[lanrete]

Hi,

i am trying to find out the content of a mixed acid made up of HF, HCl and H2SO4.

What test should i use to find out the exact amount of each acid in the mixture?

[kevins]

Dilute the sample and by anion ion chromatograph.

[labrat41]

Hi,

i am trying to find out the content of a mixed acid made up of HF, HCl and H2SO4.

What test should i use to find out the exact amount of each acid in the mixture?

You can deconstruct the mixture with thermometric titrimetry. This allows you to do all the titrations with just one titration sensor. You don't need to dilute (you can weigh your acid mixture directly into the titration vessel), since you can adjust your titrant concentration to best suit your sample, unlike other techniques where you have to dilute and dilute just to get them into range. And of course, since it is automated titration, your precision is so much better than other techniques.

Here are some ideas:
1. HF by thermometric titration with aluminium. This is based on the formation of the mineral elpasolite, NaK2AlF6. The acidic solution is suitably buffered to >pH3 and titrated with Al+++ as Al(NO3)3 in the company of an excess of Na+ and K+. The buffer is conveniently made from NaOAc, KOAc and HOAc, which neatly provides the excess of Na and K required. The method is described in Metrohm application note H-054. You use a fluoride-resistant thermometric sensor for this titration (which is also suitable for the other titrations I shall describe)
2. H2SO4 by thermometric titration with Ba++. This is the thermometric titration equivalent of the classic gravimetric procedure, except that instead of taking 4 hours to do an assay, you do it in 2 minutes.
3. HCl by thermometric titration with Ag+. Chloride thermometric titrations with AgNO3 are great, because the reaction enthalpy is relatively high.

OK, 3 separate titrations, but in each case, they're direct, and there's no interference from other ions. You could drop off one of the above titrations and do a thermometric titration of the total acids with NaOH, but from a precision point of view, it could be best to stick with the specific titrations I've analyzed.

You can check out a modern automated thermometric titration system at www.titrotherm.com

[Borek]

Thermometric titration is not a bad approach, but labrat41 will propose them as a way of dealing with any problem, so take his advice with a grain of salt

You may as well try some classic methods - determination of anions (gravimetric, or ppt titration) and alkalimetric determination of total acid.

[labrat41]

Thermometric titration is not a bad approach, but labrat41 will propose them as a way of dealing with any problem, so take his advice with a grain of salt

You may as well try some classic methods - determination of anions (gravimetric, or ppt titration) and alkalimetric determination of total acid.

I freely admit to having a business interest in thermometric titrimetry. However, I am also a professional analyst, and can personally vouch for all the procedures I discuss. Moreover, the methods are backed by the largest and most respected manufacturer of titration instrumentation in the world, and thermometric titration instrumentation is available from this company. Thermometric titrimetry is a classical method: it's been around for over 90 years. The advantages of thermometric titrimetry to the practising industrial analyst (the person who has to get high accuracy, high precision results to production and marketing as fast as possible every day) can be seen as follows:
1. One sensor suits just about all determinations that demand a result in the "g/L" or "%" range. Acid/base, redox, EDTA. precipitation and non-aqueous titrations can all be performed thermometrically.
2. This sensor does not require calibration (thermometric titrimetry is a differential method), and requires practically no maintenance. There is no reference junction to clog, and there is no electrical contact with the solution required. This means you can do titrations in totally non-conducting, anhydrous media. Having one sensor to do everything also vastly simplifies automated titrimetry. When you use a sample changer, you can put samples requiring different titration methods all mixed up in the sample rack and assign the required methods in the titration software. You don't have to change over sensors during a titration run. Chaining or linking different titrations to be performed in the same sample solution is also feasible.
3. It is able to adapt to chemistries which have traditionally required a (very slow) gravimetric finish (e.g., sulfate, phosphate, nickel, potassium) and perform titrations where there is no satisfactory potentiometric alternative (e.g., aluminium, fluoride, sodium). It is ideal for the industrial analyst charge with the responsibility for solving problems.
4. It is a very fast titrimetric technique - generally titrations take between 20 seconds to a few minutes.
5. It is also accurate and precise, with most determinations capable of precisions of 0.1% rsd or less.
6. The methodology is generally very robust. It can be used in laboratories where skilled labour may not be available (e.g., remote area minesites and refineries) and has demonstrated its worth in factory field labs where process workers perform highly reliable titrations to keep their processes on track.