPE symbol and some even had the number 2!!!So, I was using the correct bottles without even knowing.
Thanks again for you input
Mother

Whspa · Post by **Whspa** » 25 Oct 2006, 12:00

Hi Robb,
I was under the impression that "Drink bottles or food containers must never be used for storing chemicals". (CSIS 1.3.2 Storing Dangerous Goods)
Does this not refer to the HDPE water bottles you are citing? What about juice bottles?
Just wondering

Carol

Post by **Robb** » 25 Oct 2006, 13:34

Hi Carol/Whspa,

Thanks for your concern, I have been refering the solutions to be kept in HDPE bottles, of which these can be purchased from Laboratory Suppliers and are certainly adequate for the storage of Chems.

Other people have been using containers of which Distiled Water have been supplied in.

This is not nescessary a concern but certainly I would not reccomend any food based container to be used.

HDPE has a very strong resistance to chemicals like Sodium Hydroxide, it allows minimum absrobtion through the polymer due to NaOH, HCl molecules being large like H[sub]2[/sub]O molecules hence why bottles of this kind have been used for transport and storage.

Thanks for bringing this up so that all memebrs can see the relevance.

Thanks,

Robb.....

Post by **Ocker** » 27 Oct 2006, 11:23

Hi all! been offline for a week but back again.
The answer is ALWAYS clearly label every container. No exceptions!

worktofish · Post by **worktofish** » 31 Oct 2006, 15:25

Robb wrote:Hi All,

The sodium Hydroxide will keep for a good while, so the solution will remain fine if this is kept in HDPE bottles, otherwise it begins to attack the glass bottles it is kept in. The sodium Hydroxide doesn't break down as such as worktofish has suggested, as long as the base is kept as described above then there should not be a problem to store and use this over a 1-2 month period.

Stability:
Normally stable. Sodium hydroxide rapidly absorbs carbon dioxide from the air (forming sodium carbonate).(25)

This I have always beleived one of the reasons that NaOH is not a good standard. An open container left on a bench will rapidly reduce in concentration due to the above action. Any dead space left above the solution in the bottle will contain some CO2 thus reducing concentration. To perform the procedure (50 ml + 50 ml giving pH 7) will require reagents of much better purity than what will be achieved after storage for a month or so. Even from the point of standardisation to bottling a reduction in strength will occur. Addition of a Nitrogen atmosphere above the soultion will reduce breakdown if you can be bothered.

I suppose my point was also to educate your science staff into the difficulty and impracticality of that demonstration.

Post by **Robb** » 01 Nov 2006, 12:18

Hi All,

Jus to clarify one thing regarding Sodium Hydroxide, That is, it is (hydroscopic) absorbs moisture in the air. There is some absorption of CO[sub]2[/sub] but not to the extent which causes a change in pH.

Sodium Hydroxide is often used as a standard solution but is not desirable due to it being a strong Base. One must take into consideration the reaction type before using a standard solution. I have made many standard solutions of sodium Hydroxide to neutralise many strong acidic solutions.

When I prepare a standard solution of NaOH I am quick to weigh out my desired amount without any effect to concentration or reduced purity.

Once this is done standardise the NaOH solution to find the exact concentration and certainly go from there.

I am in full support of WorktoFish's comments and certainly there is no pun intended.

Hope this helps...

Robb............

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