Carbon dioxide back titration

To determine the alkalinity, dissolve a known weight of tablets in 0 1 N hydrochloric acid, boil to eliminate carbon dioxide and titrate back to phenolphthalein. 

Weigh exactly 2 g, dissolve in 25 ml of water, add 25 ml of 0-5N hydrochloric acid, boil for some minutes to drive off carbon dioxide and titrate back with 0-5N alkali to phenolphthalein. 1 ml 0-5N = 0-04201 g NaHCOa. 

Alkalinity measurement is also required for the determination of active matter by difference and equivalent weight calculations. It can be determined as two of the following compounds sodium bicarbonate, sodium carbonate, or sodium hydroxide. The sample is titrated to a phenolphthalein endpoint to determine the sodium hydroxide/sodium carbonate content. An added measure of acid converts any bicarbonate to carbon dioxide, which is subsequently removed from the solution. Back-titration of the excess acid gives a measure of the amount of bicarbonate and/or carbonate present. 

Carbon dioxide is not a common oxidation product in periodate work, but it does appear in the oxidation of ketoses,49 a-keto acids,14,39 and a-hydroxy acids,14 39 and it is often a product23 141 of overoxidation. Carbon dioxide analyses have been carried out using the Plantefol apparatus,49 the Warburg apparatus,14 23 and the Van Slyke-Neill mano-metric apparatus,39 and by absorption in standard sodium hydroxide141 followed by back-titration with acid. A most convenient method is the very old, barium hydroxide absorption scheme.16 The carbon dioxide is swept from the reaction mixture into a saturated, filtered barium hydroxide solution by means of a stream of pure nitrogen. The precipitated barium carbonate is filtered, dried, and weighed. This method is essentially a terminal assay. The manometric methods permit kinetic measurements, but involve use of much more complicated apparatus. 

Various workers have discussed the determination of total alkalinity and carbonate [ 10-12], and the carbonate bicarbonate ratio [ 12] in seawater. A typical method utilises an autoanalyser. Total alkalinity (T milliequivelents per litre) is found by adding a known (excess) amount of hydrochloric acid and back titrating with sodium hydroxide solution a pH meter records directly and after differentiation is used to indicate the end-point. Total carbon dioxide (C milliequivelents per litre of HCO3 per litre) is determined by mixing the sample with dilute sulfuric acid and segmenting it with carbon dioxide-free air, so that the carbon dioxide in the sample is expelled into the air segments. The air... 

In this experiment the neutralizing power of various antacids will be determined. Antacids contain basic compounds that will neutralize stomach acid (stomach acid is HC1). The amount of base in the antacid tablets will be determined by an acid-base titration. It is a back titration method. This method is used because most antacids produce carbon dioxide gas, which can interfere with the titration. By initially adding an excess of acid, one can drive off the C02 by boiling the solution before titrating the excess acid. There are many brands of commercial antacids with various ingredients. A few of the common ones are listed below ... 

In the application laboratory, students determine the quantity of carbon dioxide produced from the decay of leaves over a given period of time and report their results as milligrams of C02 per gram of leaf per day. They set up compost jars and a control jar a week in advance to give the leaves a chance to decay. The students back titrate to determine the number of moles of sodium hydroxide left unreacted, which enables them to ultimately determine the quantity of carbon dioxide produced from the decay of the leaves. The previously done skill -building laboratory has given them experience with back titrations when they analyzed the number of moles of hydrochloric acid neutralized by different commercial antacids. 

Fe(III) can be titrated accurately with thiocyanate as an indicator if the reagent is added slowly near the end point or the temperature raised to 50 to 60°C. Dissolved air should be removed with a stream of carbon dioxide. Cu(II) can be titrated in the presence of thiocyanate to give a white precipitate of Cu(I) thiocyanate. Some Fe(II) is added as an indicator, and the end point detected by the disappearance of the red Fe(III)-thiocyanate color from the solution. Details for the determination of many oxidants (usually by adding an excess of Ti(III) and back-titrating with Fe(III), with thiocyanate as indicator) have been given. Potentiometric end points often are used. 

