Just curious. The concentration is defined as a number of molecules (e.g. in moles) per unit of solution or gas. A molecule has a certain molecular weight. Thus, you can calculate the concentration as [C] = (weight/Fw/Volume. What is the molecular weight Fw of colloids particles?

Konstantin Rozov the traditional UV-visible molecular spectrophotometric method is not suitable because the colloids scatter the light and therefore the radiation reaching the detector does not decrease in intensity due to radiation absorption alone, but also due to the scattering effect, i.e. an erroneous absorbance is measured. The fundamentals of the UV-visible spectrophotometric method is for substances in solution. Perhaps for very small and low "concentrated" (almost homogeneous) colloids you can obtain results without so much deviation, but I think you should validate the method to ensure that it is correct. Quantification in colloidal systems depends on the nature of the dispersed material: polymeric, metallic, etc. I have seen that microscopy can be used after ultrafiltration stages, maybe that is an option, but it depends on the particle size especially because of the filtration stage. I recommend you, check specifically what methods have been applied for the colloidal system you have under study. Regards!

opposite to the stated above, as long as the particles do not settle during measurement particle concentration can be measured by determining a calibration curve of the extinction as function of particle concentration using its linear part. Extinction measures the loss of light intensity before the light reaches the detector including light scattered in other directions. For higher concentration of particles attenuating X-rays, X-ray would be applicable without needing dilution.

L. C. Resio

Dear Titus Sobisch , yes, you are indeed right. Extinction can be used. However, if you notice I referred to the "traditional" UV-visible molecular absorption method where absorbance is measured and an absorbance or transimittance versus concentration curve is constructed based on the Beer-Lambert-Bouguer Law. It is good that an expert like you has offered a solution using a spectrophotometer. That's the great thing about this site, that we can all contribute and learn. Best regards!

I'm not an expert in this area. Could someone explain me what is the "concentration" in colloids. What is the "extinction?"

Dear Yurii V Geletii extinction is the attenuation of radiation intensity by absorption and scattering in the sample. It refers to the extinction coefficient used in the fundamentals of molecular absorption spectroscopy. However, the measurement of transmittance in a traditional UV-visible spectrophotometer does not take much account of light scattering effects and the measured absorbance can be erroneous in high scattering samples. On the other hand, your question about concentration is interesting! Because I understood that the question refers to concentration in its analytical definition: mass or moles of substance/volume of mixture (IUPAC), as you defined it above.

I agree with @Titus Sobisch the turbimetric via UV spectrometry and then by the linear carliberation approach. This method has been used by several researchers in previous literatures. The colloidal particles are responsible for turbidity effect in solution. Hence if the turbidity can be determined and estimated then the colloidal particles concentration can be measured.

Dear Javier Ernesto Vilaso Cadre,

I know pretty well the fundamentals of molecular absorption spectroscopy. I asked simple questions. I have not received any response except from you.

Below are some IUPAC recommendations.

According to Article Glossary of methods and terms used in analytical spectroscop...

absorbance, A

deprecated: extinction

obsolete: attenuance

Logarithm of the division of incident radiant power (P0) by transmitted radiant power (Ptr).

A = loga(P0/Ptr) = –loga(T) = –loga(1 – αi), where T is the transmittance and αi is the absorptance.

The base a of the logarithm should be specified... decadic absorbance (log10, lg), and Napierian absorbance (ln).

The Beer-Lambert-Bouguer law can be expressed as A(λ)=lg(P0λ/Pλ)=ε(λ)cl, where the proportionality constant ε(λ) is the molar decadic absorption coefficient and P0λ,Pλ are, respectively, the incident and transmitted spectral radiant power.

ε(λ) is the molar decadic absorption coefficient commonly called an extinction coefficient

Titus Sobisch wrote: "particle concentration can be measured by determining a calibration curve of the extinction as function of particle concentration using its linear part."

Extinction is deprecated, "concentration" is not defined. How to prepare the solution for calibration?

"That's the great thing about this site, that we can all contribute and learn."

So far I learned nothing, but more confused.

Please, explain me at least what is the "concentration of colloidal particles?"

Yurii V Geletii For preparing the calibration curve you do not prepare any solution you prepare a suspension of the particles, if possible under same conditions as the particle suspension to be determined.

Scattering depends on the size and distribution of particle. How you can prepare the similar solution for calibration if you don't know what is in your sample. I doubt that it's possible to apply the photo-colorimetric method for this.

