# Is it possible to measure light absorbance for analysis of lead concentration without photospectrophotmeter

I’m designing an experiment which requires I measure final and initial lead concentrations for the determination of aqueous lead uptake by different mediums. Because lead concentrations would be measured on the ppm scale, I’d need to use a spectrophotometer to measure light absorbance by the water samplings. I could then use Beer Lambert’s Law to determine concentration.

With recent protocols surrounding COVID-19, it’s been difficult to contact an institution. I was wondering if there was any way I could measure light absorbance of my samples at home. Is there some other apparatus I could look into using (or perhaps even purchase)?

All the best!

• Unfortunately no. However small spectrophotometers are commercially available. Check Vernier's website: vernier.com/product/go-direct-spectrovis-plus-spectrophotometer – M. Farooq Mar 20 '20 at 18:12
• You still need dyes that form a colored complex with lead. – M. Farooq Mar 20 '20 at 18:14
• You need a dye, as @M.Farooq said, but there is the long shot possibility of smartphone spectroscopy here. Easy to find other articles by searching, but it would be much better to have an inexpensive real spectrophotometer. – Ed V Mar 20 '20 at 18:32
• Not an option right now, but one university where I have been working had regular "garage sales" of old-ish equipment. And there's labexchange.com – cbeleites unhappy with SX Mar 20 '20 at 21:06

Another possibility is the grease spot photometer (aka Bunsen photometer) German Wiki page. This can be homemade, you need a piece of paper, some wax or oil, two light sources, a meter stick and for the measurement of solutions also some e.g. cardboard to shield unwanted light. Also the darker the room, the better.

The underlying idea is that light intensity from two different sources can be compared (and found to be equal) with a paper with a grease (waxed) spot: the spot will vanish when as much light is transmitted through the spot as reflected around.

If you move a light source to achieve this condition, you can conclude relative intensities from the distance (intensity goes 1 / d² with d the distance from the respective light source).

### How to measure photometrically

so schematically we have:

o--U----|----o

+---d---+

with light sources o, cuvette U and the spotted paper |. Measure distance $$d$$ between equal intensity paper position and the light source with the cuvette.

We have $$I_{transmitted} \sim \frac{1}{d^2}$$

Next, a calibration series is acquired as usual: blank (which serves as $$I_0$$) and concentrations over the calibration range. Almost as usual:

$$E = -\lg \frac{I}{I_0} \sim \varepsilon l c$$ $$-\lg \frac{d_0^2}{d^2} = b c$$ $$\lg d = a + b' c$$

$$a$$ and $$b$$ are determined by calibration - I use here $$a$$ instead of $$2 \lg d_0$$ because several choices are possible and in general the fitted value should be used rather than the theoretical one since it includes some effects in which practice differs from theory (e.g. differences between % transmission in the grease spot from % reflection outside).

• This video is extremely interesting. But I don't see how this apparatus can be used to measure the absorbance of a given solution. – Maurice Mar 20 '20 at 22:01
• @Maurice: I updated with a rough outline of the procedure. – cbeleites unhappy with SX Mar 20 '20 at 22:35
• Interesting but I guess a smartphone might be better – Alchimista Mar 21 '20 at 12:58
• @Alchimista: In what respect? – cbeleites unhappy with SX Mar 21 '20 at 22:05
• In the overall procedure. I am sure the Bunsen approach might be surprisingly good if well operated but surely requiring skill. In all cases it depends on the accuracy needed. If this is for a small shop there are relatively cheap solutions on the market. – Alchimista Mar 23 '20 at 8:21

It is still possible to measure light absorbances of a colored solution without any photometer, simply by comparison.

First you prepare a reference solution with maybe 10 ppm Pb and an excess of a colorless reagent making a colored compound or complex with Pb. You prepare a series of 10 test tubes. You add 1 mL of this reference solution in the first test tube, 2 mL in the second, 3 mL in the third, 4 mL in the fourth, etc. up to 10 mL in the last. Then you add enough solvent to get a total volume of 10 mL in all tubes. You homogenize each test tube.You obtain a series of solutions. They are nearly colorless in the first tubes but the intensity of the color increases gradually from the 1st to the last one.

Then you prepare 10 mL of a new solution with exactly the same procedure used to produce the reference solution, but with the unknown solution instead of the known concentration (10 ppm). By comparison between all these tubes, you can get a good approximation of the concentration of the unknown solution.

