Help with Ph sensor pin abbreviations

Using Arduino Sensors
21

int ph_pin = A7; //This is the pin number connected to Po

void setup() {
Serial.begin(9600);
}

void loop() {
int measure = analogRead(ph_pin);
Serial.print("Measure: ");
Serial.print(measure);

double voltage = 5 / 1024.0 * measure; //classic digital to voltage conversion
Serial.print("\tVoltage: ");
Serial.print(voltage, 3);

// PH_step = (voltage@PH7 - voltage@PH4) / (PH7 - PH4)
// PH_probe = PH7 - ((voltage@PH7 - voltage@probe) / PH_step)
float Po = 7 + ((2.5 - voltage) / 0.18);
Serial.print("\tPH: ");
Serial.print(Po, 3);

Serial.println("");
delay(2000);
}

Using this code, i m not getting results
pH 20, etc is shown in serial monitor