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Author: Subject: phosphoric acid titration
CouchHatter
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[*] posted on 12-8-2020 at 18:42
phosphoric acid titration


I have some 85% phosphoric acid and wanted to titrate it. I've gone through all the numbers thrice and cannot find what I'm doing wrong. I have found its density to be that of 85% phosphoric acid by means of weighing, so I don't really need to titrate it. But I would like to know what I'm doing incorrectly. I used phenolphthalein but checked with pH paper too.

Also, is there a good way to write fractions here? I thought I could edit my post as HTML but now I can't find that capability and it's not recognizing the phpBB or MathML tags I've tried.


I used an average of 36.18mL of 0.5M NaOH in three trials. Each trial used 6 mL of diluted H3PO4 (6 mL conc. acid in 60 mL water).
(titrant molarity * titrant volume * dilution factor)/analyte volume.
(0.5M*36.18mL*10)/6mL = 30.15 M H3PO4.

Then finding % conc. by
(analyte molar mass * analyte molarity* 100)/(analyte specific gravity * 1000)
(97.994 * 30.15 * 100)/(1.6845*1000) = 175.4% concentration...

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macckone
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[*] posted on 12-8-2020 at 20:36


Phosphoric acid has three hydrogens that can act as proton donors.
The pH of monosodium phosphate is between generally between 6 and 8 ie. close to neutral.
The pH of disodium phosphate is between 8 and 11.
The pH of trisodium phosphate is between 12 and 14.

Phenolphthalein is probably giving your color change at the disodium phosphate level.
Review the color change and pH range of phenolphthalein.
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j_sum1
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[*] posted on 12-8-2020 at 22:31


I popped in to say the same thing.
(I just set this exact situation as an exam question.)

You can use congo red or bromocresol green to find the endpoint of the first transition. This will centre around pH 4.5.
You can use bromothymol blue to find the endpoint of the second transition. This will centre around pH 7.
The third transition is indistinct. Slope of the titration curve is pretty flat at this equivalence point and so you won't find a nice endpoint using an indicator. In fact you will be hard-pushed to discover it using a pH meter and analysis of the curve. (See this example.)

To find the concentration, either triple the result from your first equivalence point or ×1.5 of your result from the second equivalence point.
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macckone
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[*] posted on 13-8-2020 at 08:42


j_sum1,
check your math. based on how the OP structured his calculations it is a divide by 3 or divide by 2.
In this case divide by two gives a result that is well in line with theoretical and that corresponds to his probable end point with phenolphthalein based on the curve you provided.
I suspect the pH range is very concentration dependent.
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outer_limits
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[*] posted on 13-8-2020 at 12:06


It could be titrated as 3-protonic acid by using Ca2+ or Ag+ and NaOH

2H3PO4 + 3Ca^2+ + 6OH- -> Ca3(PO4)2 + 6H2O

Phenolphthalein can be used as indicator
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macckone
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[*] posted on 13-8-2020 at 19:58


Outer_limits,
getting a triple basic phosphate isn't that easy.
You usually have to add excess base.
But with a double basic compound it is easy and possible to dry the product and weigh it as well as titrate it.
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[*] posted on 14-8-2020 at 02:30


Quote: Originally posted by macckone  
j_sum1,
check your math.

Multiply. Divide. Same thing. Frame of reference.
Sorry for confusion.
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