Alkalinity: total, methyl orange

Commercial samples of water are frequently alkaline due to the presence of hydrogencarbonates, carbonates, or hydroxides. The alkalinity is determined by titrating a 100.0 mL sample with 0.02M hydrochloric acid using screened methyl orange as indicator (or to a pH of 3.8). To obtain the total cation content in terms of CaC03, the total methyl orange alkalinity is added to the EMA. 

Similarly, Total Alkalinity or Methyl Orange Alkalinity... 

FWTH = FW total hardness, in mg/1 (ppm) CaC03 FWMA — FW methyl orange/total alkalinity, in mg/1 CaC03 RBWMA — Required BW methyl orange alkalinity, in mg/1 CaC03 COC — Cycles of FW concentration present or desired in the BW... 

Alkalinity content may be tested and reported in various ways, but for BW, the most important tests are methyl orange/total alkalinity (reported as M or T alkalinity, expressed as mg/1, mg/kg, or ppm CaC03), and caustic /hydroxide alkalinity (reported as OH alkalinity, expressed as mg/1, mg/kg, or ppm CaC03). 

In the boiler, the BW phenolphthalein alkalinity (P alkalinity) usually is more than half of the total or methyl orange alkalinity (T or M alkalinity) ... 

Primarily the sum of carbonate, bicarbonate and hydrate ions in water, but phosphate, silicate etc. may also contribute partially to alkalinity. Normally expressed as ppm (mg/1) CaC03. Phenolphthalein alkalinity (P Aik.) is that portion of alkalinity titrated with acid to pH 8.2 end-point, while total alkalinity (T Aik. or M Aik.) is that titrated with methyl orange indicator to pH 4.2 endpoint. 

Example 6.6 A 50.00 mL water sample is titrated to the phenolphthalein end-point with 5.00 mL of 0.02 N H2SO4, and to the methyl orange end-point with additional 5.00 mL of the acid. What is the total alkalinity, in mgcaco3 /L ... 

In determining the alkalinity of a water sample having a pH of 7.5, 200 ml of the sample is titrated with 0.02 N H2SO4 (0.02 moles H+ per liter). It is found that 26.1 ml of acid is required to reach the methyl orange end-point. What is the total alkalinity ... 

This is referred to as total alkalinity. If the sample has a pH less then 8.3, a singe titration to the methyl orange end point is carried out. Alternatively, apotentiometric titration can be carried out, measuring the pH with an electrode and plotting the titration curve. A typical potentiometric titration cnrve is shown in Figure 6, illustrating the location of the stoichiometric end points and the contribution of various species to alkalinity. 

In the determination of total alkalinity a known volume of sample is titrated with a standard solution of a strong acid to a pH value in the approximate range of 4 to 5 and usually in the range 4.5 to 4.8. This endpoint is commonly indicated by the color change of the indicator methyl orange therefore, the total alkalinity is often referred to as the methyl orange alkalinity. The H added is the stoichiometric amount required for the following reactions ... 

A sample of water from the overflow of the recarbonation basin that follows a precxpitation/softening process has a pH of 9.0 200 ml of the water require 1.1 ml of 0,02 N H2S04 to titrate it to the phenolphthalein endpoint and 22.3 ml of 0.02 N H2SO4 to titrate it further to the methyl orange endpoint. Assuming the sample contains no calcite particles, what are the total and carbonate alkalinities of the sample in meq/liter and the total alkalinity in mg/liter as CaCOa ... 

Total alkalinity = the meq of acid required/liter of sample to reach the methyl orange endpoint... 

Total alkalinity is based on the volume of titrant required to lower the pH of the sample to the methyl orange endpoint, which is approximated by pH 4.3 in this problem. 

About 7-8 grammes of the sample are dissolved in water, and made up to 100 c.c., and the total alkalinity determined by titrating 20 c.c. with N/1 acid, using methyl orange as indicator. To another 20 c.c. is added barium chloride solution (10 per cent.) until it ceases to give a precipitate, the precipitate allowed to settle, and the clear supernatant liquid decanted off, the precipitate transferred to a filter paper and well washed, and the filtrate titrated with N/1 acid, using phenol-phthalein as indicator. The second titration gives the amount of caustic alkali present, and the difference between the two the proportion of carbonate. 

FIGURE 18.14 Automated analyzer system for the determination of total alkalinity in water addition of a water sample to a methyl orange solution buffered to pH 3.1 causes a loss of color in proportion to the alkalinity in the sample. 

Total Alkalinity is determined by titration with standard hydrochloric acid solution using methyl orange indicator solution or modified methyl orange indicator solution Sodium Carbonate or Potassium Carbonate (Gas-volumetric method, the lower limit of determination is 0.001 g as carbon dioxide) Carbon dioxide is evolved by acid decomposition of carbonate in the sample. The volume of CO2 is measured and calculated as sodium or potassium carbonate. 

The lactic acid may be determined from the alkalinity of the ash, using bromophenol blue or, better, methyl orange, as indicator, 1 ml 0-lN == 0-009008 g C3Hg03, calcium chloride by EDTA titration (see p. 145) and the total chlorine by the Volhard method (see p. 290). 

Magnesium oxide, MgO, Mol. Wt. 40 32. No assay is included in the JB.P. but magnesium oxide may be determined by solution of a known weight in excess of 0-5N acid and back-titration to methyl orange. 1 ifil 0 5N = 0 01008 g MgO. The U,S,P, requires the percentage of calcium oxide found by precipitation to be deducted from the total alkalinity. 

M alkalinity Methyl orange alkalinity. (See total alkalinity.)... 

A 200-mL sample of water required 25.12 mL of 0.0200N standard H2SO4 for titration to the methyl orange endpoint, pH 4.5. What was the total alkalinity of the original sample ... 

The total volume of acid [(A + B) ml] used in the two titrations, therefore, corresponds to the neutralisation of hydroxide, carbonate and bicarbonate and is, thus, a measure of Total Alkalinity, also called Methyl Orange Alkalinity. Alkalinity is expressed as parts of equivalent CaC03 per million parts of water. 

When the sample contains only Hydroxide, the neutralisation is complete at the phcnolphihalein cad-point (Reaction 3.13) and so B, the additional volume of acid needed for neutralisation to methyl orange end-point, will be zero. In this case, Hydroxide Alkalinity isecpial to Phenolphihalein Alkalinity, which is also the Total Alkalinity. 

When the sample contains only Bicarbonate, it does not give, a pink colour with phenolphthalein indicator. Therefore A, the volume of acid consumed with phenolphihalein indicator is zero. Titration to methyl orange end-point represents the complete neutralisation ofbicarbonate (Reaction 3.15). Bicarbonate Alkalinity is therefore equal to Total or Methyl Orange Alkalinity. 

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