#define sensorPin A0

unsigned int voltage;
float        average;
int cnt = 50;

void setup(){

	Serial.begin(9600);

}
void loop() {

	average = 0.0;
	for(int i=0;i<cnt;i++){
		voltage = analogRead(sensorPin);
		average += voltage;
		delay(10);
	}
	average /= cnt;
	Serial.print("average = ");
	Serial.println(average);

	delay(1000);
}

#define sensorPin A0

float pH_400     = 4.0;
float pH_686     = 6.86;
float pH_918     = 9.18;
float analog_400 = 674.86;
float analog_686 = 558.57;
float analog_918 = 476.98;
int   loopCnt    = 50;
float slope_acidic;
float slope_alkaline;
float pH;
float average;

void setup(){

	Serial.begin(9600);
	slope_acidic   =  ( pH_400 - pH_686) / (analog_400 - analog_686);
	slope_alkaline =  ( pH_918 - pH_686) / (analog_918 - analog_686);
}

void loop() {

	average = 0.0;
	for(int i=0;i<loopCnt;i++){
		average += analogRead(sensorPin);
		delay(10);
	}
	average /= loopCnt;
	Serial.print( "pH = " );
	if (average > analog_686) {
		pH = slope_acidic * ( average - analog_686 ) +  pH_686;
	} else {
		pH = slope_alkaline * ( average - analog_686 ) +  pH_686;
	}
	Serial.println(pH);
	delay(1000);
}