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K12Chemistry

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posted on 14-2-2013 at 08:12

Sulfuric Acid Unknown Concentration?

I boiled down some sulfuric acid I made electrochemically and I stopped it after about 5 minutes after it started releasing SO3. How can I find the concentration? I don't have bromothyl blue and I don't have phenolphthalein. Would red cabbage juice work? I'm not looking for a very accurate measurement I just want something that can give me an idea of the level of concentration.

Is there any other way apart from titration with NaOH or a base?

Thanks!

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Mailinmypocket

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posted on 14-2-2013 at 08:20

Quote: Originally posted by K12Chemistry

I boiled down some sulfuric acid I made electrochemically and I stopped it after about 5 minutes after it started releasing SO3. How can I find the concentration? I don't have bromothyl blue and I don't have phenolphthalein. Would red cabbage juice work? I'm not looking for a very accurate measurement I just want something that can give me an idea of the level of concentration.

Is there any other way apart from titration with NaOH or a base?

Thanks!

The way to finding the concentration of an acid is titration... Cabbage juice won't really cut it. There is an easy enough to follow video here by Robert Bruce Thomson that demonstrates how to determine the concentration of sulfuric acid in a home lab, via titration.

http://m.youtube.com/watch?v=-bweWvb0wws

It only requires common chemicals, you will never be able to determine the concentration with only an acid base indicator and nothing else!

K12Chemistry

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posted on 14-2-2013 at 08:22

thanks ill try and follow the video

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IanCaio

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posted on 14-2-2013 at 08:36

Maybe if you have a precision balance you can try defining the solution density, and compare it to have an idea on the concentration.

I found this table, it might be useful. It just wouldn't be precise if you have really small ammounts of acid solution I guess.

http://www.generalchemical.com/assets/pdf/Sulfuric_Acid_Spec...

MrHomeScientist

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posted on 14-2-2013 at 09:29

Density is probably the simplest, if you don't have proper indicators. There are extensive charts online for sulfuric acid density at various concentrations, since it's such a widely-used chemical in industry. Here's a good one I've used: http://www.sschemical.com/wp-content/uploads/conversions.pdf

It's in weird units, though, so you'll need to do conversions. To measure density accurately, you'll need a good milligram scale and volumetric glassware (ideally).

K12Chemistry

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posted on 14-2-2013 at 10:03

I don't have a precision scale

. I just normally use a scale that just goes from like 1 to 2 to 3 instead of milligrams. I measured it any way and I got 9 grams for 10ml. I did it 4 times and the mean was 10 to 10. So the density is the volume divided by mass so 1? That is reaaaaaaly low concentration. That is just not right.

Anyway I'm going to try and buy phenolphthalein.

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Hexavalent

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posted on 14-2-2013 at 10:28

No, density is calculated by dividing mass by volume...

ρ = ^m_v

Phenolphthalein solutions are widely sold on eBay, often for biodiesel titrations.

[Edited on 14-2-2013 by Hexavalent]

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bfesser

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posted on 14-2-2013 at 11:13

Quote: Originally posted by K12Chemistry

I don't have bromothyl blue and I don't have phenolphthalein.

<a href="http://en.wikipedia.org/wiki/Bromothymol_blue" target="_blank">Bromothymol blue</a> <img src="../scipics/_wiki.png" /> (which I assume is what you meant) is not suitable for this particular titration. <a href="http://en.wikipedia.org/wiki/Phenolphthalein" target="_blank">Phenolphthalein</a> <img src="../scipics/_wiki.png" /> would be a better choice.

[Edited on 7/9/13 by bfesser]

feacetech

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posted on 14-2-2013 at 19:35

heres an analytical method, you can modifiy this to a crude method

This method is applicable to concentrated sulphuric acids, specifically to the 98.5%

Sulphuric acid is weighed out in a dry stoppered vessel and transferred quantitatively to a 500 mL conical flask using deionised water. The solution is then titrated against standard NaOH solution to a pink end point.

Apparatus

Three stoppered weighing vessels (plastic or glass) per acid sample
Dry 2 mL graduated pipette or Pasteur pipette
Three 500 mL conical flasks per acid sample
A grade burette
Funnel

Reagents

Standardised Sodium Hydroxide Solution (1.0 mol NaOH L-1)

Weigh 81-83 g sodium hydroxide into a 250 mL tall form beaker. Add 100-150 mL distilled water and stir till dissolved. Cool and transfer to a 2000 mL volumetric flask. Dilute to volume with distilled water. Store in a plastic bottle and standardize

Phenolphthalein Indicator (1%)
Dissolve 2.000 g phenolphthalein in 200 mL ethanol

1. Accurately weigh a dry stoppered vessel and record this initial mass to 0.1 mg. Add from a clean dry pipette about 2 mL of acid sample or alternatively for a known 98.5% acid strength, weigh 3.55 – 3.65 g of acid using a Pasteur pipette.

2. Stopper immediately and record the final mass to 0.1 mg.

3. Carefully add deionised water drop wise using a fine tip wash bottle. Slightly tilt the lid of the vessel and add the water slowly until spitting has stopped. The acid should then be allowed to cool to room temperature before transferring to the conical flask.

4. Using a funnel, transfer acid carefully to a 500 mL conical flask, washing thoroughly.

5. Titrate with standardized 1.0 mol NaOH L-1 using a few drops of phenolphthalein indicator until a pink end point is achieved.

6. Repeat twice or more if required to within 0.1% accuracy.

Calculation

Mass (g) H2SO4 = Final mass – Initial mass

% H2SO4 = [(Molarity NaOH / 2000) x mL NaOH x 98.07 x 100]/Mass (g) H2SO4

Notes
When analyzing acid samples of a known 98.5% acid strength it can be expected that ~71 mL 1.0 mol NaOH L-1 will be required when using the above procedure.

Standardisation of NaOH

Reagents For Standardisation

Carbon Dioxide - Free Water
Boil 1000 - 1500 mL distilled water for 20 minutes in a 2000 mL conical flask. Cool in a stoppered container. This must be prepared fresh.

Potassium Hydrogen Phthalate
Dry a quantity of AR grade potassium hydrogen phthalate (KHC8H4O4) in a weighing bottle for at least two hours, but preferably overnight at 100 oC. Cool in a desiccator.

Phenolphthalein Indicator (1%)
Dissolve 2.000 g phenolphthalein in 200 mL ethanol.

Procedure

Weigh to the nearest 0.1 mg, an aliquot of potassium hydrogen phthalate into a 250 mL stoppered conical flask.

for 1.0 mol NaOH L-1 use 3.5-4.5 g KHC8H4O4

Add 50 mL warm carbon dioxide-free water. Stopper the flask and swirl gently until the salt dissolves. Titrate against the unknown sodium hydroxide solution using three drops of phenolphthalein indicator.

Repeat as necessary to give satisfactory replication.

Calculation

NaOH (mol L-1) =
(g KHC8H4O4 x 1000)/(mL NaOH x 204.229)

[Edited on 15-2-2013 by feacetech]