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. 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.