IiVDUSTRIAL AND ENGINEERING CHEMISTRY
October. 1927
1159
Experiments in Wood Preservation‘ 111-Preservative Properties of Basic Substances By Leo Patrick Curtin ENGINEERING LABORATORIES, WESTERN UIION TELEGRAPH Co., A-EW YORK,N. Y.
1925, p r e l i m i n a r y t e s t s were made with sodium carbonate and barium hydroxide which justified this opinion in a c o n c l u s i v e manner.
Sodium Salts ‘Odium is regarded as practically nontoxic toward fungi,2 its kill-
, I t is shown t h a t the fungus Fomes onnosus and probably all other wood-rotting fungi may be inhibited by alkaline, or basic, substances. The fungus normally grows in an acidic environment and is, itself, ordinarily capable of producing t h e required degree of acidity in its medium of growth. If the basic material is present in concentrations beyond t h e capacity of the fungus t o neutralize, no growth takes place.
-
making it difficult to sell. They do not injure m e c h a n i c a l l y , homTeT7er, since the fiber is not attacked. The process of inhibiting the blue stain by sodium and bicarbonate was patented by Cowles4 in 1903. The inventor states that the blue stain is “due
INDUSTRIAL AND EAVGILVEERILVG CHEMISTRY
1160
of a salt of a feeble acid, such as carbonate or arsenite, evolution of acid by the fungus will proceed a t this high rate until normal acidity is reached or, if toxic materials soluble in the acid are present, until the fungus is killed by the dissolved material.
Vol. 19, No. 10
PRESERVATIVE: SrClz (Cultures made with hexahydrate and calculated t o SrClz) No growth 10.0* Growth 8.0 6.0
Slow healthy growth. On molar basis, carbonate is three times as toxic as chloride of strontium.
S a l t s of Alkaline-Earth M e t a l s PRESERVATIVE: CaC03
The other alkali-metal carbonates are too expensive for use as wood preservatives; therefore the investigation was turned toward the carbonates of the alkaline-earth metals. Barium ion is toxic toward animal life, and, to a lesser extent, toward the higher forms of plant life. It was expected that it would be more toxic toward fungi than the other alkaline-earth metallic ions. There was no reason for suspecting that Sr*, Ca*, and Mg++ would be toxic. Bateman and Baechler3 show that Fomes annosus will grow in the presence of 8.3 per cent BaC12.2H20, also that it is not completely inhibited by 7 per cent Ca(CHaC00)2.2Hz0or 10 per cent MgS04.7H20. This information was not available, however, at the time the following experiments with chlorides were made. It was decided to make a series of cultures of the chlorides and carbonates of barium, strontium, and calcium. Magnesium was left out, as it seemed to hare no advantage over calcium. The tests with the chlorides would indicate the toxicity of the metallic ion, while the tests with the corresponding carbonates would show the effect due the base. As before, Fomes annosus was the test fungus: No growth Growth
PRESERVATIVE: BaC03 0.65*
0.625*** 0.600***
0.50,
O.SO*
0.70 0.40
Growth sickly and discolored at 0.50. Partial inhibition much below killing point. Size of transplants is important, as acid in a large transplant will neutralize carbonate in adjacent culture and permit growth for several millimeters before finally checked. Working with very small transplants, it was found that concentrations of 0.55 per cent completely inhibited growth. Preparation of Cultures. A double-strength nutrient solution is prepared and to 10 grams of this are added 5 grams of sodium carbonate solution containing the required quantity of carbonate; then, after shaking, 5 grams of solution containing the proper amount of barium chloride are added, with shaking. The barium is immediately precipitated as carbonate in the hot solution and the cooling of the jelly holds it in place with a minimum of aggregation and settling out.