Rayees Ahmad Rather

I agree with you that first other techniques should be used to characterize the size and distribution of particles. Then, for routing experiments with the similar solution, UV-vis can be used.

size and distribution of particles is good to know but not necessary if it can be ensured that it kept constant. Again, particle solutions do not exist. https://drive.google.com/file/d/1z7nM0U8vOJ2uKuoDGIZyf07WnWK8xOUo/view?usp=drive_link

I feel that part of the confusion is that using a spectrophotometer is not the same as using the photocolorimetric method. Using a (spectro)photometer does not necessarily imply using the colorimetric method. The term colorimetry is in disuse and refers to the analytical method of molecular absorption in the visible spectrum and therefore in a colored solution that can be colored intrinsically or by a color development procedure. Turbidimetry is not the same as colorimetry.

When I said from the beginning that the traditional molecular absorption method is not useful, it is because I was answering the question where photo-colorimetry is mentioned. The problem does not only lie in whether or not a stable colloidal dispersion can be prepared, because in many cases it can, the problem goes beyond that, the absorbance is a measure of the amount of radiation absorbed, although there are always scattering effects that can be minimized to a certain extent. However in suspensions where there is strong light scattering effect, the Lambert-Beer law is not fulfilled because the asborbance is wrong due to the predominance of scattering over radiation absorption. Everything I said from my first answer is clearly stated in the literature. Here is an example:

https://pubs.aip.org/aip/rsi/article/92/12/123103/283238/Absorbance-spectroscopy-of-light-scattering

In any case, the effect that light scattering can have on the measured absorbance depends on the particle size and the number of particles, so I said that it cannot be ruled out that in poorly scattering systems it can be applied. But I referred to the molecular absorption method where transmittance is measured. There may be other ways of using a spectrophotometer that do not involve the absorption method.

Javier Ernesto Vilaso Cadre

I agree that "a spectrophotometer is not the same as using the photocolorimetric method." Just an additional clarification.

Three types of instruments are used in colorimetric methods for color measurement: tristimulus, densitometer, and spectrophotometer.

Now back to the original questions.

"the concentration of colloidal particles in an aqueous solution" Particles in water is a heterogeneous system, the term "concentration" is applicable for homogeneous solutions." The question is ill formulated and does not have an answer.

The discussion was mostly educational, but very helpful. I learned much. Thanks

The idea of using a UV spectrometer is to choose a wavelength where molecular absorption is absent. The signal is then entirely due to particle scattering and the technique is turbidimetry, not colorimetry. Strictly one should use a narrow slit so that only a narrow range of scattering angles is accepted. The late, great Kees de Kruif used this method very effectively to measure the aggregation kinetics of casein micelles. A particular geometry can be made absolute by calibrating with sample of particles with known concentration, size and refractive index, typically polystyrene latex. The results can then be interpreted for other samples using either the formula for Rayleigh scattering or Mie theory, using a formula given by van de Hulst in his book "Light scattering by small particles".

The concentration of colloidal particles in an aqueous solution can be determined using various methods. The choice of method depends on the characteristics of the colloids without leaving out the information you seek.

Just as Yurii V Geletii i V Geletii rightly pointed out, you would need somewhat information about the colloidal sample you hope to determine the concentration. However, some methods that can be used are:

I believe, (in theory), other analytical techniques such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS) or Atomic Absorption Spectroscopy (AAS) can be also be used to analyze specific elements in colloidal solutions, providing information about concentration.

But I believe somewhat information must be provided in order to decide on the best method for determining the concentration for the colloidal particles in aqueous.

I hope you find this helpful.

Best Regards!

Titus Sobisch I disagree. Particle solutions do exist in theory and in practice, and they are a fundamental aspect of many scientific and industrial processes. A particle solution typically refers to a mixture in which particles are dispersed in a solvent or liquid medium. The particles may vary in size, composition, and origin, and they can be classified into different categories such as suspension, emulsions, solutions as well as colloidal solutions, the elephant in the room

You can use ultrasound technique

Marvellous Eyube

particle dissolution exist, meaning molecules forming the particles are transfered into solution and finally the particles distinct.

Colloidal solutions are solutions of macromolecules or micellar solutions. All others belong to dispersions. For details see the definition of a solution, which are also characterized by the nonpresence of interfaces. See also details given in the link before.

Titus Sobisch Thank you very much!