2 EXPERIMENTAL                
2.1 Equipments and reagents
XRC-1 melting point apparatus (uncorrected), Perkin-Elmer 2400 elementary analysis instrument, EQUINOX55 Infrared spectrometer (KBr disks), FX-90Q NMR spectrometer and RBC-1 Rotating-bomb calorimeter were used for the measurement.
    4-amino-6-tert-butyl-3-mercapto-1,2,4-triazin-5-(4H)-one (trizine) was prepared by literature procedure^[3], recrystallization twice from methanol. m.p.212°C-214°C(Lit. ^[3] m.p.212°C-214°C). All reagents used are A.R. grade.
2.2 Experimental procedure              
40g (0.2mol) of trizine in 90% H₂SO₄, the reaction mixture was subsequently stirred, heated until all the solid dissolved. Methanol 10ml (0.3mol) was added dropwise. The reaction temperature was kept at 60°C-70°C. After the mixture stirred for 2h-3h, the reaction mixture was cooled and poured into ice-water. The Metribuzin sulfate as white crystal was collected, yield>95%. Recrystallizaton from ethanol.

3 RESULTS AND DISCUSSION
3.1 Analysis results
The melting point of the Metribuzin sulfate is 218°C-219°C. Elementary analysis (%): C 30.80(30.77), H 5.15(5.13), N 18.00(17.95) . So, the composition of the Metribuzin sulfate is C₈H₁₆N₄O₅S₂ M.W. 312. IR (cm^-1): 2750-3500 (m, NH₄SO₄), 2970 (CH₃), 1730 (C=O), 1639 (C=N), 1361,1218 (C-N) , 1066,679 (C-S). ¹H NMR (TMS, DMSO-d6):δ1.35 (9H, s, C(CH₃)₃),δ2.48 (3H, s, SCH₃),δ9.20 (4H, s, -NH₄SO₄). Fig.1 is the ¹H NMR of Metribuzin sulfate. Because the -NH₂ and the H₂SO₄ formed -NH₄SO₄, this decreased the electric density around the N atom, the peak moved to downfield in δ9.20. Fig.2 is the ¹H NMR of Metribuzin sulfate, D₂O as solvent. No peak for protons of -NH₂ group can be found in Fig.2. Because these protons are substituted by the deuterium atoms of D₂O.

Fig.1 ¹H NMR spectra of Metribuzin sulfate    Fig.2 ¹H NMR spectra of Metribuzin sulfate
                                                                                  (added D₂O)

Using acidimeteric method, the content of H₂SO₄ was determined. The result was Metrabuzin : H₂SO₄ = 1 : 1（mol）.
    All the analytical results indicates that the structure of the Metribuzin sulfate is as bellow:

3.2 The standard enthalpy of formations of Metribuzin sulfate and Metribuzin
Same method of reference^[4] was used to determine the combustion energy of the sample,DcU = (- 15909±15.05) J^.g^-1 . The standard combustion enthalpy of the sample, DcH⁰ , refers to the combustion enthalpy changes of the following ideal combustion reaction at 298.15K and 101.325kPa：
C₈H₁₆N₄O₅S₂(s) + 23/2 O₂(g) = 8CO₂(g) + 2SO₂ (g) + 2N₂(g) + 8H₂O(l)
The standard combustion enthalpy of the Metribuzin sulfate was calculated from the combustion energy using the following equation: DcH⁰ = DcU +DnRT, DcH⁰ = (-4964.46±4.70) kJ^.mol^-1. According to the above thermochemical equation, standard enthalpy of formation,
D_fH⁰, was calculated by Hess's law. The result of the calculation was D_fH⁰ = (-1063.88±4.83) kJ^.mol^-1.
    The standard combustion enthalpy of the Metribuzin was determined by the same method as DcH⁰ = (- 5294.85±3.50) kJ^.mol^-1. The standard enthalpy of formations of the Metribuzin was D_fH⁰ = (- 150.85±3.66) kJ^.mol^-1.
    The results indicate that the Metribuzin sulfate is more stable than the Metribuzin. It is very useful for the storage of the pesticide.
3.3 The biological activity test
Small cup method was used and the same gemmation barnyard grass seeds were selected for the test. Distilled water was used as contrast. The concentration of both Metribuzin sulfate and Metribuzin was 2.5%(W/V) in ethanol solution and then diluted to 25 and 50ppm. The temperature was kept at 25°C±3°C under daylight lamp for 72 hrs. The heights of the grass were measured. According to the following equation, the inhibition percentage of the growth Dh% were calculated.
Dh% = ×100%
Where, h₀ and h refer to the height of the sample and the contrast. The results showed: the heights of the grass using Metribuzin were 13.30% (25ppm), 28.42% (50ppm); the Metribuzin sulfate were 13.21% (25ppm) and 28.40% (50ppm). Biological tests showed that it has herbicidal activities corresponding to the Metribuzin.

REFERENCES                  
[1] Zhang Dianjin, Chen Renlin. Encyclopedia of Chemical Weeds Control on Farmland. Shanghai: Shanghai scientific and technological Literature Publishing House, 1994: 625-626.
[2] Wang Yu, Chen Tiebao, Huang Chongyan. Pesticide (Nongyao), 1998, 37 (4): 27-28.
[3] Herbert K, Werner S, Wolfgang L. US Patent,1979,  4 175 188.
[4] Gao Shengli, Yang Xuwu, Ren Dehou et al. Thermochimica Acta, 1996, 287: 177-182.

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