215.2 Approved for NPDES (Editorial Revision 1978) - CAS Lab

1 METHOD #: for NPDES (Editorial Revision 1978) TITLE:Calcium (Titrimetric, EDTA)ANALYTE:CAS # Ca Calcium 7440-70-2 INSTRUMENTATION:TitrationSTORET (mg/L CaCO ) 009103 Calcium, Total (mg/L Ca) and method is applicable to drinking and surface waters, domestic andindustrial lower detection limit of this method is approximately mg/l as CaC0 ;3the upper limit can be extended to all concentrations by sample dilution. It isrecommended that a sample aliquot containing not more than 25 mg CaCO of ion is sequestered upon the addition of disodium dihydrogenethylenediamine tetraacetate (EDTA). The titration end point is detected bymeans of an indicator which combines with calcium and barium interfere and alkalinity in excess of 30 mg/L may causean indistinct end point.

2 Magnesium interference is reduced or eliminated byraising the pH between 12-13 to precipitate magnesium laboratory titrimetric hydroxide, NaOH, 1 indicators are available, both laboratory prepared andcommercial, and may be used. Two are described (ammonium purpurate) indicator: This changes from pink topurple. Dissolve 150 mg of the dye in 100 g absolute ethylene glycol. Ifa dry powder is preferred mix 200 mg murexide with 100 g solid NaCland grind to 40 to 50 mesh. Titrate immediately after adding indicatorbecause it is unstable under alkaline Blue Black R (sodium-1-(2-hydroxy-1-naphthylazo)-2-na phthol-4-sulfonic acid) indicator: This changes from red throughpurple to bluish purple to a pure blue without any trace of red orpurple tint.

3 The pH of some waters must be raised to 14 (rather than12-13) by the use of 8 N NaOH in order to get a good color in a mortar 200 mg powdered dye and 100 g solid NaCl to 40 to50 mesh. Store in tightly stoppered bottle. Use g of this mixture EDTA titrant, N: Place g analytical reagent grade disodiumethylenediamine tetraacetate dihydrate, Na H C H O NC2H O in a 1 liter221012822volumetric flask and dilute to the mark with distilled with standard calcium solution ( ) by titration ( ). Store in periodically because of gradual calcium solution, N: Place g anhydrous calciumcarbonate(primary standard low in heavy metals, alkalies andmagnesium) in a 500 mL flask. Add, a little at a time 1 + 1 HCl ( )until all of the CaCO has dissolved.

4 Add 200mL distilled water. Boil3for a few minutes to expel CO . Cool. Ada a few drops of methyl red2indicator ( ) and adjust to intermediate orange color by adding 3 NNH OH ( ) or 1 + 1 HCl ( ) as required. Quantitatively transfer4to a 1 liter volumetric flask and dilute to mark with distilled acid solution, 1 + red indicator: Dissolve g methyl red in distilled water in100 mL volumetric flask and dilute to hydroxide solution, 3 titration procedure: Place 10 mL standard calciumsolution ( ) in a vessel containing about 50 mL distilled water. Add1 mL buffer solution ( ). Add 1-2 drops indicator ( ) or smallscoop of dry indicator ( ). Titrate slowly with continuous stirringuntil the last reddish tinge disappears, adding last few drops at 3-5second intervals.

5 At end point the color is blue. Total titration durationshould be 5 minutes from the time of buffer solution: Dissolve g ammonium chloride in 143 mL concammonium hydroxide in a 250 mL volumetric flask. Add g ofmagnesium salt of EDTA ( ) and dilute to the mark with distilledwater. Store in tightly stoppered plastic : Commercially available Eriochrome Black T is used in one ofthe three methods described. All gradually Mix g dye with g hydroxylamine hydrochloride. Dissolvein 100 mL of 95% ethanol or Place g dye in 100 g of triethanolamine Mix g dye and 100 g NaCl for dry Magnesium Salt: Commercially drinking waters, surface waters, saline waters, and dilutionsthereof, no pretreatment steps are necessary.

6 Proceed to most wastewaters and highly polluted waters, the sample must bedigested as given in the Atomic Absorption Methods section of thismanual, paragraphs and Following this digestion, proceed calcium content of the 50 mL aliquot to be titrated should be 5-10mg, therefore dilution should be used for high calcium the alkalinity is > 300 mg/L CaCO and cannot be reduced by 3dilution because of low calcium concentration, the alkalinity must bedecreased by acidifying, boiling one minute and mL NaOH solution ( ), or a volume sufficient to produce pH12 to 13, to 50 mL of Add to g indicator ( or ) or 1-2 drops if indicatorsolution is titrate with continuous stirring. Check to see that nofurther color change occurs when using murexide ( ) by adding 1 to2 more drops of titrant after recording milliliters of titrant at firstjudged end calciumwhere:A = mL titrantN = Normality of EDTA hardnesswhere A and N are the same as in and synthetic unknown sample containing 108 mg/L Ca, 82 mg/L Mg, mg/LK, mg/L Na, 241 mg/L chloride, mg/L nitrate N, 250 Fg/L nitrite N,259 mg/L sulfate, and mg/L total alkalinity in distilled water wasdetermined by this method with a relative standard deviation of and arelative error of in 44 Methods for the Examination of Water and Wastewater, 14th Edition, p 189,Method 306C (1975).

7 Book of ASTM Standards, Part 31, "Water", Standard D511-76, Method B, p253(1976).