Perhaps you can add H2S to create PbS (as a starting step). Then, based on an extract from Wikipedia on Lead Sulfide:

Although of little commercial value, PbS is one of the oldest and most common detection element materials in various infrared detectors.[12] As an infrared detector, PbS functions as a photon detector, responding directly to the photons of radiation, as opposed to thermal detectors, which respond to a change in detector element temperature caused by the radiation. A PbS element can be used to measure radiation in either of two ways: by measuring the tiny photocurrent the photons cause when they hit the PbS material, or by measuring the change in the material's electrical resistance that the photons cause. Measuring the resistance change is the more commonly used method. At room temperature, PbS is sensitive to radiation at wavelengths between approximately 1 and 2.5 μm. This range corresponds to the shorter wavelengths in the infra-red portion of the spectrum, the so-called short-wavelength infrared (SWIR). Only very hot objects emit radiation in these wavelengths.

some ideas. Perhaps examining a comparative study (based on known concentrations of Pb) of the changed in measured electrical resistance of a sample of PbS suspension taken from the solution after treating with the same volume of H2S followed by irradiation may provide a path. This not meant to be a complete answer, but a suggestion on a possible path to be explored.

[EDIT] To be clear, I am assuming that one cannot measure light absorbance, due to lack of equipment, for the purpose of establishing Pb concentration. But, one can perhaps measure electrical resistance. The question is can one construct a path (actually, more likely a graph to which a curve is fitted) to determine Pb concentration from the change in electrical resistance recorded upon applying a fixed amount of infrared radiation to a solution where varying amounts of PbS is introduced. If experimenting with known starting Pb concentrations, converted to PbS, suggests a detectable and measurably path with some degree of goodness-of-fit, then yes, else no (which is the only answer provided so far).

Here is some background on PbS based photodetectors where irradiation exposure varies and the amount of PbS concentration remains fixed.

PbS is a standard SWIR semiconductor detector (1 - 3.3 µm) whereas PbSe is used in the MWIR range (1 - 4.7 µm when uncooled; up to 5.2 µm when cooled). Our lead salt detectors are photoconductive; the detector resistance is reduced during illumination. The crystal structure is polycrystalline and is produced via chemical deposition.

Please note the actual size of the PbS based photodetector.

Note, my proposed study of feasibility avoids the use of a nephelometer (which can measure the concentration of some inert suspended particulates in say a liquid colloid by employing a light 'source' beam and a second light detector at 90° to the source beam) by taking advantage of the properties of irradiated PbS particulates. Also, it should be noted that a spectrophotometer, based on a quantitative measurement of the reflection or transmission properties of a material as a function of wavelength, based again on light, is also presumably not an available option.

[EDIT EDIT] Just found a reference, THE RELATIONSHIP BETWEEN ELECTRICAL RESISTANCE AND DISPERSED PHASE CONCENTRATION IN OIL IN WATER EMULSIONS to quote:

The electrical resistance of a series of oil‐in‐water emulsions has been measured, and used to test the validity of four equations relating resistance with the concentration of dispersed phase. A modification of one of the equations was found to give the best relationship.

In the current context, we have a suspension of irradiated PbS in water (releasing electrons, per this 2013 article 'Photoinduced electron transfer from PbS quantum dots to cobalt(III) Schiff base complexes: light activation of a protein inhibitor'), which I would have guessed is more promising in determining the PbS concentration than say of oil in a water emulsion.

Also found a new method to quote a source:

In summary, we have demonstrated a practical way to measure nanoparticle concentration in a colloidal solution using quartz crystal microgravimetry. Application of a small drop of the nanoparticle colloid in a volatile organic solvent to the crystal surface leaves a dry nanoparticle film after solvent evaporation. Crystal resonant frequency shifts obeyed Sauerbrey’s equation for the dry nanoparticle concentrations up to 1300 µg/mL, as calibrated using a set of serial dilutions of Si and Ag nanopowders in methanol, rhodamine B in methanol, and ferrocene in cyclohexane.

• In aqueous solution, adding $\ce{H2S}$ would create a precipitate of $\ce{PbS}$ which would be very fine particulates. If any kind of quantitative analysis could be done on the particulates it would be using a nephelometer. I'll point out though in some sort of indeterminate solution that $\ce{H2S}$ would precipitate many heavy metals. – MaxW Mar 20 '20 at 18:48
• I have addressed your comment by an edit in the thread. If light is NOT an option, it follows that a nephelometer is not likely a viable option either, and certainly show NOT be a rationale for dismissing electrical resistance paths. – AJKOER Mar 20 '20 at 20:10