Decarboxylation, another traditional method for pectin quantitation, is achieved by treatment of pectins with 19% hydrochloric acid—evolved carbon dioxide is collected in a standard solution of sodium hydroxide and back-titrated with acid (Schultz, 1965). This method is considered most reliable but is time-consuming and needs a relatively large quantity of samples. 

As an alternative, the acid can be introduced in an amount slightly in excess of that needed to convert the sodium carbonate to carbonic acid. The solution is boiled as before to remove carbon dioxide and cooled the excess acid is then back-titrated with a dilute solution of base. Any indicator suitable for a strong acid/strong base titration is satisfactory. The volume ratio of acid to base must of course be established by an independent titration. 

Carbon dioxide reacts with primary amines to form a carbamic acid (Pine et al., 1980). Carbamic acids are unstable the very act of dissolving the carbamic acid in a solvent for titration or for conducting NMR studies, for example, converts the acid back to amine plus CO2 (Elsbernd, 1988). Heating the acid to about 50-100°C can force the reaction in the reverse direction, and if the CO2 that is generated is removed, the amine can be recovered. Although the conversion of carbamic acid back to amine plus CO2 occurs quite readily (Pine et al., 1980) the reversion is not instantaneous (Gallagher and Krukonis, 1987). What can also occur during the reverse reaction is reaction between any unconverted carbamic acid and the amine that has just reformed to form a urea. 

The establishment of the structure of monocrotic acid as a, 3-dimethyl-levulinic acid (LXIV) and its formation from monocrotalic acid by loss of carbon dioxide limits very dehnitely the possible structures of the latter. Monocrotalic acid was known to contain one carboxyl group and the presence of a lactone group was indicated on back titration of the acid... 

Back-titration method for the determination of "free, dissolved carbon dioxide"... 

The free dissolved carbon dioxide level is measured in the field (sampling location) by means of back titration (Chapter 1) (b). 

The back titration method can be used to measure total alkalinity with simpler equipment than that required for the potentiometric titrations described above, but at the cost of a poorer precision. The method described is that of Gripenberg (1936) as modified by Koroleff (1972). The sample is acidified with hydrochloric acid to a pH of about 3.5, and the carbon dioxide is driven off by boiling. The solution is then back-titrated to pH 6 with sodium hydroxide solution using bromothymol blue as indicator. The alkalinity is estimated simply as the difference between the amounts of hydrochloric acid and sodium hydroxide added. 

To determine both chlorine and carbon dioxide, the chlorine is absorbed by a solution that contains known amounts of acid and potassium arsenite, and the chlorine is determined by back-titration of the arsenite. The carbon dioxide, which is not absorbed by this solution, is then absorbed by potassium hydroxide solution. 

In organic analysis, most of the time the determinations involving dichromate ions are back titrations. The dichromate in excess is determined by indirect iodometry. Several organic compounds can be determined by chromimetry, including ethylenics, alcohols, carboxylic acids, and aldehydes. The reaction products are usually water and carbon dioxide. Ethanol gives acetic acid. This reaction is used to determine the alcohol level in blood. 

Ammonium bicarbonate, NH4HCO3, Mol. Wt. 79 06, is weighed for determination by difference from a weighing bottle into excess of N acid the solution may be titrated back direct to methyl orange, or, preferably, after boiling off the carbon dioxide, titrated back with N alkali to methyl red. 1 ml N - 0-07906 g. 

Boil 100 ml of re-distilled industrial methylated spirit (95 per cent) in a 400-ml flask, add 0 5 ml of a 0 5 per cent phenolphthalein solution, allow to cool to 70° and neutralise at that temperature with O IN ethanolic potassium hydroxide. Add 10 g of the soap in thin shavings and dissolve it as quickly as possible by heating. Immediately after complete solution of the soap add 3 ml of N sulphuric acid and boil on a water-bath for at least ten minutes to ensure complete removal of carbon dioxide. If the solution is colourless, cool to 70° and titrate back with N sodium hydroxide until the pink colour reappears. If, after the boiling with acid, the pink colour returns, a further quantity of N sulphuric acid must be... 

Fundamental work was carried out by Fischer and Schmidt (8) using a tube furnace and combustion boats of pythagoras-mass. Carbon dioxide is absorbed in barium hydroxide solution (pH = 9.9) and determined by back titration... 

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