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I’m trying to make a 10 ppb Lead Nitrate solution as part of a concentration calibration step for a Beer Lambert’s Law experiment. I plan on performing a dilution in three steps. The calculations have been evaluated using:

$c_{1}v_{1}=c_{2}v_{2}$

Step 1: Creating a 1000 ppm (=1000 mg/L) stock solution (let’s call it A) by dissolving 1 gram (1000 mg) of Lead Nitrate in 1 L of distilled water [creating 1L of stock solution A]

Step 2: Creating an even more dilute stock solution B of concentration 1 ppm (=1 mg/L = 1000 ppb) by diluting 1 mL of stock solution A with 1999 mL of distilled water. [creating 2L of stock solution B]

Step 3: Creating the desired solution C of concentration 10 ppb by diluting 0.25 mL of stock solution B with 24.75 mL of distilled water. [creating 25 mL of desired solution C)

Does this seem like an adequate method to obtain the desired solution? Should I add 1% v/v of Nitric Acid between every dilution step as a stabilizer to keep the lead ions in solution for as long as possible?

I only plan on using these solutions for about 10 days, so I’m not too concerned about the longevity of the solutions.

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    $\begingroup$ Your plan makes sense. And the idea of adding dilute nitric acid is also a good idea. $\endgroup$
    – Maurice
    Commented May 17, 2020 at 9:22
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    $\begingroup$ Note that, for ppb-level concentrations, you should prepare a blank of your nitric acid and water in the same way. $\endgroup$
    – user7951
    Commented May 17, 2020 at 9:54
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    $\begingroup$ I wasn't sure how accurate you want your final solution. Serial dilution is always risky. But, I'd recommend do the last dilution 1 mL to 100 mL instead of 0.25 mL to 25 mL. Measuring 0.25 mL makes a lot of error. $\endgroup$
    – Mathew Mahindaratne
    Commented May 17, 2020 at 9:58
  • $\begingroup$ @MathewMahindaratne That makes sense. Is there perhaps a less risky alternative to serial dilution? $\endgroup$
    – Mas
    Commented May 17, 2020 at 11:02
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    $\begingroup$ In general, to obtain a reliable 10ppb solution of lead nitrate (do you want 10ppb lead or 10ppb lead nitrate, actually?), you'll need water which has been certified for trace analytics regarding lead content. The other thing that might go wrong is adsorption of lead ions to your container (glass?), esp. upon storage of the solution. You might be better off using a suitable ionic matrix to prevent that, e.g. 100mM (or so) sodium nitrate, possibly in dilute nitric acid, to prevent the solution being alkaline and extracting stuff from the glass containers. $\endgroup$
    – imalipusram
    Commented May 17, 2020 at 15:42

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Your dilution approach can be heavily improved.

Rule no. 1: 2+2 $\neq$4 in solution preparation from an analytical chemist's perspective. An extreme example is that you add 50 mL of water+50 mL of ethanol $\neq$ 100 mL solution. If you add a solid NaCl to 1 L of water, the volume may not remain 1 L. It may decrease!

Analytical dilutions are always prepared in highest quality volumetric glassware (Called class A glassware), which comes in fixed volumes. You might have 2 mL, 5 mL, 10 mL, 25 mL, 50 mL, 1000 mL flasks in the university. Similarly, pipettes comes in fixed volumes. The graded ones (so called Mohr's) are not that great.

With these limitations, you design dilutions, in such a way that volumes are not fractional so a volume of 24.75 mL for dilution is a no-no.

The dilution relation is handy $$\mathrm{C_iV_i=C_fV_f}$$, where i and f indicate initial and final concentrations (C) and volumes (V).

So think about it now in order to prepare 1000 mg/mL you would like to dilute it 1000-fold in steps to make it a 1 ppm solution or 1000 ppb $\ce{Pb(NO3)2}$. From there you would like to dilute it 100-fold to make it 10 ppb.

The first question you would like to think now, is what volumetric glassware is available.

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  • $\begingroup$ Thank you for the clarification! Is it then that dilutions should only be performed with whole number volumes of diluent? Further, I only have access to standard glassware and was wondering if there was a way to quantify uncertainty on the concentration (because I don’t think my procedure will be near 100% accurate due to the materials I have available to me)? I’ve also looked into purchasing different micro-pipettes in the 0.1-2 micro liter range, so I don’t think the pipetting should be too huge a problem. $\endgroup$
    – Mas
    Commented May 18, 2020 at 1:35
  • $\begingroup$ Smaller the volumes, larger the errors. What are you trying to do with 10 ppb $\ce{Pb^2+}$? $\endgroup$
    – ACR
    Commented May 18, 2020 at 2:46
  • $\begingroup$ I was trying to create a calibration curve for Beer’s Law to determine Lead Nitrate concentration in solution, using 1,3,4 dimercapto 2,5 thiadiazole as the colorimetric complexing agent. The standardizing Lead Nitrate solutions are of concentrations 10 ppb, 20 ppb, 30 ppb, and 40 ppb. There is a calibrated curve published already, however. Would it be wise to simply use that calibration curve given the large margin of error in trying to create these select, very dilute solutions? $\endgroup$
    – Mas
    Commented May 18, 2020 at 2:51
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    $\begingroup$ No, you should not use a published calibration curve. $\endgroup$
    – ACR
    Commented May 18, 2020 at 4:19

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