Department of Microchemistry, Brussels University.

Unterzaucher's Titrimetric Oxygen Determination. (Report of informal discussions during the International Congress on

Analytical Chemistry, Oxford, 1952".)

By

A. Lacourt.

(Received June 14, 1954.)

The points discussed included:

1. Scavenging gases and their purification. 2. Quality of carbon. 3. Quartz used for tubes. 4. Temperature. 5. Preparation and quality of iodine pentoxide. 6. lVlethod of packing carbon. 7. Blanks.

1. Scavenging gases: Nitrogen originally containing 0.01 per cent of oxygen is used by Unterzaucher. (Argon has no particular advantages. I ts replacement in the U. S. A. by very pure helium - - 0.0001 per cent of oxygen - - is understandable since the latter gas is readily available there. This is not the case in Europe.) The purification of the nitrogen is therefore very important. Unterzaucher uses hot reduced copper (1.2-1.5 kg) over which the nitrogen passes at the rate of 10 ml per minute. The temperature of the copper is maintained at 500 r day and night. The metal is only replaced after being in use for 9 months.

Unterzaucher prepares his copper from copper oxide that has been granulated in a mortar, washed with acetic acid to remove all alkMi, and then washed with water. I t is then heated in a quartz tube to 800 ~

* Delay in publication is clue to the fact that these discussions took place subsequently among 22 interested microchemists who wished to share their experience of this new technique. These notes could therefore not be added at the time to Dr. Unterzaucher's paper 1 and discussion.

736 A. Lacourt : [Mikrochim. Acta

and cooled. A slow stream of hydrogen is passed over it and the temperature is slowly raised until reduction occurs. I t is important tha t reduction should be carried out at as low a temperature as possible, and always below 300 ~ . The contact t ime between the copper oxide and the hydrogen should be at least 5 seconds. Some workers recommend the use of a mixture of nitrogen and hydrogen for reduction.

Nitrogen purified over this copper, when tested, is so pure tha t oxygen is no longer detectable. I t is advisable to use paraffin oil in the flow- meter rather than sulphuric acid, to prevent reduction of acid and production of sulphur dioxide, leading to high blanks.

2. Quality o] carbon: The origin, ash content and surface capacity were discussed.

The ash content appears to be very important , as well as the nature of the elements present in the ash. Very little seems to be known about the composition of the ash. The Degussa C. K. 3 in Pellets recommended by Unterzaucher contains 0.01 per cent ash, and is obtained from mineral off. I t contains sulphur impurity, and therefore should be heated for some time, as recommended by Unterzaucher, before use.

Careful pretreatment of carbon from other sources seems to give an adequate product. Whatever its origin, the carbon should be packed so tha t the temperature is constant throughout, at about 1120 ~ . Both ends should be in the furnac% otherwise low results arise from reversible equilibria. Silver wire packing is not recommended in preference to quartz fibre. A constant temperature is more important than the precise temperature over the range 1115-1150 ~ .

Unlike earlier types of carbon, the Degussa carbon is not treated with hydrochloric acid before use. I t was suggested tha t it might be advisable to t rea t it in this way so as to reduce ash and alkali content. After being dried and sieved the carbon is heated for 48 hours at 110 ~ and is then filled into the analysis tube.

3. Quartz tubing: Bayer, ]=[ereaus and Vitreosil quartz have all proved satisfactory. The original source and the composition are unknown. Devitrifieation arises primarily from the presence of sodium and phosphorus. Nickel and platinum plating make the quartz less resistant. I t would be helpful to know more about the composition of the carbon and the quartz. Contact of all metals with the quartz should be avoided.

Quartz treated with hydrofluoric acid for 5 hours before use, then heated with a blast burner to red heat, is highly resistant and transparent.

The tube walls should be at least 1.5 mm thick. Such tubes, when attacked, become covered by a resistant coating, whereas thin-walled tubes have little resistance and break.

4. Temperature: This should be as constant as possible, at the opt imum value, for all reagents throughout the whole procedure.

1954/6] Un te rzauehe r ' s T i t romet r ic Oxygen De te rmina t ion . 737

5. Iodine pentoxide: Samples of iodine pentoxide have often failed to give satisfaction because of high blanks. Some commercial products (e. g., B. D. H., Arthur tL Thomas Company) have proved satisfactory if protected from direct sunshine. Leverkusen proposes to market an Iodine Pentoxide Recryst. which remains completely white even in direct sunshine.

6. Carbon paclcing: If the packing is not tight at both ends low results are obtained. Coarse quartz fibre is preferable to silica wool. Carborundum chips have also proved satisfactory.

7. Blanks: Following Unterzaucher's directions, no blank is obtained after 15 minutes. Substances with high chlorine content can lead to reduction of silica and a consequent high blank. In such cases hydro- fluoric acid treatment of the quartz tube reduces the blank.

Organic substances with low oxygen content may give appreciable blank values through interaction of the excess hydrogen and the iodine pentoxide. So far it has proved impossible to detect less than 0.1 per cent of oxygen in hydrocarbons such as anthracene.

There may be considerable difference in the blank obtained in presence and in absence of organic substances. Even powdered quartz may give a considerable blank since it gives off water when heated; and most substances, because of the difficulty of removing adsorbed gazes, give a blank when tested.

Bibliography. l j . Unterzaucher, Analyst 77, 584 (1952).

~ikrochim. Acta 1954/6. 4;8