During heavy metal analysis we need to run standards which are generally prepared in De-ionized or Distilled water. But I gone through literature in which 5-10% or 1-5% of HCl or HNO3 has been used.
Hi Sana,
For toxic metals analysis (ICP-OES and ICP-MS) of mining-impacted water, I have always acidified standard solutions and SAMPLES using HNO3 (1-2%) analytical grade (67%). I guess the acid percentage depends also on the nature of the samples. Which kind of samples are you going to analyse? which concentration?
As far as I know, water samples are acidified to:
- preserve most trace metals in solution prior to being nebulised;
- avoid or reduce precipitation.
I am interested on other answers as well, thanks for asking this.
Hi Sana Ullah
We digest soil or plant tissue materials in HNO3acid as it can convert the metal ions in the sample into their nitrate salts which is highly soluble. Plus, addition of HNO3will help to destroy the matrix (organic matter in the soil) which can interfere the chemical analytical process. After dilution, the final acid concentration would be more or less 2% acid.
Therefore, when we prepare the metal standards we need to match the matrix of unknown samples and SRMs. This is the prime reason to prepare the working standards in nitric acid.
Further, heavy metal standard preparation in nitric acid provides greater stability for the working standards to ensure that the metal ions will remain in the solution for a longer period of time. This is more important for transition metals which can hydrolysis, precipitate or degrade at lower acid concentration.
Hope this would help you. Cheers
When you prepare standards of (heavy) metals with concentration of 1000 ppm, some metal species can have low solubility at pH 4,5 ou more. So you need to add acid in the standard solution to ensure its solubility and the correct measurement.
You could see the free-software Hydra/Medusa to simulate some single-solution of CuSO4 or Cu(NO3)2, for exemplo. In this software, you can see the fraction of solubilized metal as a function of the pH.
Regards.
Our samples and standards are prepared in 1% nitric acid to preserve the samples and improve the analysis. However, some standards are in HCl because some elements (Hg, Au, probably others) need Cl to stay in solution at high concentrations (like the 1000 ppm in stock solutions).
Nitric acid is a strong oxidant for wet and dry oxidation of organic samples for metal analysis. it helps in the ionization of samples for easier detection during spectrophotomeric analysis. it does not necessarily result to loss of metal ion concentrations during use for digestion procedure. it quickens the digestions of samples for cationic dterminations
Hi Sana!
The ions of the analysed metals will remain in dissolved form to reliably determine their concentration.
For accurate and reliable analysis of the heavy metal to be analyzed, ionization in the water is required. In addition to ensuring matrix compliance of digested samples and calibration standards, acidification of water with 1-2% HNO3 and HCl aims to ionizzation of heavy metals. The type of acid or acid mixture to use depends on the type of heavy metal to be analyzed. The solubility (ksp) value of the heavy metal salt in water is also important in the choice of acid type. The main reason for the use of HNO3 is generally due to the almost whole dissolution of the nitrate salts of heavy metals in water. Since the solubility of chloride salts of heavy metals such as Hg, Au etc. in water is high, it is necessary to add HCl. Depending on the type of heavy metal to be analyzed and the type of medium(liquid or solid), it is sometimes preferred to mix HNO3-HCl (3/1 v: v) to increase the efficiency of digestion/ionization . I prefer adding a low percentage of HF(about 0.1%) to the this acid mixture to digest the rigid solid materials especially contain Si. When extraction or acidification is carried out with such mixtures, you avoid the precipitation can be observed.
In order to release the metals from the mesh they form in water, I use 1:4 ratio of nitric acid with perchloric acid (APHA). Often strong acids are suggested for preparing the standards.
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