This is a very satisfactory method of preparing cultures of insoluble substances. Certain compounds, such as cupric aceto-arsenite, cannot be handled in this way, because of other undesired chemical reactions which take place. Such materials must be treated in powder form. PRESERVATIVE: B a ( 0 H ) 2. 8Hz0 1.0 Growth 0.90
hTo growth
PRESERVATIVE: BaClz (Dihydrate was used, then calculated t o BaCId ll.O** 12.0 5.0 3.0 9.0 7.0 Growth
? i o growth
Growth feeble and discolored. Barium carbonate is sixteen times as toxic to Fomes annosus as a molar equivalent of barium chloride. PRESERVATIVE: SrCOa 7
o*
2.5*
2.0*
2.0* 0.75*
I . 5* 1.0*
2.5 0.60
Slow healthy growth. N o growth
Growth
PRESERVATIVE: CaCh 8.0*
6.0*
10.0 5.0
Very slow but healthy growth a t 6 per cent. Calcium a s carbonate is about five times as toxic as in the form of chloride. Table I shows that sodium carbonate is sixteen times as toxic to Fomes annosus as sodium chloride, owing solely to its alkaline character. Barium carbonate is likewise sixteen times as toxic as barium chloride. Here the increased toxicity is due partly to the capacity for neutralizing acid possessed by the carbonate, and partly to a certain toxicity inherent in barium ion. The low solubility of the alkalineearth metal carbonates is also a factor. The toxicity of barium ion is not very great, as is shown by the toxicity tests with barium chloride. However, the growths of the fungus a t concentrations much below the killing point were sickly and discolored in all the barium cultures, while the strontium and calcium cultures showed slow but healthy growths just under the killing point. T a b l e I-Comparative Toxicities of C a r b o n a t e d and Chlorides of Sodium, Barium, S t r o n t i u m , a n d Calcium RATIO on SUBSTANCESOLUBILITY KILLIXG POINT TOXICITIES Mol/liter Pev cenf Mol/lifer (COd--:CliVaC1 5.5 1.000 16 h'azCOa 0 . 3 3 0.062 ( 0 . 5 mol) BaCIz 11.0 0.53 16 BaC03 0.00011 0.625 0.032 SrClz 10.0 0.63 0.20 3 SrCOa 0.00007 3.0 8.0 0.72 CaClz 1.5 0.15 5 CaC03 0,00013
Strontium and calcium carbonates are, respectively, three and five times as toxic as the corresponding chlorides. These toxicities are too low to be of interest in wood preservation. Sodium carbonate is fully as toxic as zinc chloride, but its solubility renders it undesirable for timbers which are exposed to rains. For this reason it was not further investigated. Barium carbonate, however, seemed to have possibilities as a wood-preservative since it is moderately toxic, of very low solubility, inexpensive, and obtainable in large quantities. T r e a t m e n t of Wood w i t h B a r i u m C a r b o n a t e
1.1 0.80
The hydroxide is entirely converted to carbonate by atmospheric carbon dioxide. The killing point of 1.0 checks that of 0.625 in the barium carbonate series.
No growth Growth
No growth Growth
1.0
Slow but healthy growth. Growth of fungus in the highly opaque alkaline-earth carbonate cultures makes the jelly translucent by dissolving the precipitate of carbonate.
The most convenient method of precipitating barium carbonate in wood is to treat with a solution of barium hydroxide and allow the hydroxide to be converted to carbonate by atmospheric carbon dioxide. Such a solution is most readily prepared by dissolving barium hydroxide in warm water. The water should be reasonably pure with no more than traces of carbonates and sulfates. It must also be somewhat above the ordinary temperature, since barium hydroxide is not sufficiently soluble in cold water. While barium hydroxide forms a saturated solution of but 3 per cent strength a t 15" C., its solubility increases rapidly with rising temperature and quite concentrated solutions may be made with warm or hot water. Wood treated with the barium hydroxide solution has, after diying out, a n electrical conductivity almost as low as that of untreated wood. This is, of course, due to the low solubility of the barium carbonate which has been precipitated from solution. This feature is of importance in
I S D U S T R I A L S N D ESGISEERING CHEMISTRY
October, 1927
the treating of railroad ties, telegraph poles, and cross-arms which carry electrical conductors. Where the conductivity of the wood is not of interest, it is preferred to prepare the solution by dissolving barium chloride and sodium hydroxide in the proper portions, a small amount of by-product sodium chloride being formed. Such a solution costs slightly less than if prepared from barium hydroxide. Barium hydroxide solution, if concentrated and hot, is injurious to wood, resembling other caustic alkalies and zinc chloride in this respect. For this reason it was decided to treat a t 125" to 135" F. (52' to 57' C.). I n 1926, one hundred poles were satisfactorily treated a t this temperature, the concentration of barium hydroxide corresponding to 5 per cent barium carbonate. There was no evidence that the mood had been injured in any way by the solution and the treatment could probably have been made with safetyat a somewhat higher temperature. The disadvantages of the barium carbonate preservative are its very moderate toxicity, 55 per cent that of zinc chloride, and the fact that the hydroxide must be protected from carbon dioxide before injection into wood. Its advantages are its simplicity, cheapness, permanence, good penetrating properties, low electrical conductivity, and the fact that both barium hydroxide and carbonate are noncorrosive toward iron. This last property makes it available for use in standard iron pressure-treating cylinders. It is also of interest to users of treated wood who have found wood treated with zinc ch!oride and certain other halogen salts to be somewhat corrosive toward iron spikes, bolts, and nails. T e s t s w i t h Zinc S a l t s
In connection with the work on the alkali and alkalineearth metal carbonates, some toxicity tests were made on zinc oxychloride and zinc hydroxide. The object of these tests was to show that insoluble zinc salts of feeble acids are as toxic as zinc chloride because of the solvent action of acids produced by fungous growth. Zinc chloride is a very soluble salt, 100 grams of water dissolving 209 grams of it a t 0" c. This would appear to make it valueless as a wood preservative for outdoor timbers; yet it has met with some success in this field, particularly in regions of low rainfall. Bwause of the high solubility of zinc chloride, it seems unlikely that the resistance t o decay shown by wood treated with this salt is entirely due to zinc in the forrn of chloride. After several years' exposure, it is probable that zinc in a difficultly soluble form is the principal preservative substance. Zinc chloride is capable of reacting with hpdioxyl and amino groups in wood, thereby producing zinc compounds which would be of considerable permanence. By a series of hydrolyses due to dilution by rainwater, resulting in loss of hydrochloric acid, it is also capable of forming oxychloride, hydroxide, and oxide-all compounds of low solubility.
