How the CaO would be present? For example if it is crystalline, your technique of choice could be X-ray diffraction. If not crystalline, you can only infere it from the stoichiometric relations. EDS and XRF essentially are the same technique, they analyze the X-rays produced by the sample when irradiated either by electrons (EDS into a SEM or TEM) or by X-rays (the case of XRF). Both techniques give you atomic composition (i.e. at% or weight % with detection limits around 1-5% depending on the atom weight), thus, you will know that you have CaO only if the Ca:O ratio is 1:1 and there are not other possible compounds to be formed by the rest of elements (lets say, CaCO3 if you have Ca, Ca and O). An issue of XRF is that is not frequently used for oxygen, regular XRF equipments work from Na or even Ti to higher Z numbers. Another issue is which matrix do you have, because lets imagine a quite complicated matrix such as a soil where you could have nanocrystalline CaO not detectable by XRD. Then XANES (X-ray Absorption Spectroscopy, done in a synchrotron only) could be a better option.

So, first tell us in which state your CaO is expected to find the most suitable technique.

Hello Antonio,

XRD and EDX are non destructive techniques, and can be used as powder samples. XRD will tell you whether it is crystalline or not. EDX will tell you the composition of the chemical but I do not think it is representative of the whole sample as it samples a tiny area. On the other hand, XRF will give you the composition of the sample as a percent and it is also non destructive if analyses are done using powder pellets. One can also do a fusion of a sample to give you a glass disk which will be analysed using XRF. The latter technique is destructive, but you get to keep the sample and re-run it later on. XRF gives results in terms of oxides, but one can easily work back to get the elemental value. EDX will give you results in element composition.

My samples are oak wood-based CO2-activated carbons. I have already performed EDX and I got % of Ca up to 15% for some samples for some spots. I also performed XRD and I found CaCO3 as main crystalline phase of the carbons but not CaO.

So my aim would be to see if there is any non-crystalline form of CaO onto the carbons. Is XRF suitable to my purpose? If not, which other technique could I use?Also, unlike EDX, is XRF giving an estimation of the chemical composition over the whole sample?

As explained above, XRF and EDX (EDS) are in principle the same technique. They will only give the elemental composition (e.g. Ca, Fe, O etc), not what type of compound you have (e.g. CaO or CaCO3). The EDX and ERF results are however sometimes reported as oxides, where the elements are assumed to be in the form of their most abundant oxides. This also happens when other techniques have been used for analysis of the elemental composition (e.g. ICP-OES and ICP-MS) and it can sometimes be misleading.

If you have 15% Ca in your sample and find CaCO3 as a main crystalline compound with XRD, you can probably assume that (at least most of) your Ca is in the form of CaCO3. If you want to be more certain you can use a quantitative XRD, where you add some extra CaCO3 to your sample to see how much a (non overlapping) CaCO3 peak increases in intensity.

You can also try to wet your sample to see (with XRD) if any non crystalline CaO reacts with water and forms (crystalline) Ca(OH)2. This will however not tell you if there is any Ca-silicate, which is also a possibility. For that you would probably have to go to a synchrotron and use x-ray absorption techniques (XANES and EXAFS). That is however easier said than done.

CaO is usually unstable in presence o f water ornother carbon comppunds, it would react to form CaCO3 as you already found. If you really suspect CaO presence Xanes is possiible the only with enough sensibility and possibility to see phases not elements, to tell you. With elemental analysiis such as xrf or eds you would not able to tell it

In accordance with colleague Caballero-Briones and his statements, i) XPS or ii) XRD-EDS are well known techniques that make noticeable the presence of Ca-O bonding with XPS; and correspondingly the presence of polymorphs of CaO compound as their respective crystallographic phases by XRD as well as the presence of Ca through EDS elemental analysis. Hope this helps. Best regards and researches.

The method, called a complexometric titration, is used to find the calcium content of milk, the ‘hardness’ of water and the amount of calcium carbonate in various solid materials.

Solid samples, such as limestone and eggshell, must first be dissolved in acid.

1. Accurately weigh about 0.5 g of the solid into a small beaker or conical flask, add about 20 mL dilute hydrochloric acid and allow the solid to completely dissolve (this may take several minutes).

2. Neutralise the unreacted acid with dilute sodium hydroxide solution until the pH of the solution is almost 7 (according to pH indicator paper). With eggshells, the inner membrane will not dissolve and should be carefully removed from the solution.

3. Transfer the solution to a 100 mL volumetric flask

Titration

For undiluted seawater, undiluted milk, eggshell and limestone samples.

1. Pipette a 10 mL aliquot of the sample solution into a conical flask.

2. Add 40 mL of distilled water and 4 mL of 8 mol L−1

sodium hydroxide solution (see safety notes), and allow solution to stand for about 5 minutes with occasional swirling. A small of magnesium hydroxide may precipitate during this time. Do not add the indicator until you have given this precipitate a chance to form.

3. Add 0.1 g of Patton-Reeder indicator and swirl the solution to dissolve the indicator.

4. Titrate the sample with the EDTA solution. The endpoint is a colour change from pink/red to blue. Repeat the titration with further samples until concordant results (titres agreeing within 0.1 mL) are obtained.

For tapwater the method is modified due to the much lower Ca2+ concentration.

1. Dilute the ETDA standard solution by a factor of 1/50 by pipetting 10 mL into a 500 mL volumetric flask and making up to the mark with distilled water. This will give a 0.005 mol L−1 solution.

2. Pipette a 50 mL aliquot of tapwater into a conical flask. Add 4 mL of 8 mol L−1 sodium hydroxide solution, and allow solution to stand for 5 minutes with occasional swirling.

3. Add 0.1 g of Patton-Reeder indicator and swirl the solution to dissolve the indicator.

4. Titrate the sample with the diluted EDTA standard solution to the blue endpoint.

Result Calculations

1. Calculate the average volume of EDTA solution used from your concordant titres.

2. Calculate the moles of EDTA required to complex the Ca2+ ions in the sample.

3. Using the method ratio Ca2+: EDTA = 1 : 1, calculate the concentration in mol L−1 of Ca2+ in your sample solution.

4. Calculate the concentration, in mg/L (parts per million or ppm), of Ca2+ in your sample solution.

5. In the case of a solid sample which has been dissolved in acid, the concentration of Ca2+ in your sample solution may be used to calculate the percentage, by weight, of CaCO3 in the solid sample. This assumes that all the Ca2+ found has come from CaCO3

So, just to recap, besides XANES, might I be able to discriminate Ca-O bonds due to a different phases (CaO, Ca(OH) or CaCO3) by using FTIR or XPS maybe?

As regards FTIR, does anybody know which wavenumbers Ca-O absorbance bands occurs at?

Dear Antonio,

Qualitatively if your sample constitutes only Ca compounds, you should at least get the CO3 and OH vib bands from FTIR (For CaO...it MAY be beyond the normal IR range...you have to search for it)...but I doubt whether you can get a proper quantitative estimate.

XPS gives the valency states...but you have Ca (2+) and O (2-) in all your mentioned compounds.

Xps will be the best indeed, besides xanes.  Regards

If get xps spectra and want some help to interpretation dont hesitate to ask, regards 

Thanks! I'm going to try this as soon as I can. So, ideally, with a good spectra interpretation, I should be able to identify different phases (e.g. CaO, Ca(OH)2, CaCO3), right?

XRD (crystalline phases) and XANES (mapping of chemical species-dependence of near edge X-ray absorption on chemical environment) should be complementary and ideal.