Wall-Wash Methods

The main methods are summarised below. More detailed protocols will be added over the course of time.

The investment in a flame photometer for Na and K means that all the major electrolytes are being independently measured, and thus I can check the ion balance to verify the overall accuracy. Typically, the cations and anions are charge-balanced within 5%. If there is an unusual electrolyte, or an unusual interference with one of the analyses, it would be obvious.

Work Up:
The sample, as received, is washed from the sample container, filtered and made up to 100ml. This is the soluble fraction which forms the basis for most of data that is reported. However, a further check is always made for insoluble sulphate components, and thus, the residue is digested with HCL 1.0M for 2 hours at 90°C, filtered, and made up to 100ml. This is the acid soluble fraction (AS).

A 25ml aliquot is titrated against standard silver nitrate using a chromate indicator – Mohr’s method.
LOD = 5nmole/sqcm

A 25ml aliquot is treated with excess barium chloride and the precipitate is assayed for barium by indirect EDTA titration using magnesium nitrate and eriochrome black T as indicator at pH10
LOD = 5nmole/sqcm

A 10ml aliquot is titrated against standard hydrochloric acid using a mixed methyl red, bromocresol green indicator.
LOD = 25nmole/sqcm

Sodium and Potassium:
Analysis by dual channel flame photometer calibrated against multipoint standards for both sodium chloride and potassium chloride.
LOD Na = 0.3nmole/sqcm and LOD K = 0.04nmole/sqcm

Calcium and Magnesium:
Two 10ml aliquots are firstly freed of organic matter by oxidation with nitric acid, prior to a dehydrating with hydrochloric acid to precipitate silicate, and finally held at pH6 to precipitate aluminium and iron. Thus freed of potential interfering species, one aliquot is used to determine calcium and magnesium in combination by indirect EDTA titration against standard magnesium nitrate using an eriochrome black T indicator at pH10. The second aliquot is used to determine calcium alone by direct titration against EDTA using Paton and Reeder’s indicator at pH13.
LOD Mg = 13nmoles/sqcm, LOD Ca = 13nmole/sqcm

Iron and Aluminium:
One 10ml aliquot is used to determine combined iron and aluminium by indirect EDTA titration against standard lead nitrate using xylenol orange as the indicator at pH 5. An excess of sodium fluoride is then used to decomplex the aluminium, but not the iron, and titration continued to measure aluminium alone.
LOD Fe = 25nmoles/sqcm, LOD Al =13nmole/sqcm

A 1ml aliquot is used in the analysis of low order silicate species by the standard reduced silicomolybdate method. A single standard and blank is then used to calibrate spectrophotometric response at 810nm.
LOD (as SiO2) = 1nmole/sqcm

A 1ml aliquot was used in the analysis of free phosphate by the standard phosphomolybdate method. A single standard and blank is then used to calibrate the spectrophotometric response at 820nm.
LOD = 0.6nmole/sqcm

Calcium and Magnesium – confirmatory method:
This method provides a cross-check on the accuracy of the compleximetric method where interference by organic species can be a problem. A 10ml aliquot is processed, firstly to precipitate calcium as the oxalate followed by precipitation of magnesium as the ammonium phosphate. The calcium oxalate is taken up in sulphuric acid, and the oxalic acid released is then determined by titration against standard permanganate. The magnesium ammonium phosphate is dissolved in excess EDTA at pH 10 with the excess determined by back-titration against standard calcium nitrate using eriochrome black T as an indicator.
LOD = 10nmole/sqcm