Water tests (1 Viewer)

[tennille]

Does anyone know how to do the pracs for dissolved oxygen, total dissolved solids (I know it is by evaporation but what equipment do you use?) and water hardness?

I have a general knowledge of how to do it, but what substances do you use to determine DO (it has to be somehting with OH and I-) and water hardness (an EDTA salt, but what's it called)?

Sorry.

 

[Xayma]

Dissolved oxygen normally has a substance placed in it (cant remember from memory) that reacts with it fixing the amount.

You then do a titration to find it.

TDS you can use a conductivity meter, or filter then boiler the water.

 

[tina_goes_doo]

Dissolved oxygen - Winkler Method (back titration)

This was the prac we did for it:

1. Fill three 300mL flasks will the sample and stopper them, ensuring no air bubbles are present.

2. Using a gradual pipette or syringe, add 2mL manganese sulfate solution, then 2mL iodide solution. Put the tip of the dropper in the water so that only the water overflows.

3. Stopper and invert several times to mix. A brown floc will form.

4. Add 4mL of 9M sulfuric acid. The floc will dissappear.

5. Pour each sample into a 500mL flask and add 0.5-1.0 mL starch solution as an indicator.

6. Titrate with 0.0365M sodium thiosulfate solution until blue colour dissapears.

I'll probably type up the results of that later.


For water hardness you can use stiochiometry (titration) with EDTA as an indicator. I doubt you could use this in a school lab as EDTA from memory was a pretty bad substance. It can cause cancer? Whatever. It's bad....but we played with it at the 'Experiment Fest' at Newcastle Uni!


And a conductivity meter works well for TDS.

 

[tennille]

Thanks so much for that!!!

 

[Paroissien]

DO you can just use a calibrated oxygen sensor electrode.
But I'm assuming you are referring to the Winkler method, in which Mn2+ ions are added to the sample. And of course the oxygen oxidises it. The sample is then treated with a known amount of potassium iodide. It is then titrated with standard sodium thiosulfate, using starch as an indicator.
That's a lot to remember, so I prefer the sensor electrode method.
Hardness is a measure of conc. of magnesium and calcium ions, which is determined by titration with EDTA salt as you said (buggered if I know what it is called).
Or you can just say you tested it for lather, which I've seen in exemplar responses.

EDIT: God, now I look like a tool. But anyway, I'm pretty saw the EDTA is not the indicator. What I've got here says it binds to the calcium and magnesium ions. Not sure why that is though

 

[tennille]

Thaks guys.

 

[tina_goes_doo]

EDTA. As you guys are all so interested, it's full name is ethylenediammine tetraacetic acid. That's almost as fun to remember as the full formula for citric acid!

 

[xeriphic]

besides the titrating with EDTA for the water hardness, I read somewhere also how many soap drops which takes for the water to lather, hmm not sure though

 

[Paroissien]

Not exactly sure what you mean... but soft water lathers (eg. Sydney) and hard water doesn't (or at least not easily, eg. Adelaide)