SALT LAYDOWN PROJECT - BONNEVILLE SALT FLATS
1997-99 Progress Report

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APPENDIX 9

Table A9.1. - Comparison of EDTA and ICP analytical results for Laydown-brine sample triplicates B-1 and B-9.

Table A9.2. - Reilly EDTA (Mg+Ca & Mg-Ca) values and U of Az, and CMS Mg values for A-1 through A-9.

Table A9.3. -Reilly EDTA (Mg+Ca & Mg-Ca) values and U of Az, and CMS Mg values for A-12 - A-22 (EDTA corrected with CMS calcium values).

Table A9.4. -Reilly EDTA (Mg+Ca & Mg-Ca) values and U of Az, and CMS Mg values for A-12 - A-22 (EDTA corrected with U of Az calcium values).

VOLUMETRIC ANALYSIS FOR MAGNESIUM - EDTA [1] TITRATION

Reconciliation of Differences Between Reilly and Independent Laboratories Mg Analyses

Reilly MgCl₂ values for the AA@ sample triplicates averaged about 5 times independent laboratory [2] values for AB@ and AC@ sample triplicates. The reason for this marked difference is mainly due to differences in analytical methods used by Reilly, CMS, and U of Az. Reilly used a traditional volumetric analysis method (EDTA titration), while CMS and U of Az used instrumental analysis (ICP-spectrometry).

The differences in Reilly's titration-method results versus CMS - U of Az results are believed to be due to an interference caused by the natural presence of calcium in the analyzed brine. This is suggested because of the following characteristics of the EDTA titration method:

During EDTA titration of a brine sample that contains both calcium and magnesium, calcium ion is preferentially complexed by the EDTA before any magnesium ion can be complexed. Therefore, the total EDTA volume added to the unknown solution is comprised of an EDTA volume consumed by calcium complexation and an EDTA volume consumed by magnesium complexation. If the analyst is unaware of calcium presence in the brine sample, an error will be introduced because the total EDTA volume added to the unknown solution will be assumed to have complexed with magnesium only. This will result in an artificially-elevated magnesium concentration. Because Reilly does not include calcium in its suite of brine analytes, calcium is not accounted for in Reilly's titration calculations.

To confirm that presence of calcium was the source of bias in the AA@ triplicate EDTA results, BLM requested that CMS perform EDTA magnesium determinations on AB@ sample triplicates whose magnesium concentrations had previously been determined by ICP. CMS selected sample triplicates B-1 and B-9 for the comparison. Results of the EDTA titration of B-1 and B-9 were processed in equations 1 - 3 to obtain the respective magnesium and magnesium chloride concentrations.

  (2)

   (3)

Equations 4 - 7 show CMS computation results for B-1 and B-9 using general equations 1 and 2 to obtain Wt% Mg + Ca and Wt% Mg:

  (4)

  (5)

   (6)

  (7)

Table A9.1 shows good agreement between the CMS EDTA and ICP results for B-1 and B-9. Calcium-corrected EDTA results (i.e., Mg-Ca) are within 15% of ICP results as compared with uncorrected EDTA values (i.e., Mg+Ca) which average 5 times ICP values. This comparison illustrates the importance of accounting for the presence of calcium in the Laydown-brine sample triplicates.

Calcium Adjustment to Reilly AA@ Triplicate EDTA Values

Reilly=s reported EDTA magnesium values (Reilly/17Mar99/EDTA Mg+Ca) for the AA@ sample triplicates have been adjusted (using equations 2 and 3) to account for the presence of calcium. Table A9.2 uses CMS calcium analyses (as Wt%) from B-1 through B-9 triplicates as the calcium input to equations 2 and 3, and results are listed under the heading AEDTA (Mg-Ca)@. Reilly=s calcium-adjusted magnesium values (i.e., Wt% Mg and MgCl₂, respectively) for the AA@ triplicates are then compared with respective CMS and U of Az analyses for AB@ and AC@ triplicates.

Tables A9.3 and A9.4 are in the same format as Table A9.2 and list the calcium-adjusted EDTA magnesium values for 8 samples from AA@ sample triplicates A-10 through A-24. Table A9.3 uses CMS calcium values from AB@ triplicates, and Table A9.4 uses U of Az calcium values from AC@ triplicates as input to equations 2 and 3. While both sets of calcium values from the 8 triplicates bring Reilly=s EDTA (Mg-Ca) values in much closer agreement with U of Az and CMS magnesium values, the best agreement is obtained using U of Az calcium values.

[1] EDTA - Ethylenediaminetetraacetic acid
[2] Chemical and Mineralogical Services (CMS), and University of Arizona, Dept. of Chemistry (U of Az)

Project Contact: Bill White

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