+
ZnCL H2O F-L Zn(OH),Cl Zn(0H)CI H20 Zn(OH)?
+
+ HC1 + HCl
The oxychloride and hydroxide are difficultly soluble and would resist the leaching effect of rains for a long period. Toxicity tests were made on these two materials in standard nutrient gel, following the procedure outlined in connection with barium carbonate. I n precipitating the zinc in the gel as oxychloride, one molar equivalent of sodium hydroxide was used and in preparing the zinc hydroxide cultures, two molar equivalents of base were added. Sogrouth Growth
PRESERVATIVE: ZnClz 0 li* 0.40* 0 50 0.30 0 20 0.15
0.10*
0 05
Very slight growth a t 0.30, slow healthy growth a t lower concentrations. The hot nutrient was quickly (cooled after
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addition of zinc chloride to reduce to a minimum reactions with organic materials. PRESERVATIVE: Zn(0H)CI (Percentages calculated on ZnClz equivalent; exactly enough NaOH was added to precipitate the zinc a s oxychloride) No growth 0,3* 0.4 0.5 Growth 0.2 0.1 0.05 PRESERVATIVE. Zn(OH)z (Percentages calculated on ZnC1z equivalent) No growth 0 3 0 4 0 5 Growth 0 2 0.1 0.05
A comparison of the 0.2 and 0.1 cultures in the zinc chlorides, oxychloride, and hydroxide series showed that the growths in the zinc chloride cultures were most vigorous and those in the oxychloride cultures least T.71'gorous. The rates of growth in the series of 0.2 per cent cultures were interesting. After 14 days from inoculation the radial increases of the growths were: ZnC12, 22 mm.; Zn(OH)2, 14 mm.; Zn(OH)Cl, 9 mm. All the growths were healthy. I n the less concentrated cultures all grew rapidly and there were no important differences. Tests were also made on powdered zinc oxychloride, prepared by adding to a zinc chloride solution the calculated quantity of sodium hydroxide solution. ZnCll
+ NaOH
Zn(0H)Cl
+ SaCl
The precipitate was dried for 7 days over calcium chloride, then introduced, in powder form, into a neutral, nutrient gel. PRESERVATIVE Zn(OH)CI (Percentages calculated as ZnCIz) No growth 0.70 Growth 0.60
1.0 0.43
The same lot of material was again tested after standing
2 months. I n this test the killing point was between 0.70 and 1.00 per cent. I n testing insoluble preservatives in powder form, an apparent decrease in toxicity is noted due to mechanical enclosure of some of the material inside the grains of powder. For the same reason, organic preservatives dissolved in petroleum usually show but one-third to one-half their toxicity in the free state. Inorganic preservatives in powder form show a similar relation, since it is impossible to get the perfect distribution obtainable when the preservative is precipitated in the nutrient gel. Summary
1--A culture of the wood-rotting organism Fomes unnosus, starting from a very small transplant, produced, in 14 days, acid equivalent to 113 mg. of acetic acid. 2-The carbonates of sodium and barium are sixteen times as toxic to Fomes annosw as the corresponding chlorides. The carbonates of strontium and calcium are, respectively, three and five times as toxic as the corresponding chlorides. 3-The "killing points" with respect to Fomes annosus of the various salts under test were found to be as follows:
4-The hydroxide and oxychloride of zinc, although difficultly soluble, are fully as toxic as the highly soluble zinc chloride, because of the solvent action of acid evolved by the fungus. .?-Sodium carbonate is as toxic as zinc chloride but ie lacking in permanence when exposed t o rains. 6-Barium carbonate is 55 per cent as toxic t o Fomes annosus as zinc chloride, but otherwise, because of its non-corrosive nature, low electrical conductivity, and permanence. seems to be worthy of test as a wood preservative.
