Cannot read modbus data repetitively
I have found why !!!

1st: buf[8] instead buf[7] in declaration
2nd: reinit byte buf[8] before EC request part.

So the code who works fine is:

byte buf[8];
void loop(){

      Serial.print("pH: ");   
      digitalWrite(modbusModuleRE, RS485Transmit);  // Init Transceiver
      digitalWrite(modbusModuleDE, RS485Transmit);  // Init Transceiver
      byte  pHrequest[] = {0x00, 0x03, 0x00, 0x00, 0x00, 0x01, 0x85, 0xDB}; 
      RS485Serial.write(pHrequest, sizeof(pHrequest));
      RS485Serial.flush();
      digitalWrite(modbusModuleRE, RS485Receive);
      digitalWrite(modbusModuleDE, RS485Receive);
      RS485Serial.readBytes(buf, 8);
     Serial.print(buf[3], HEX);Serial.println(buf[4], HEX);

byte buf[8];

      Serial.print("EC: "); 
      digitalWrite(modbusModuleRE, RS485Transmit);  // Init Transceiver
      digitalWrite(modbusModuleDE, RS485Transmit);  // Init Transceiver
      byte  ECrequest[] = {0x01, 0x03, 0x00, 0x00, 0x00, 0x02, 0xC4, 0x0B};
      RS485Serial.write(ECrequest, sizeof(ECrequest));
      RS485Serial.flush();
      digitalWrite(modbusModuleRE, RS485Receive);
      digitalWrite(modbusModuleDE, RS485Receive);
      RS485Serial.readBytes(buf, 8);
      Serial.print(buf[3], HEX);Serial.print(buf[4], HEX);Serial.print(buf[5], HEX);Serial.println(buf[6], HEX);

Serial.println();

}